reprocessing of high resolution crustal seismic reflection ... j... · crustal seismic reflection...

Reprocessing of High Resolution

Crustal Seismic Reflection Data

from the

Abitibi Greenstone Belt

b y

Jan Oxbow Kozel

Geophysics Laboratory

Department o f Physics

University o f Toronto

A thesis submitted in conformity with

the requirements for the degree of

Master of Science

at the

University of Toronto

© by Jan Oxbow Kozel

1990

R e p ro d u c e d with p e rm iss ion of the copyright ow ner. F u r th e r reproduction prohibited without perm iss ion .

Abstract

Deep crustal reflection seismic data were collected in the Abitibi Greenstone

Belt during the winter of 1987-8 as part of the Lithoprobe Project. Although much

reflection energy was visible after initial processing, it was apparent that the stacked

section could be improved. In particular, very little reflection energy was imaged

in the upper 1 s, making it difficult to correlate deeper reflections with the mapped

surface geology. The high resolution line 12A was reprocessed with a particular

effort to obtaining a better image of the shallow data.

Spectral equalisation of the upper 3 to 4 s and an accurate, complete static

solution were found to be the most important steps in the reprocessing. Ground

roll proved difficult to attenuate, with velocity filtering showing some promise, al

though not entirely satisfactory. Neax offset, low frequency vibrator noise was only

partially attenuated through spectral equalisation, and may have been caused by

subhaxmonic distortion in the vibrator signal. The dip moveout effect was substan

tial for shallow data in the southern part of the line.

Reflection energy is apparent at all travel times in the unstacked data, although

most of it does not stack coherently because its amplitude and phase characteristics

vary with offset. Reflection energy above 100 Hz could not be imaged as coherent

reflection energy after stacking.

Reprocessing resulted in great improvement throughout the stacked section,

with considerable reflection energy being imaged to 0.5 s. The results of this thesis

illustrate ihe value of detailed processing of deep crusted reflection data in Archaean

cratons.

R e p ro d u c e d with perm iss ion of th e copyright ow ner. F u r th e r reproduction prohibited without perm iss ion .

Acknowledgements

I am most of all grateful to my supervisor, Prof. Gordon tyv>>f. His keen

interest in this project as well as his able help have committed these year? fondly

to my memory. I admire his extremeljr broad knowledge and int?i'«*ts, and he has

been a fine example to me.

Next I would like to thank Dr. Kris Vasudevan of the Lithoprobe Seismic Pro

cessing Facility (LSPF) in Calgary. Not only was he an eager participant in many

discussions about my work, he also worked very hard to expedite my processing

on the Lithoprobe computer. The rest of the staff at the LSPF also deserve my

thanks for all of their work in keeping the facility operating smoothly. They are

Glen Lines, Angela Dutoit, and Todd Clark. I especially thank Todd for patiently

wrapping and sending to me many scores of plots, keeping them remarkably free of

footprints. There are also the many (unfortunately) nameless and faceless operators

\vh i mounted so many tapes for me.

Prof. Luck Bailey and Dr. Gerhard P ratt were the readers of this thesis, and I

f canK ' ■ ~ for their careful reading and insightful comments.

Claire Samson has been of much help to me, having read portions of this thesis

thoroughly, and has been a participant in many discussions, both seismic and non-

seismic in nature. Peter Hurley was most helpful in guiding me through my first

experiences with seismics both in the field during the chilly December of 1987 and

in processing.

I appreciate Shell Canada’s allowing me to use in my thesis some of the results

of my work there during the summer of 1990. Gary Billings was very kind in

helping secure permission to do this, as well as in sticking around after work to

- ii -


discuss aspects 01 chis thesis.

For helping to put my work into geological perspective, both specifically and

in general, I thank: Drs. Richard Sutcliffe and Steve Jackson of the Ontario Geo

logical Survey for showing me how geologists think (and behave) in the field during

the summer of 1988; John Varsek at the University of Calgary, for many eclectic

discussions; and Prof. Fred Cook, also at Calgary for opening a global geological

perspective to me through his course in 1989.

I would like to take this opportunity to thank the many who have been impor

tant to me personally during my days (and nights) at school: my family, Dianne,

Rob, Geoff, Norrie, Hans the Guide, my longstanding officemate Wank, my various

tennis partners and hockey players, the Unmade Beds, Rudolf Pez, and the Saigon

Palace Restaurant. My guitar thanks Neil Young. My piano thanks Mozart, Bach,

and Gershwin.

- iii -


Table of Contents

Chapter 1 : Introduction

1.1 B ackground to the Seismic Reflection S tudy ............................................. 1

1.2 Geology of th e A bitibi G reenstone B e l t .......................................................... 2

1.3 C haracteris tics of the D a t a .................................................................................5

1.4 O bjectives ................................................................................................................7

1.5 O u tline .................................................................................................................... 7

Chapter 2 : A cquisition and Preliminary Processing

2.1 A c q u i s i t i o n ................................................................................................................8

2.2 P relim inary P r o c e s s in g ..........................................................................................9

Chapter 3 : Reprocessing o f Line 12A

3.1 Frequency C ontent of the D a ta ......................................................................15

3.2 A m plitude Com pensation and CM P B inning ........................................... 17

3.3 A tten u a tio n of the G round R o l l ......................................................................19

3.4 V ib ra to r Noise .....................................................................................................26

3.5 S ta tic C o r r e c t io n s ................................................................................................ 29

3.6 Velocity Analysis ................................................................................................ 36

3.7 V ariability o f Reflection Energy w ith O f f s e t ............................................... 37

3.8 Inclusion of High Frequencies in the Processing ...................................... 39

3.9 S tacked S e c t io n s .................................................................................................... 40

Chapter 4 : Conclusions

4.1 S u g g e s t io n s ............................................................................................................. 42

4.2 E valuation and In terp re ta tion of the Reprocessed S e c t i o n ....................44

Appendix

D ISCO Jo b Decks ..................................................................................................... 47


Chapter 1

Introduction

1.1 B ackground to th e S eism ic R eflection S tu d y

The use of reflection seismology to explore the crystalline basement of the

continental crust dates to the 1950s and 60s (Junger [1951], Richards and Walker

[1959], Widess and Taylor [195:1], Dix [1965], Dohr and Fuchs [1967]). These initial

results were most encouraging, showing that coherent reflections could be obtained

from within crystalline igneous and metamorphic rocks (which had been in doubt),

and eventually led to the establishment of COCORP (Consortium for Continental

Reflection Profiling) in the United States in 1974. The goal of this program is

to conduct and interpret deep crustal reflection surveys throughout the U. S. The

successes of this program resulted in Em increased pace of scientific exploration

around the world in the early 1980s, with deep crustal programs being established

in many countries. Canada joined the scene in 1984 with the institution of Phase I

of Lithoprobe.

The results achieved with the seismic imaging of the subduction zone beneath

Vancouver Island (Clowes et al. [1987]) led to the establishment of phase II of

Lithoprobe: the probing of the continental crust throughout Canada, concentrating

on a number of “transects” , each focussed on particular geological environments and

problems. The program’s philosophy is to coordinate studies using a great variety of

geophysicEd and geological methods (seismic reflection and refraction, potential field,

electro-magnetic, radio-isotopic, structural, and geochemical studies), integrating

the results to give a coherent tectonic interpretation of each region.

- 1 -


Chapter 1 Introduction 2

The Kapuskasing Structural Zone (KSZ) Transect, initiated in 1987, was the

first of phase II. Five hundred and seventy three line kilometres of vibroseis data

were acquired in the central Canadian shield during the autumn and winter of 1987-

8 by Veritas Geophysical Ltd., with nine lines being acquired in the KSZ, and four

lines (1 2 ,12A, 14, end 14AB) in the Abitibi Greenstone Belt (AGB; see figure 1.1).

The data studied in this thesis are from line 12A in the AGB. Although collected

as part of the KSZ Transect, the AGB lines are actually preliminary studies for

the Abitibi - Grenville Transect. The reflection data for this transect me being

acquired in late 1990. Lithoprobe’s primary goal in acquiring the data in the AGB

was to determine optimal acquisition and processing methods, with a secondary

goal of evaluating existing tectonic interpretations of the AGB.

1.2 G eology o f A b it ib i G reen sto n e B elt

Archaean cratons constitute the bulk of the continental crust (Kroner [1981]).

Having experienced very little deformation since Precambrian times, they are cru

cial to our understanding of crustal formation during the early history of the earth.

These cratons are predominantly composed of two types of terrains, granite - green

stone and granulite - gneiss, with these two types likely being formed in different

tectonic environments (Kroner [1981], Windley [1981]). Greenstone material, which

comprises approximately 20% of the granite - greenstone terrains, is characterised

by mafic volcanic material which has undergone regional low pressure, greenschist

facies metamorphism (Goodwin [1981]). These greenstone belts are common in

Precambrian terrains, but axe often considered to be different from Phanerozoic

volcanic material, although m odem volcanic arcs and back axe basins show some

similarities to Archaean greenstone belts (Windley [1981]). This suggests that at

least some aspects of the Archaean tectonic regime differed from the modern Wilson

Cycle.


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Figure 1.1 Simplified geological map of Central Abitibi Greenstone Belt (AGB). The seismic lines acquired as part of the Kapuskasing Transect in 1987-88 are marked. LCF = Larder Lake - Cadillac Fault Zone; PDF = Porcupine - Destor Fault Zone; BR = Blake River Group; Ki = Kinojivis Group; Ti = Timiskaming and lithologically similar groups; PO = Pontiac Group; HM = Hunter Mine Group; LL = Larder Lake Group; SK = Skead Group; PA = Pacaud tuffs; CA = Catherine Group. From Green et al. [1990].


99


A number of settings have been proposed for the formation of greenstone belts

based on some similarities with Phanerozoic structures. These include various

rift environments, both in continental and oceanic crust, volcanic arcs, back arc

marginal basins, and crusted shortening regimes (Garson and Mitchell [1981]). One

of the purposes of the Lithoprobe seismic program is to evaluate hypotheses regard

ing the formation of the AGB.

The AGB is located in the Superior province of the Canadian shield, which

is composed of a number of east - west trending granulite - gneiss and granite -

greenstone subprovinces. Since the time of the formation of the AGB circa 2.7 Ga

ago (e. g. Ludden et al. [1986], Jackson et al. [1990]), there has been relatively

little deformation, intrusion, and erosion, making the AGB not only the largest,

but also one of the best preserved greenstone belts in the world (Goodwin [1981]).

Moreover, the presence of notable economic mineral deposits has prompted many

studies of the region, so the geology is relatively well explored. Therefore, this is a

prime research area, and we can expect the seismic data to reveal structures extant

from the formation of the AGB.

Earlier studies have shown that the AGB is composed of regions of varying

composition and style of emplacement. Ludden and Hubert [1986] divided the

region into four zones as shown in figure 1.2: the northern volcanic zone (NVZ),

the centred granite - gneiss zone (CGG), the southern volcanic zone (SVZ), and the

southern granite - gneiss ;:one (SGG). Dimroth et al. [1982] have divided the AGB

into an internal zone (towards the interior of the craton, corresponding to the NVZ

and CGG), and an external zone (towards the margin of the craton, corresponding

to the SVZ). Greenstone material from the internal zone was possibly emplaced on

continental crust, whereas material from the external zone appears to have been

emplaced on oceanic crust (Jackson and Sutcliffe [1990]). The SGG corresponds to

the Pontiac Metasedimentary Belt (PMB). Dimroth et al. [1982] interpret the PMB

as being the sedimentary foreland of the AGB.


ONTARIO

(M A P 'j AREA* J r\

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f t ***’

P O N T I A d ^ GRANITOID ROCKS

MAFIC INTRUSIONS

ULTRAMAFIC LAVAS

SEDIMENTSPOST

ARCHEAN| | MAFIC-FELSIC

£ MASSIVE GNEISSIC ' - TONAUTE100 km

............. rf > TECTONIC FRONT

PARAGNEISS- _ 3 MIGMATITE

Figure 1.2 Map of the Abitibi Greenstone Belt, composed of the folowing: 1 = Northern Volcanic Zone; 2 = Southern Volcanic Zone; 3 = Central Granite - Gneiss Zone; 4 = Southern Granite - Gneiss Zone; dashed lines separate these zones. KSZ = K v puskasing Structural Zone. Box indicates area detailed in figures 1.1 . From Ludden and Hubert [1986].



1.2.1 Volcanic Material

Volcanic rocks of the AGB were erupted in two cycles. The following description

of this volcanic material is summarised primarily from Dimroth et al. [1982, 1983]

and references therein.

Despite limited exposure of the first cycle, it is known to consist of interdigi-

tating rhyolite and basalt flows. The age of the top of this cycle is 2711 +6/-3 Ma

as determined from U - Pb dating of zircons. The bulk of the southern AGB is

composed of material from the second cycle, which unconformably overlies the first

cycle and is subdivided into three stages. The lowest is an extensive ultramafic (ko-

matiitic) lava plain of 7 km thickness, formed in a deep (~ 2 km) sea environment,

and underlies much of the southern AGB. The second stage involved the eruption

of the Kinojevis Group, composed of uniform and differentiated tholeiites. This

material is 5 to 7.5 kilometres thick and was erupted as deep marine lava plains in

the west, and central volcanic complexes in the east. These complexes were built up

nearly to sea level in some areas. The upper stage involved the eruption of the Blake

River Group (BRG) from central volcanic complexes, with material building up to

sea level. The maximum thickness of the BRG is 10 km in the west, with it lensing

out to the east. This group has a complex interned stratigraphy and very diverse

composition (mafic to felsic), being formed of alternating tholeiitic and calc-alkalic

differentiation suites. The age of the top of this group is 2700 +4/-3 Ma, so the

entire 15 to 25 kilometres thickness of the second cycle erupted over approximately

10 Ma. The emplacement of these large volumes of volcanic material may have

resulted in large scale subsidence and a synclinal structure of the BRG (Jensen and

Langford [1985]). Isotopic evidence indicates that this volcanic material does not

have a significantly evolved crustal component, indicating that this materiel was

not emplaced on evolved (continental) crust (Jackson and Sutcliffe [1990]).



1.2.2 Deformation Structures

The AGB is transected by two regional steeply dipping mineralised deformation

zones, the Destor - Porcupine Deformation Zone (DPDZ) and the Larder Lake -

Cadillac Deformation Zone (LCDZ). These have been interpreted in a number

of ways. Dimroth et al. [1982] believe them to be normal faults resulting from

subsidence during the eruption of the ultramafic basalts at the beginning of the

second cycle. Hubert et al. [1984] and Ludden et al. [1986] have interpreted them

to be sinistral strike slip faults caused by oblique convergence between different

terrains to the north and south, resulting in rifting and the eruption of the second

cycle volcanics. This was cotemporal with or followed by north - south compression,

resulting in thrust movement along the the DPDZ and LCDZ. Jackson and Sutcliffe

[1990] infer north side down motion along the LCDZ after 2680 Ma, followed by

south side down motion in the early Proterozoic (24S0 to 2460 Ma). Thus, these

zones appear to have been active at a number of stages with composite dip and

strike slip movement. The presence of such regional deformation zones throughout

the Abitibi - Wawa subprovince with an apparent absence in the Kapuskasing Uplift

suggest that these may not extend to the lower crust — they either become listric

with depth, or are overprinted (Jackson et al. [1990]).

1.3 C h aracteristics o f th e D a ta

All of the data collected as part of the KSZ Transect was initially processed

by Veritas Seismic Ltd. using relatively basic procedures to give a preliminary set

of seismic profiles. This processing scheme will be described in section 2.2 . From

these stacks, we shall now make general observations about the characteristics of

the data.

One is immediately struck by the number and quality of reflections in the line

12A preliminary section (see plate 1). It is apparent that there are two styles of



reflectors in these data, which are typical of all of the data from the KSZ Transect:

1) in the mid crust (2 to 5 s), reflecting zones are continuous over long distances and

are relatively short in time duration (see figure 1.3(a)); and 2) in the deeper crust (>

5 s), there is higher overall reflectivity with thicker reflective zones being composed

of many interweaving reflections of short spatial extent (see figure 1.3(b)). It is

worthy of noting that throughout the KSZ dataset, there are no strong indications

of a reflection Moho. In the AGB, there is a decrease in reflectivity at 11.0 to 11.5

s which has been interpreted to be the Moho (Green et al. [1990]), but this is not

a sharp, clear boundary. This is in agreement with Meissner's [1986] observation

th a t the Moho is less prominent or not at all detectable from reflection data in

old cratons. The lower crust in such areas is of high velocity (~ 7 km /s), with a

gradual transition from crustal to mantle velocities. Meissner attributes this velocity

structure to higher heat flow in the Archaean, which could have resulted in more

melt being extracted from the mantle, forming a thicker, less differentiated, higher

velocity crust.

There is good penetration of the energy, as is demonstrated by the high reflec

tivity up to 8 s. Only in the upper 1 s of the section is there very low reflectivity.

This may not be a true reflection of the geology because there axe penetrative

structures visible at the surface which one might expect to be capable of producing

reflections. Some notable examples are 1) the Crosby Sill, 2) the contact between

the Kinojevis and Blake River Groups, and 3) a nearby deformation zone, all of

which are possible candidates for the dipping reflection noted in figure 1.4 (Jackson

et al. [1990]). If not due entirely to the low reflectivity of the rock, then the appar

ent low reflectivity in the shallow data is likely the result of problems in the data

processing.

This zone of apparent low reflectivity causes problems in the interpretation of

the section. Direct observations of the geology have only been made near the surface,

whereas reflections from the seismic section cannot generally be traced to shallower


300

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Reproduced

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VP 400£ KINOJEVIS GROUP

CROSBY

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600 800BLAKE R IV ER GROUP

0.0 8

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Figure 1.4 Reflection projecting to the surface location of the Crosby Sill.


than 1 s ( 3 km). Thus, there is a gap of several kilometres between the two. The

situation is further complicated because many structures at the surface are steeply

dipping (Jackson et al. [1990]), whereas many of the reflections are sub-horizontal,

causing difficulty in correlating surface geology with deeper structures.

1 .4 O b jectives

The primary objective of this thesis is to reprocessing line 12A, thereby evalu

ating the need for detailed processing of crustal seismic data. Therefore, the bulk

of this thesis will involve an analysis of processing methods used in obtaining an

improved stacked section. Due to the importance of the shallow data in interpreting

the section, special effort was spent in reprocessing the upper 1 to 2 s.

An evaluation of the acquisition is beyond the scope of this work. However,

certain aspects of the acquisition method will be examined where they may have

contributed to difficulties in the processing (for example, problematic noise which

could be attenuated through design of the survey).

1.5 O u tlin e

In chapter 2, work performed by the contractors will be reviewed (the acquisi

tion method in section 2.1, the preliminary processing in section 2.2). Some weak

nesses in the processing will be identified which will guide efforts to reprocess the

data, detailed in chapter 3. Also in chapter 3, some aspects of the acquisition will

be examined where these aspects have significant effects on the processing. In chap

ter 4, the results of the reprocessing will be examined and evaluated in comparison

with the results of the preliminary processing. Conclusions about the processing of

seismic data in the AGB will also be detailed, hopefully guiding future seismic work

in the Lithoprobe Abitibi - Grenville Transect.

R e p ro d u c e d with p e rm iss ion of th e copyright ow ner. F u r th e r reproduction prohibited without perm iss ion .

Chapter 2

Acquisition and Preliminary Processing

2.1 A cq u isitio n

Two sets of parameters were used in acquiring the KSZ reflection data: “re

gional” , used for most of the data, and “high resolution” , used in selected areas of

special interest (lines 12A and 14AB in the AGB — see figure 1.1). The subject of

this study is the high resolution line 12A. The two sets of parameters are detailed

in table 2.1, with the most significant aspects being contrasted in the following

paragraphs.

Vertical motion recordings from 240 receiver groups were acquired using two

DFS-V recording systems and OYO McSeis III (14 Hz) geophones. Most of the data

in the AGB were collected using an asymmetric receiver spread with 60 channels to

the north and 180 to the south of the vibrator point (VP). Six (sometimes seven)

stations on each side of the VP were not recorded for all lines except 12A and 14A,

where this gap was increased to eight stations on each side. At the ends of the

lines, this shot gap was removed, and the recording spread was stationary as the

VP advanced along the line (known as “roll-on” or “roll-off”). The spacing between

receivers was 20 metres for the high resolution, and 50 metres for the regional.

The source array consisted of two Mertz 18 vibrators (each with 20 072 kg

peak force) for the high resolution data, and four for the regional (although much

of the regional data was acquired using only three vibrators due to phase control

problems). For the high resolution, an 8 s, 20 to 120 Hz upsweep was used, with

a 14 s, 12 to 52 Hz upsweep for the regional. The toted record length of the high

resolution records is 8 s, and 16 s for the regional. There was a VP at every station

- 8 -


Regional High Resolution

Number of Vibrators 4 2

Sweep 12 - 52 Hz 20 - 120 Hz

Sweep Length 14 sec 8 sec

Record Length 16 sec 8 sec

Vibration Point (VP) Spac 100 m 20 m

ingNumber of Sweeps per VP 8 8

Number of Receivers 240 240

Receiver Spacing 50 m 20 m

Subsurface Coverage 60 Fold 120 Fold

Table 2.1 Acquisition parameters for regional and high resolution surveys.

R e p ro d u c e d with p e rm iss ion of th e copyright ow ner. F u r th e r rep roduction prohibited without perm iss ion .

1) STACK OF ADJACENT SHOT GATHERS

2) AUTOMATIC GAIN CONTROL

3) TRACE EDITING

4) CROOKED LINE GEOMETRY

5) REFRACTION STATIC CORRECTIONS

6) CMP SORT

7) VELOCITY ANALYSIS

8 ) NMO CO RRECTIO N

9) FIRST BREAK MUTE

10) NON SURFACE-CONSISTENT RESIDUAL STATIC CORRECTIONS

11) STACK

Table 2.2 Preliminary processing sequence.


Chapter 2 Acquisition and Preliminary Processing 9

for the high resolution, resulting in 120 fold data with 10 metre common mid-point

(CMP) spacing. For the regional, there was a VP at every second station, resulting

in 60 fold data and 25 metre CMP spacing.

2 .2 P ro cessin g

Veritas Seismic Ltd. processed the data to obtain initial seismic sections using

the processing sequence listed in table 2.2 . There were two main reasons for

producing preliminary seismic sections: 1) preliminary geological interpretations

may be made, which would, in turn, guide future work, and 2) as an intermediate

result, they can guide further processing.

In the following section, significant procedures will be examined with possible

weaknesses being highlighted. In section 2.2.2, possibilities for improvement during

reprocessing will be outlined.

2.2.1 Preliminary Processing Scheme

11 Stacking of Adjacent Shot Records

The KSZ seismic data form an enormous dataset with a very high CMP fold

of 120. There were 1530 shot records of 8 s length with 2 ms sampling for line

12A alone. Thus, the processing of the data is a formidable undertaking, consid

erably more costly and problematic than standard processing in industry which

often involved 4 s of data with a CMP fold of 24 or 48 at that time. In order to

alleviate the cost, adjacent shot records were summed before further processing.

This has two detrimental effects on further processing: 1) the quality of the first

arrivals is degraded due to interference effects, resulting in poorer estimates of the

static corrections using a refraction static routine (see step 3), and 2) the quality of

the signal is degraded in the shallow data where the differential moveout between



summed traces could be significant. Thus, the stacking of adjacent shot gathers is

likely a partial reason for the low apparent reflectivity in the shallow section.

21 Crooked Line Geometry

The definition of the geometry of the survey involves the specification of the

ground surface geometry (the location and elevation of each station, and the record

ing spread configuration), and the sub-surface geometry (the locations and sizes of

the CMP bins — reflection points which lie outside these bins are not included in

further processing). Because the seismic data were acquired along existing minor

roads, the seismic lines are crooked, resulting in a wide scattering of CMPs wher

ever there were substantial bends in the line, spanning a width as great as several

kilometres in certain areas. Thus, the choice of a CMP line is important, with many

traces falling outside of the CMP bins and being dropped from further processing.

The line defined for preliminary processing was selected to maintain as high a fold

as possible (i. e. so that as many traces as possible fall within the defined CMP

bins).

31 Static Corrections

Due to the varying elevation of the sources and receivers along the line, elevation

static corrections must be applied to reposition the data to a datum level. The

surface conditions along the line are variable, ranging from minimal overburden

to over ten metres of loose glacial till. This results in varying travel time delays

in addition to the elevation static correction. Refraction static corrections were

applied to account for this varying overburden thickness and for lateral bedrock

velocity variations. However, the accuracy of the corrections is dependent on the

accuracy of the picking of the first arrivals. From a visual examination, the picks

made during preliminary processing were found to be unsatisfactory. Partly, this

is because of a relatively high noise level, which is known to cause difficulties for

automated picking algorithms (Mayrand et al. [1987]). This is aggravated by the



variable surface conditions which may result in variable source - ground coupling

and a variable wavelet. Furthermore, as was mentioned in step 1, the stacking of

adjacent shot gathers causes a deterioration in the quality of the first arrivals. Due

to the unsatisfactory quality of the first arrival picks, one would expect that the

refraction static corrections Eire also unsatisfactory.

Refraction static corrections do not form the complete solution to the statics

problem; the method cannot determine very short wavelength statics (less than one

receiver spread). For this reason, non surface - consistent residual static corrections

were also determined for the data. These corrections are not well founded in physical

reality since Ein independent correction is determined for every trace in the dataset

in order to align reflections across a CMP gather. No regard is given to the surface

locations which correspond to the static corrections. To be more physically valid,

the residual corrections would have to be constrained to be surface - consistent

(i. e. each surface location would have an associated static correction representing a

near surface travel time delay). This method will be summarised in section 3.5.3 .

41 Velocity Analysis

An EuiEilysis must be performed to select stacking velocities which are used

to remove the normal moveout (NMO) effect. By selecting the highest coherency

of events on a number of stacks made using different stacking velocities, a velocity

function for the line was defined. One would expect deep reflections to be insensitive

to the choice of velocity in this survey, but shallower reflections were surprisingly

insensitive as well. This leads one to believe that the pre-stack processing was not

optimal. In particular, a poor solution to the statics problem and the reverberatory

character of the wavelet could lead to velocity insensitivity. The latter suggests the

need for deconvolution or another method of spectral balancing. Furthermore, there

Eire few coherent reflections above 2 s, resulting in poor shallow velocity control.



S'! Stack

The stacking of CMP gathers is meant to increase the signal to random noise

level (S/N). In addition, coherent noise which does not have hyperbolic moveout

corresponding to the stacking velocity should also be attenuated. There are two

forms of coherent noise which are common to the entire KSZ dataset: 1) ground

roll, and 2) neax offset, low frequency source generated noise, termed “vibrator

noise” . The stronger of the two is the ground roll — a high amplitude, low velocity

wavetrain found in the upper 1.0 to 1.5 seconds in all shot records (see figure 2.1).

Vibrator noise, appearing as ghost images of the ground roll (see figure 2.2), is not

as pervasive as the ground roll, but often dominates the near offset traces.

Since both types of noise are of much higher amplitude than primary reflections,

the reflection energy is obliterated by the noise, even after stacking. Moreover, the

autom atic gain control (AGC) reduces the amplitudes of shallow primary reflections

in the vicinity of the ground roll, forming an “AGC shadow”. Thus, the coherent

noise can degrade data quality, especially in the shallow data.

2.2.2 Strategy for Reprocessing

Having outlined the preliminary processing scheme, it is clear that reprocessing

should improve reflection energy on the seismic section. Modifications to the steps

which were taken and additional procedures which could be used are summarised

in the following sections.

1) Stacking of adjacent shot records

This was done solely to reduce the data volume for processing. Due to the

number of processing problems which resulted from this, this data reduction should

not be done during reprocessing.


TRACE 50 100

REFRACTED P WAVE ARRIVAL

GROUNDROLL

Figure 2.1 Ground Roll.


TRACE 50 100

0.0 s

1.0

2.0

Figure 2.2 Near offset, vibrator related noise. Trace spacing is 20 m. The vibrators were located between traces 60 and 61.



21 Spectral equalisation

Higher frequency reflection energy is attenuated more than low frequency en

ergy. The amplitude spectrum of the data should be equalised in order that the

weaker high frequency information is boosted, resulting in broader signal bandwidth.

3) Crooked line processing

Since the CMP scatter can be as large as severed km, there is freedom in the

choice of a CMP line. The choice made during preliminary processing should be

reexamined to determine if a better CMP line could be defined.

41 Attenuation of coherent noise

As was mentioned in the preceeding section, stacking is not a satisfactory means

of attenuating coherent noise where it dominates the reflection energy. Thus, par

ticular effort should be spent in attenuating the ground roll and vibrator noise.

51 Static corrections

A new refraction static solution should be calculated after more accurate pick

ing of first arrivals. Then surface - consistent residual static corrections should be

determined rather thai. the less physically valid non surface - consistent method

which was used during preliminary processing.

61 Problems particular to the shallow data

Why might we expect poor imaging of the shallow data after preliminary pro

cessing when it is clear that signal is strong enough to image deep reflectors? 1)

High amplitude ground roll energy obscures signal in the upper 1.0 to 1.5 seconds of

the record. Thus its attenuation should lead to improved imaging of the near sur

face. 2) Shallow data has a greater sensitivity to NMO velocity than does deep data.

After reprocessing, there will likely be much better velocity control on the shallow

reflections. 3) Standard CMP processing assumes that the earth has a laterally

homogeneous, horizontally layered velocity structure. Then the surface projection

of the poir/ where the seismic energy is reflected (the common reflection point,

with p erm iss ion of th e copyright owner. F u r th e r rep roduction prohibited without perm iss ion


or CRP) for a particular shot - receiver pair will coincide with the geometric mid

point, the CMP. Where the velocity structure deviates from this ideal structure, the

CRP and CMP do not coincide, with the discrepancy being greater for shallower

structures. In particular, where reflecting horizons are dipping, the shape of the

reflection hyperbola is changed, with the curvature decreasing and the reflection

points being moved updip (Yilmaz [1987]). This necessitates that a dip moveout

(DMO) correction be applied to accurately image the sub-surface. Since dipping

structures are visible in the surface geology, one would expect that DMO removal

would be necessary during reprocessing.


Chapter 3

Reprocessing of Line 12A

The resources at the Lithoprobe Seismic Processing Facility (LSPF) at the

University of Calgary were available for use during the course of this thesis work.

The computer hardware consists of a CYBER 835 computer with a MAP-V array

processor; the available software is the DISCO seismic processing package.

The intended reprocessing sequence is listed in table 3.1 . In many cases, results

had to be determined using the bulk of the processing flow in order to determine

the results of individual processes. In the following sections, we shall examine the

individual steps.

3.1 Frequency C on ten t o f th e D a ta

The first step in processing seismic data should be an examination of the fre

quency content. It is most useful for further processing to know the frequency

characteristics of the signal and of the noise.

3.1.1 Reflection Energy

Can reflection energy over the full sweep bandwidth t f 20 to 120 Hz be imaged?

The amplitude spectrum in figure 3.1 shows that most of the reflection energy is

between 25 and 50 Hz. Above 70 Hz, amplitudes are over 20 dB below the peak.

Even if the high frequency components were boosted, it would still be very difficult

to enhance through stacking whatever weak signal that may be present at high

frequencies due to its sensitivity to static corrections, stacking velocities, and the

presence of dipping reflectors. Limiting the frequency bandwidth of the data to 20

- 1 5 -


1) NEW CROOKED LINE GEOMETRY

2) TRACE EDITING

3) ATTENUATION OF GROUND ROLL

4) ATTENUATION OF VIBRATOR NOISE

5) REFRACTION STATIC CORRECTIONS

6) SPECTRAL EQUALISATION

7) AUTOMATIC GAIN CONTROL

8) CMP SORT

9) SURFACE-CONSISTENT RESIDUAL STATIC CORRECTIONS

10) VELOCITY ANALYSIS

11) NMO AND DMO CORRECTION

12) STACK

T able 3.1 Intended reprocessing sequence.


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0

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0 80 160

FREQUENCY (HZ)Figure 3.1 Amplitude spectrum of da ta between 1 and 4 s from a shot record with good reflection quality, i. e. the spectrum indicates reflection energy and background

240

Chapter 3 Reprocessing o f Line 12A 16

to 60 Hz would likely result in little loss of reflection energy. The data could then be

resampled from 2 ms to 4 ms, reducing the data volume by half, making subsequent

processing less cumbersome and time consuming. Since low frequency energy still

dominates within the 20 to 60 Hz bandwidth, the amplitude spectrum of the data

should be balanced over this range.

The spectral equalisation method which was used operates as follows: 1) the

data are filtered using three zero phase band-pass filters (20-30, 35-45, 50-60 Hz);

2) a 150 ms AGC (see section 3.2) is then applied to each of the resulting band-

limited records; 3) the 35-45 and 50-60 Hz band records are scaled to have equal

root-mean-square (RMS) amplitudes, with the 20-30 Hz record scaled to one half

of the amplitude of the other two (to reduce contamination by low frequency noise

— see section 3.1.2 below); and 4) these band-limited records are stacked.

To show the effect of spectral equalisation, similarly stacked data with and with

out the application of this process are compared in figure 3.2 . Clearly, the imaging

of some shallow reflections has been improved. For data below 3 s, the spectral

equalisation process was found to make very little improvement, suggesting that

the higher frequency reflection energy within this bandwidth has been attenuated

to below noise levels. Therefore, since it is an extremely time consuming process,

spectral equalisation was only applied to the upper 3 to 4 s of data.

S.1.2 Noise

There are three primary types of noise evident in the pre-stack data: 1) ground

roll; 2) near offset, low frequency vibrator noise; and 3) miscellaneous noise which

contaminates entire traces (e. g. 60 Hz noise, cultural noise, or poor receiver cou

pling). The third type of noise can be removed quite simply by trace editing and

applying a 60 Hz notch filter. The first two types of noise require further examina

tion.


400

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The ground roll was found to be of a bandwidth comparable to that of the

reflection energy, suggesting that there will be difficulty in attenuating this noise.

This issue will be examined in detail in section 3.3 .

On shot gathers where vibrator noise is significant, it was found to be of pre

dominantly low frequency (20 to 40 Hz). This noise will be discussed further in

section 3.4 .

3 .2 A m p litu d e C o m p en sa tio n an d C M P B inn ing

One has two options with regard to amplitude compensation in the process

ing of seismic data: 1) to preserve the relative amplitudes by applying gain to

compensate for spherical divergence and varying coupling between the ground and

the sources and receivers, or 2) to lose relative amplitude information by applying

non-deterministic scaling (AGC). By preserving the relative amplitudes, different

reflectivity levels throughout the stacked section may constrain lithological inter

pretations and correlation of reflection events across the section. However, this

requires more careful editing to remove noise from the data, which is extremely

time consuming and beyond the scope of this thesis.

A pre-stack AGC was applied; in the upper 3 s, a 150 ms window was used in

order to minimise the size of the AGC shadow of the first arrivals and ground roll,

with a 750 ms window used below 3 s. By balancing the amplitudes both within

and across traces, the level of incoherent noise becomes comparable to that of the

reflection energy and will not dominate after stacking. Nevertheless, traces with

obviously low S/N were killed at an early stage during the processing. Anderson

and McMehan [1989] and Mayrand and Milkereit [1988] have found that cursory

trace editing provides an increase in S/N comparable to that provided by very

careful trace editing provided that the fold is not low and that an AGC is applied.

R e p ro d u c e d with p erm iss ion of th e copyright ow ner . F u r the r reproduction prohibited w ithout perm ission .

C hapterS Reprocessing o f Line 12A 18

The choice of a CMP line and bin sizes determines which traces will be dropped

from further processing, and may be particularly important where CMP scatter is

great. In such areas, the CMPs corresponding to different offsets will trace different

lines. By stacking traces within various offset ranges, the effect of using different

CMP lines is approximated. In this way, the data need not be sorted into different

CMP gathers a number of times, which is an extremely time consuming process.

Sections stacked using only very long offsets (over 2 km) or very short offsets

(up to 500 m) showed poorer reflection quality than those using only medium offsets.

At long offsets, S/N is lower because an array of vibrators emits most of its energy

vertically downwards, with less energy being emitted at shallow angles (Sheriff and

Geldart [1982]). At short offset, the data Eire often contaminated by vibrator noise.

Reflection energy w eis not sensitive to offset within the range of 500 to 2000 m,

in part due to the need for more comprehensive processing in order to image the

weaker reflections (especiEilly in the shallow data). Therefore, an intermediate offset

value of 750 m was selected in order to form the new CMP line; the line formed by

CMPs corresponding to traces with this offset was defined to be the new CMP line.

It now remEiins for the bin sizes to be selected. During preliminary processing,

the bins were 10 m (in-line, equal to the CMP spacing) by 800 m (cross-line). After

processing, every three adjacent traces were stacked to give one final trace. For this

reason, a CMP spacing Eind in-line bin length of 30 m was selected. This makes

reflection energy on CMP gathers more easily observed — since there are more

traces within a narrow offset range, reflection energy which is intermittent with

offset (see section 3.7) is consistent over a greater number of traces.

The selection of the cross-line dimension of the CMP bins is only important in

areas cf large CMP scatter. When bins are very narrow (less than 400 m), the fold

in such areas becomes very low, resulting in much poorer imaging of reflections.

When bins of greater than 800 m width are used, there is slightly poorer imaging of



reflections because reflection information from very widely varying reflection points

is stacked, resulting in some destructive interference. Deterioration in reflection

quality is more marked if the bin sizes are extremely small than if extremely large.

In addition, if bins are wider than 800 m, the residual static solution becomes

unstable (see section 3.5.3). This prompted the selection of 30 x 800 m CMP bins.

3 .3 G round R oll

S.3.1 Characteristics

The upper one second of the shot gathers is dominated by coherent, source

generated surface energy which is known as “ground roll” (see figure 3.3). Any

shallow reflection energy present is obscured within the “AGC shadow” of this

noise. Even after stacking using an AGC window of 150 ms, reflection signal in

the shallow data is obliterated. Thus, the attenuation of this noise before AGC is

im portant in improving the image of shallow structures.

In the Abitibi data, the ground roll is characterized by a high amplitude, broad

band (20 to 60 Hz) wavetrain of roughly 100 ms duration which shows linear moveout

corresponding to a velocity of 3.5 to 4.0 km /s . Its amplitude spectrum is very sim

ilar to that of the reflection energy (see figure 3.4). It is composed of surface wave

energy, shear wave energy (probably converted from P wave energy at the interface

between the bedrock and the overburden), or a combination of the two.

In general, ground roll comprised of surface wave energy may be dispersed, i. e.,

different frequency components travel at different phase velocities. This can occur if

the medium shows anelastic attenuation, or if layers of varying velocity axe present

near the surface, with dispersion being greater in the latter case (Al-Husseini et

al. [1981]). Strongly dispersed ground roll can obscure a large portion of the record

because different frequency components arrive at different times. There exist a


TRACE 50 100

Figure 3.3 Ground Roll.


Reproduced

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copyright ow

ner. Further


without

permission.

0

coWPQHHU

8

-20

-40

0 40 80 120

FREQUENCY (HZ)

Figure 3.4 Amplitude spectrum of ground roll (dashed line) compared with tha t of d a ta between 1 and 4 s (solid line).


number of methods which distinguish such dispersive surface noise from reflections

on the basis of dispersion characteristics, thereby allowing one to attenuate this

energy. See, for example, Beresford-Smith and Range [1988], and Saatgilar and

Carntez [1S38].

In the Abitibi data, the ground roll is not significantly dispersive over the

bandwidth of the d ata as is verified by means of bandpass filter panels (figure 3.5).

Note that neither the arrival time nor the phase velocity of the wave train varies

with frequency content, the former indicating that the group velocity is constant

with frequency. Note also that the energy (or the envelope of the ground roll) is

travelling at the same apparent velocity as the individual peaks (the phase velocity),

indicating that the group and phase velocities are equal. Although the duration

of the ground roll is substantial (100 ms), this is not because the wavetrain is

dispersive, but rather it is due to the energy reverberating between the surface and

the bedrock-overburden inte-face. This can be seen by noting that the wavetrain

is comprised of a series of parallel, similar wavelets. If the near-surface velocity

structure leads to significantly dispersive propagation, then the wavelet broadening

can be much more pronounced; for example, Al-Husseini et al. [1981] found ground

roll in Saudi Arabia with an onset time of 0.5 s to have a duration of half of one

second. Since the data do not show dispersion over the bandwidth of the data, and

DISCO does not contain procedures for rejecting dispersive waves, such methods

were not attempted.

3.3.2 A ttenuation through Processing

A number of traditional methods were attempted in order to attenuate the

ground roll prior to stack:

1) low cut frequency filtering

2) spectral equalisation


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2 0 - 3 0

FREQUENCY BAND

(HZ)

3 0 - 4 0 4 0 - 5 0 5 0 - 7 0

10 .0 s

0.5

Figure 3.5 Filter panels of ground roll. Note tha t the arrival time does not vary with frequency content, indicating th a t dispersion is not significant over the data bandwidth.

Chapter 3 Reprocessing o f Line ISA 21

3) minimum phase deconvolution

4) velocity (f-k) filtering

5) surgical muting in the t-x domain

Of course, these axe not the only methods which exist. The possibility of muting

the data in the r-p domain can be investigated. Another possibility is to use a

multi-channel median filter, which is commonly used in vertical seismic profiling to

remove unwanted coherent noise (Stewart [1985]). The latter method would involve

applying time shifts so that the ground roll is horizontal, passing only horizontal

energy using the median filter, then subtracting the resulting record from the origi

nal data, and removing the time shifts. However, software for only the five methods

listed above was available, so the other possibilities must be left for future research.

11 Low cut frequency filtering

The ground roll has a very broad amplitude spectrum, with a shape similar to

that of the reflection energy (figure 3.4). This indicates that low cut filtering will

attenuate signal as much as noise, and so will not be helpful.

21 Spectral equalisation

This process equalises the spectrum by applying an AGC to band-limited

records as described in section 3.1 . The net effect is similar to that of a zero

phase deconvolution process. Again, since the spectrum of the ground roll is similar

to that of the reflection energy, this method does not significantly attenuate the

noise (figure 3.6).

31 Minimum phase deconvolution

This process also equalises the amplitude spectrum of the data, and so was

tested for the attenuation of ground roll. There are two notable differences from 2)

above: 1) this method is a deconvolution process, i. e., it determines an inverse filter,

which is then applied to the data, whereas the above method balances the spectrum

in a non-convolutional, non-linear way; and 2) this method assumes that the data


TRACE 50 100

Figure 3.6 Shot after spectral equalisation.The bandwidths used were 20-30 Hz (weight=0.5), 35-45, 50-60 Hz (each with weight=1.0). A 750 ms window length was used.



axe minimum phase. Although the vibroseis wavelet is zero phase, minimum phase

deconvolution can work well with careful selection of parameters.

Spiking deconvolution was attempted with a range of parameters, but the

ground roll could not be attenuated (figure 3.7). Moreover, reflection energy has

been decreased throughout the record. The spectral equalisation method described

in section 3.1 was preferred as a method of balancing the spectrum.

4) Velocity (f-k) filtering

Two methods of velocity filtering exist in the DISCO processing package: 1) a

multiplicative method, operating in the f-.k domain, and 2) a convolutional method,

operating in the t-x domain. These methods were tested using a series of cut-off

slopes and tapers. The best results will be presented here.

The f-k spectrum of the upper one second of a shot gather is shown in figure

3.8 . The ground roll can clearly be seen to dominate, being 12 to 24 decibels

above the reflection energy. Figure 3.9 shows the result of applying the f-k domain

filter; the ground roll has been somewhat attenuated, but still shows considerable

amplitude. Note that around channels 80 to 90, the ground roll flattens due to a

bend in the line, resulting in a greater amount of energy leaking through the filter.

Results from the t-x domain filter (see figure 3.10) were found to be less satisfactory

than the f-k domain filter.

Although the ground roll was partially attenuated, the data have been contam

inated with filter artifacts. This limited success did not warrant the large amount of

computer time required in applying the f-k filter before stack (to process the entire

line would be extremely problematic, and would take over 24 hours of CPU time,

or four days of elapsed time at best).

In order to determine the reason for the limited effectiveness of the velocity

filter, a number of tests on synthetic data were run using the t-x domain filter.

Ground roll was simulated as a 40 Hz sine wave of 100 ms duration. This “event”


TRACE 50 100

0.0 8

1.0

2.0

Figure 3.7 added).

Shot after spiking deconvolution (56 ms filter length, 1 % white noise


G R O U N D R O L L

3 . 9 k m/ s

0 dB

60

40

20

- 0 .5 0.5WAVENUMBER ( t r a c e )

Figure 3.8 f-k spectrum of the upper 1 second of one side of a shot record, containing the ground roll. Trace spacing is 20 m.


FREQ

UEN

CY

(HZ

)

TRACE 50 100

S t e m

0.0 8

1.0

2.0

Figure 3.9 Velocity filter (applied in the f-k domain), rejecting slownesses between 3.5 and 10 m s/trace. A 60 Hz high-cut filter was applied afterwards.


Figure 3.10 Velocity filter (applied in the t-x domain), rejecting slownesses greater than 3.5 ms/trace. A 60 Hz high-cut filter was applied afterwards.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.


was made to have alternating apparent slownesses of 5 and 10 m s/trace as shown

in figure 3.11(a); the filter cut-off used was 3.5 m s/trace. This wavetrain was then

superposed on weaker events with infinite apparent velocity and of similar frequency

content as the ground roll. As can be seen in figure 3.11(b), the filter had limited

success in attenuating the ground roll. Reducing the amplitude of the ground roll

results in improved attenuation.

From this, it can be concluded that the high amplitudes of the ground roll

and the varying apparent velocity (due to the crooked line) contribute to the poor

performance of the f-k filter.

5f Surgical muting

Having rejected the above methods as a means of attenuating the ground roll,

and having decided that some means of removing ground roll energy is necessary,

the only remaining option is to mute this wavetrain prior to AGC (see figure 3.12).

Except where the seismic line hats large bends, the CMP fold should be sufficiently

high that shallow reflection energy could be imaged.

After reprocessing, one can see regions where there is no data coverage due to

bends in the line. One can also see that shallow reflections can be imaged to 0.4 s

travel time (figure 3.13), so the AGC shadow has been significantly reduced. Above

this, the data are contaminated by near offset noise and remnants of the ground

roll energy. Therefore, a better method of attenuating the ground roll would be

required to extend images closer to the surface.

S.S.S Attenuation during Acquisition

Since the ground roll posed such difficulties during processing and is such a

serious source of noise, it will be appropriate to discuss the possibility of attenuating

the ground roll during the acquisition of the data.

The attenuation of coherent noise through careful design of field arrays has


TRACE 2 5 5 0

W T O W W m W ‘

TRACE 2 5 5 0

(b) ■«?< •<««

0 .0 s

1. 0

Figure 3.11 Synthetic ground roll, (a) before and (b) after t-x domain velocity filter. The ground roll is a 100 ms wavetrain with alternating slownesses of 5 and 10 ms/trace. Slownesses greater than 3.5 m s/trace were rejected. The filter length was 10 time samples by 6 space samples. Note that considerable energy at 5 m s/trace has been passed by the velocity filter.


TRACE 50 100

V < v±?2!i

o .o s

1.0

2.0

Figure 3.12 Pattern for muting first arrival and ground roll energy. Although an AGC has been applied for display purposes here, the mutes were applied prior to AGC during the reprocessing.


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VP 300 400 500

0.0 s

0.5

1.0

Figure 3.13 Portion of the reprocessed stacked section showing regions of no data coverage where there are bends in the line (a result of surgical muting). Shallow north- dipping reflections can be traced to shallower than 0.5 s in some cases.


become a topic of some interest in the geophysical literature in recent years. For

example, a receiver array will attenuate energy in certain spatial wavenumber, k,

ranges. The attenuation characteristics depend on the length of the array, L, as

shown in figure 3.14. Spatial wavelengths greater than twice the array length (for

k < ^ ) will be passed by the array with little reduction in amplitude, while shorter

wavelengths will be attenuated significantly. Thus, the receiver array length can be

chosen so that ground roll wavelengths are attenuated. Note that the spacing of

the array elements (the geophones), I, must be fine enough that there is no spatial

aliasing for the smallest wavelength of the ground roll (i. e., kmax < jj)-

However, one must exercise caution in selecting the length of the array. As is

illustrated in figure 3.15, the angle between the wavefront of the reflected energy

and the surface becomes greater as the depth to the reflector decreases, resulting

in a smaller spatial wavelength. Thus, this energy is attenuated and smeared in

time, reducing the resolution of wide offset, shallow time data. This principle is

illustrated in an experiment conducted in the Albertan prairie in which a survey

was re-shot with smaller array lengths (Taylor [1989]). Indeed, the level of resolution

was increased, but at the cost of a lower S/N (which was partially attributed to less

attenuation of the ground roll).

In some cases, the ground roll energy may not be travelling in the line of the

rec fiver arrays. For example, changes in topography or shallow features can result

in the scattering of surface energy from off the line (Martel et al. [1977]). Another

possibility is that energy can arrive obliquely to the line of the arrays when the

seismic line is very crooked, as is the case with KSZ data. Such broadside ground roll

energy has a greater apparent spatial wavelength and so may be attenuated less by

the array. Moreover, it will have a higher apparent velocity on a shot gather, which

can make attenuating this energy through velocity filtering very difficult (see section

3.3.2). To avoid such difficulties, this energy should in principle be attenuated in the

field using three dimensional arrays; however, this can be impractical as it involves


Figure 3.14 Receiver array response (from Morse and Hildebrandt [1989]). The horizontal axis is wavenumber, fc, with nodes at multiples of where L is the receiver array length.

F igure 3.15 Wavefront geometry for energy arriving from different depths. On the left, shallow reflection energy has a short spatial wavelength; on the right, deep reflection energy has a long spatial wavelength.


Chapter 3 Reprocessing of Line 12A 25

much greater cost.

Figure 3.8 shows that the ground roll energy for line 12A has wavenumber

between 7.0 X 10“3 and 1.4 x 10-2 m -1 . In order to attenuate this, the receiver

array length, L, would have to be at least 70 m. However, the purpose of this line

was to acquire high resolution shallow information, which prompted the selection

of a 20 m receiver array, which passes wavenumbers below 2.5 x 10-2 m-1 , and so

passes significant ground roll energy.

It is not enough to be concerned only with the length of the receiver arrays;

the shot - receiver configuration (which determines the offset increment, or spatial

sampling, within CMP gathers) is also im portant. The combined response of the

field arrays and the spatial sampling within the CMP gathers is known as the

stackarray response. It is described by Anstey [1986] and Morse and Hildebrandt

[1989], with field examples in the latter and in Prskalo [1988]. A brief summary of

stackarray theory will be given here; if the reader desires more information, then he

is referred to the cited works.

Just as the field array has a response which is defined by its length, L, there is

also a “CMP array” response which is defined by the spatial sampling rate within the

CMP gather, D, i. e., the offset increment (figure 3.16). The stackarray response is

simply the product of the receiver and CMP array responses. If the survey geometry

is designed so that D = L, then the resulting stackarray will pass only energy with

k a 0, as shown in figure 3.16 . In this case, the equivalent receiver array for a

CMP gather (i. e. the stackarray) is in fact continuous and of uniform weight, and

is said to satisfy the stackarray criterion. If a configuration results in a stackarray

which is not continuous and uniformly weighted, then it will pass significant energy

with k 0 (figures 3.17 and 3.18).

In practice, the application of the stackarray criterion has been found to im

prove data quality (Prskalo [1989]). However, crooked Lithoprobe lines result in


C M P-array s ta c k a rra y

1 4 4 • • • 4 4 = " I 4 4 • • • 4 . A .“ I 60m r

6 0 m field array re sp o n se CMP a rray re sp o n se s ta c k a rra y re sp o n se

.4 1 1 1 H(m'*) o .02 .033 (m*’) q

Figure 3.16 The top line shown 60 m receiver arrays with a 60 m spatial sampling within a CMP gather, resulting in a continuous, evenly weighted stackarray. The bottom line shows the corresponding wavenumber response. Note that the stackarray only passes energy with k ~ 0. From Morse and Hildebrandt [1989],

15 -e lem en t CMP a rra y s ta c k a rra y

* j ^ 4 . . . ^ ^ ... ^ ^

30m fiald a r ray re sp o n se CMP a rra y re sp o n a e s ta c k a rra y re sp o n se

Figure 3.17 As in figure 3.8, but with a 30 m receiver array, resulting in a discontinuous stackarray. The stackarray passes energy with k ^ 0. From Morse and Hildebrandt [1989],

C M P-array

4 4 4 . . . ' 4 4 =

-1 60m r

00m field a r ra y re sp o n se CMP a rray re sp o n ae s ta c k a rra y re sp o n se

L , _ 4 i . 1 1 H . ______<mM) 0 .02 .033 (m*1) o

F igure 3.18 As in figure 3.8, but with a 90 m receiver array, resulting in a continuous but unevenly weighted stackarray. The stackarray passes energy with k ^ 0. From Morse and Hildebrandt [1989],



uneven spatial sampling within CMP gathers, thus complicating the stackarray

theory. Nevertheless, a brief analysis of line 12A was made.

For line 12A, the offset increment, D, is 40 m whereas the receiver array length

is 20 m. Therefore, the stackarray is discontinuous, having the response shown in

figure 3.19. Note that the danger of this stackarray configuration is that any noise

with k ~ 0.05 m -1 will not be attenuated after stacking. However, the ground roll

energy, with wavenumber between 7.0 x 10~3 and 1.4 X 10~2 m-1 , is attenuated by

40 dB after stacking even if no other attem pt is made to attenuate it. Nevertheless,

reflection energy within the AGC shadow is still of very low amplitude, so only

allowing the stackarray to attenuate the ground roll is not enough. Other methods,

such as those discussed in section 3.3.2 should be attempted.

3 .4 V ib rator N o ise

3.4-1 Attenuation

The near offset coherent noise illustrated in figure 3.20 is termed “vibrator

noise” due to its apparent spatial relation to the vibrators. This noise has linear

moveout equal to that of the ground roll, and is dominated by low frequency en

ergy (20 - 35 Hz) when compared with uncontaminated records (figure 3.4). The

magnitude of this noise is variable from one shot to another, often contaminating

offsets as great as 800 metres.

The muting of the near offset traces did not result in improvement of reflection

quality, which suggests that these noisy traces are attenuated through stacking.

However, reflection energy on these traces has been reduced in amplitude by the

application of an AGC, and so it does not contribute to the stack. Thus, it would

be better if reflection energy were enhanced before AGC.

with permission of the copyright owner. Further reproduction prohibited without permission.

AM

PLIT

UD

E

WAVENUMBER ( l /m )

F igure 3.19 Stackarray response for line 12A. Computed from a receiver array with 12 elements at a sp u in g of 1.67 m, and from a CMP array, 120 fold with an offset increment of 40 m.


TRACE 50 100

0.0 s

F igure 3.20(a) Near offset, vibrator related noise. TVace spacing is 20 m. The vibrators were located between traces 60 and 61.


o<s o

siaaiDaa


Figu

re

3.20

(b)

Am

plitu

de

spec

trum

of

the

vibr

ator

no

ise.


Velocity filtering was not a viable solution, due both to its limited success and

the large computer effort required. Since the vibrator noise is a “ghost” image of

the ground roll, applying time shifts so that the ground roll is horizontal on a shot

record will also result in horizontal vibrator noise. Thus, the multiple trace median

filtering method discussed in section 3.3.2 would attenuate this noise while passing

reflection energy.

Since the vibrator noise is of predominantly low frequency, the spectral equal

isation applied in section 3.1 results in an improved S/N as is shown in figure 3.21,

although there is still some contamination by the noise. The improved quality of

the section after spectral equalisation was discussed in section 3.1, and shown in

figure 3.2 .

S.4-2 Causes of Vibrator Noise

Since vibrator noise was such a prevalent problem throughout the KSZ data, it

is relevant to examine the possible causes of this noise. It is apparently an echo of the

high amplitude ground roll, so it is likely an artifact of the cross-correlation process.

Seriff and Kim [1970] pointed out that harmonic distortion (i. e. of higher frequency

than the fundamental) early in the sweep correlates well with the fundamental

frequency later in the pilot sweep if a downsweep is used (as was customary in the

1960s). This results in high amplitudes at positive time lags in the cross-correlation

function between the distorted sweep and the theoretical pilot sweep. Thus, the

correlation process will produce ghost energy following high amplitude energy such

as the first arrivals or ground roll.

The use of an upsweep rather than a downsweep results in sidelobes at negative

time lags. Thus, the correlation ghosts of a reflection will be superposed on earlier,

stronger reflection energy, so signal quality does not deteriorate appreciably in the

presence of harmonic distortion. However, now the presence of low frequency rather


TRACE 50 100

rs-'li'-

0.0 8

1.0

2.0

F igure 3.21(a) Shot gather contaminated by vibrator noise, before spectral equalisar tion.


TRACE 50 100

VS*****' >. "-<<v'<.--i»

0.0 8

1.0

2.0

Figure 3.21(b) Shot gather contaminated by vibrator noise, after spectral equalisation.


C h a p ters Reprocessing of Line 12A 28

than high frequency distortion results in low frequency sidelobes at positive time

lags. This suggests that the KSZ data may have been subject to low frequency

distortion in the emitted vibrator signal.

By what mechanism can low frequency distortion be produced? Martin and

W hite [1989] examine the occurrence of noise with a very similar appearance which

was found to have been caused by the loss of phase control in one of the vibrators.

This problem was caused by hardware failure in the vibrator, and resulted in low

frequency subharmonic or random noise. Vibrators in the KSZ transect did have

phase control problems (sometimes resulting in only three vibrators functioning

during the regional surveys). However, there was no obvious connection between

the amount of phase error and severity of vibrator noise. Furthermore, because this

type of noise is so common in vibroseis data (Billings [1990]), it is unlikely that it

is exclusively caused by hardware failure, but rather is more fundamental.

Uncorrelated vibroseis data acquired by Shell Canada Ltd. in 1984 and analyzed

by the author confirm that prominent subharmonic energy can be produced by the

vibrators which results in correlation noise. Amplitude spectra of progressive time

intervals of an uncorrelated shot record (figure 3.22) show that this energy remains

at half of the fundamental frequency as the latter varies between roughly 60 and

70 hertz. After correlation, this noise has an appearance similar to that of line 12A

data (figure 3.23).

Many physical systems generate harmonic energy, but it is far more unusual

for subharmonic energy to be generated. W hat mechanism might be involved in

the case of vibrators? One possibility is that the vibrator pads decouple from the

ground over a range of frequencies comparable to the natural resonant frequency

of the near surface. This could produce subharmonic energy in two ways: 1) the

pad may bounce every second cycle, or 2) the pad may rock back and forth, being

tilted in one direction during one cycle, then in the other direction during the next


0 10 20 30 40 SO 60 70 80 90 100Frequency

5

4e■O

I 21

0Frequency----------

20 30 SO 60 Frequency

80 90 100

1.6 1.4-

. 1.2 - % 1.0 -

S 0.8- go.e-

0.4-0 .2 -

0.0100805030 40

Figure 3.22 Amplitude spectra from uncorrelated vibroseis data collected by Shell Canada Ltd. in 1984. Spectra are of progressively later 1 s data windows, with the sweep frequency marked by an arrow. Subharmonic energy is generated when the sweep is between 55 and 70 Hz.


1.4-1.2 -

• I.O- *o2 0 .8 -

’Eo.e-l 0.4-

0 .2 -

0 .0 -20 30 40 100Frequency

2.0

1.5-

3 l.o-

0.5-

0.020 30 50 100

Frequency2.0

1.5-

3 1.0-

0.5-

0.020 30 40 80 90 100

Frequency

1.5-

§ 1 0 -

0.5

0.0 90 1008050 603020

Figure 3.22 (Continued)


8 - 9 s

9 - 10 s

2.5

2.0 -

1.0 -

0.5-

0.080 00 10040 5020 60

Frequency

100Frequency

3.0

2.5-

0.5-

0.020 30n in 40 SO 60 70 80 90 100

Figure 3.22 (Cc.itinued)


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SHT

J R<P

0 .5

1 .0

1 .5

2. 0

2 .5

TIME

IN

SEC0NDS

SHTx ao to

4 .5

5 .0

5 .5

^SS^SSSSSs&C

mmsSS00^$0 S M p W i i p l ®

Figure 3.23 Vibroseis record acquired by Shell in 1984 showing timilar vibrator noise to that present in the Kapuskasing data.

woz

onm

w

2—

ns

Chapter 3 Reprocessing o f Line 1ZA 29

cycle (Lansley [1988]). Such a possible resonance was visible when the pads were

observed during acquisition of the data in 1987 (West [1990]).

The Shell data were acquired using vibrators which did not use ground force

control, whereas the vibrators in the KSZ transect did. D ata acquired by Shell in

1990 using ground force control show similar vibrator noise. The cause of this noise

has not yet been determined, but is known not to be subharmonic distortion. This

does not rule out the possibility of subharmonic distortion in KSZ data; it only

demonstrates that there are different mechanisms for producing such noise. The

cause of this noise is a topic worthy of future investigation. If it is not possible to

record uncorrelated data in the field so that such distortion can be removed before

further processing, then uncorrelated data should be recorded at least in a region

affected by this noise for later examination to determine possible causes.

3.5 S ta tic C orrection s

3.5.1 Elevation Statics

The elevation static correction, teiev, is given in Yilmaz [198'. j as:

2 Ed — (E s + Er)*eicv —

Vb

where Ed , Es , and Er are the elevations of the datum, shot, and receiver respec

tively, and Vb is the velocity of the bedrock (see figure 3.24). This formula for t eje„

assumes that that the raypaths are vertical near the surface and that the shallow ve

locity structure is uniform along the line. The first assumption is not unreasonable

because the depth of penetration of the seismic rays is greater than the recording

spread length for most of the seismic section. For line 12A, the maximum offset

was 3.6 km; a ray arriving at 45° at this offset will hav ’ been reflected at a depth

of 1.8 km, which corresponds to a travel time of 0.85 s assuming a typical bedrock


RECEIVER

SHOT

D A TUM

Figure 3.24 Elevation static correction.

3.6 km

SHOT RECEIVER A R R A Y

1.8 km

v = 6 km /sec

t =0.85 sec

Figure 3.25 Depth of penetration for a seismic ray incident at 45° at the maximum offset.



velocity of 6 km /s (see figure 3.25). Only for smaller travel times might one ex

pect seismic energy to deviate from the vertical by at least 45 degrees. Moreover,

the weathered, near-surface bedrock has lower velocities them the deep unweathered

bedrock, causing the seismic energy to be refracted towards the vertical. So, a static

correction of 30 ms determined assuming vertical raypaths for a reflector at 0.35 s

will be underestimated by less than 4 ms. Where this assumption is not valid, such

as when extremely large maximum offsets are used, there is no longer one single

time correction for the entire trace. In this case, one can determine time corrections

which Eire dependent on the ray parameter p (related to the angle of incidence), and

apply the corrections in the r-p domain. For example, Wenzel [1988] applied such

a method to DEKORP data collected in the Rhinegraben region in West Germany

with an offset range of 66 to 82 kilometres.

Although the verticeil raypath Eissumption is reasonable, the assumption that

the near-surface velocity function is laterally homogeneous can be significantly in

error. There is a layer of glacial till of variable thickness and of very low velocity

overlying the bedrock (Haeni [1986] found velocities for glacial till ranging from 0.3

to 2.4 km/s , whereas the bedrock velocity was determined from the first arrivals

to be approximately 6 km /s in the Abitibi). Thus, there will be an additional

time delay for the energy to travel through this overburden, with this delay being

proportional to the thickness of the low velocity cover. In order to determine this

additional delay, one must perform a refraction static emalysis.

3.5.2 Refraction Statics

There are a number of ways in which refraction static corrections can be deter

mined (see Russell [1989] for a review of a variety of methods); all of them involve

“picking” the first arrivals on shot gathers, then determining the static corrections

from these travel times. The method available in the DISCO processing package

involves the inversion of the first arrival times to determine a velocity model of the



near-surface, then calculating the corrections from this model.

I t Picking of the first arrivals

The picks made with an automatic picker during the preliminary processing

were found to be of poor quality, so new first break picks were made using the DISCO

automatic picker. This routine computes the envelope of the seismic data over a

time gate centred on a user-supplied estimate. The first break was picked where the

envelope reached 50% of the peak amplitude within this time gate. Although the

vibroseis wavelet is zero phase and therefore should in principle be picked on the

positive peak, the picking routine performed poorly if one picked the maximum value

of the envelope. This difficulty arises because the first arrival energy reverberates

between the surface and bedrock-overburden interface resulting in an extended first

arrival wavetrain. The envelope of this energy has a very flat peak, so that there is

ambiguity in selecting the time with the maximum value. Picking the wrong phase

of the wavelet is not critical — it is the relative pick times which axe important.

The relative static corrections will be accurate provided that the same phase of the

wavelet is picked on all traces.

The process of picking arrivals is computer intensive (picking all shot records

would take 24 hours of CPU time, which corresponds to at least four days of elapsed

time). Thus, it was decided that not all offsets from all shots would be picked since

the additional redundancy did not significantly change the results of the inversion

during testing. For this reason, picks were only made for data in the offset range

500 to 1340 metres (corresponding to traces recorded by only one DFS-V recorder,

which simplified the processing considerably). For offsets of less than 500 metres,

the data are characterized by high amplitude precursors which result in inaccurate

picks. At far offsets, there are two reasons for disregarding the first breaks: 1)

the data have lower S/N, and 2) the refracted energy penetrates more deeply as

the offset is increased, so at the farthest offsets, it may be sampling bedrock of a

R e p ro d u c e d with p erm iss ion of th e copyright ow ner. F u r the r reproduction prohibited without perm iss ion .


greater velocity (Mayrand et al. [1987]). By using the middle offsets, only the most

accurate picks are retained. Picks were made only for every sixth shot, resulting in

an average of twelve picks per station for use in the inversion routine.

In order to quantify the reliability of the picks, the reciprocal time errors were

determined as in Mayrand et al. [1987]. These errors are the differences in pick

times for reciprocal shot-receiver configurations. A histogram of errors for all of

the picks is shown in figure 3.26(a). From the total of 1526 pairs of reversed picks,

the mean reciprocal error is -2.0 ms. and the variance is 50 ms2. Figure 3.26(b)

shows the histogram of errors if one ignores the picks which were rejected by the

inversion routine as being unreliable. The mean is -2.0 ms, and the variance is 19

ms2 for 1460 picks. Thus, the mean error is only one sample, with roughly 80% of

the reciprocal picks being within two samples of each other.

21 Inversion of the first arrivals

The second step in the refraction static method is to determine the static

corrections from the first arrival times. The refraction static inversion routine from

the DISCO seismic processing package utilizes the delay time method as described

by Barry [1967], modified so as to determine a velocity model iteratively. From

this model, the static corrections are then calculated. At each iteration, a bedrock

velocity is fitted to the first arrival times; any picks which fall more than a specified

tolerance away from this curve are rejected, thus ensuring that spurious picks do

not contaminate the results. For a description of the method, see the DISCO User’s

Manual.

R ather than requiring an initial velocity model, the DISCO inversion routine

requires the user to specify the overburden velocity (1.6 km /s) and the upper and

lower bounds on the bedrock velocity (5.3 to 6.8 km/s). Unlike many other inversion

routines, the DISCO routine allows for only one weathering layer.

W hat is the validity of the assumption that there is only one weathering layer?


Num

ber

of Pa

irs

Num

ber

of Pa

irs

400

200 -

-8 0 -4 0 0 40 80Reciprocal Error (m s)

400

200 -

-8 0 -4 0 0 40 80Reciprocal Error (m s)

Figure 3.26 Histogram of reciprocal pick errors (a ) all picks (b ) not including picks rejected in the inversion routine.



In reality, one would expect to find a layer of very low velocity glacial till (less than

2 km/s) overlying weathered, fractured bedrock (roughly 5 km/s). This weathered

bedrock would grade downwards into unweathered, higher velocity bedrock (roughly

6 km/s). Thus, one would expect that a more reasonable model would include at

least two low velocity layers. However, the first breaks in the range of offsets which

were picked (500 to 1340 metres) generally can be approximated by one straight

line, which suggests that a model of one layer over bedrock is satisfactory (see figure

3.27).

The resulting near-surface velocity model and static corrections are plotted in

figure 3.28 . The best method of testing whether the inversion routine performed

well is to make stacked sections with and without the corrections being applied

(see figure 3.29). The overall improvement shown after the application of refraction

statics is clear.

3.5.3 Surface Consistent Residual Statics

In the days before the digital recording of seismic data, refraction static cor

rections were commonly believed to be the complete solution to the statics problem

(Russell [1989]). In reality, the refraction static method has an inherent limitation in

that the traveltimes of the refracted first arrivals reflect the average velocity struc

ture along the entire raypatfi, thus limiting the i ̂ solution of the statics solution.

Moreover, the earth model used (one constant velocity layer over laterally vary

ing bedrock) is admittedly simplified. Any complexities in the velocity structure

(e. g. varying overburden composition and water table levels resulting in irregular

velocity variation in the overburden, and vertical velocity variation in the bedrock)

will result in complexities in the raypaths, deviations from the vertical raypath

assumption, and inaccurate static corrections.

In the early 1970s, a new method of determining surface consistent residual


Reproduced

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copyright ow

ner. Further


without

permission.

OFFSET a t/1 t/l ( j \ 0 ) 0 ) ( J ) 0 ) O ) « < J < J « > J o J s J 0 0 0 Q Q ) 0 0 C D ( 0 ( 0 ( 0 ( 0 ( 0 t*k t a M t B t , a * M M ^ M M M M M W Wo i ' j C b — Cj i / i ' - j aD — o)C/i * ^ cd — C 5 c n - g i D — o j t / i - j o o s c a s s Q ^ — — — r u r o / u r v j r uu ) C i ) n ) ^ s ^ ^ ^ u ) 9 s s s ) M ^ t u ^ i \ i > u i u ^ ^ s a ) B n j & o ) ( O K O ) ( / i s i ( o ^ u ) u t > j u )

RED-STBTGO CO GO OO QO 03U\ cn o) 5) oi ct)od to s n j u) ^ u i c n N i c o u ) S H i \ j o ) ^ t n o ) s j a ) ( o c a ^ i u u ^ u i o ] s j c o ( o

200

400

cnz

LU

Figure 3.27 First arrival picks are marked as x . The plain, straight line is the initial estimate used in the picking routine. For the offsets used in the inversion routine, one straight line fits the fimt arrivals very well, indicating that the assumption of one overburden layer over a half-space bedrock is reasonable.

Reproduced

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without

permission.

QU JCD

200 200OVERBURDEN

t E 100 1 0 0BEDROCK

30f e l 20

3020

6250 6250

6000 6000

5750 5750

550055001 1 0 0 1200 1300

STATION1400 1500

Figure 3.28 Near surface velocity model as determined from the inversion routine. Param eters for the inversion routine are as follows: overburden velocity = 1.6 km/s; allowable bedrock velocity = 5.3 km /s to 6.8 km/s; bedrock velocity model is smoothed over 50 stations; delay times are smoothed over 5 stations; 3 iterations in the inversion routine.

900

1000

11

00


Figu

re

3.29

(a)

Stac

ked

sect

ion

with

no sta

tics

appl

ied.

900

1000

11

00


Figu

re

3.29

(b)

Stac

ked

secti

on

with

with

elev

atio

n st

atic

s ap

plie

d.

Q.>


Chapter 3 Reprocessing o f Line 1ZA 34

statics was developed (Taner et al. [1974], Wiggins et al. [1976]). This method,

which is used in the DISCO processing package, is non-deterministic in that the

corrections are determined directly from the data so that reflectors are aligned

within a CMP gather. It has the capability of resolving statics with a wavelength

less than one receiver spread, complementing the refraction static solution. The

following outline of the method is summarised from Taner et al. [1974].

Each trace in an NMO-corrected CMP gather is correlated with a pilot trace

(the stacked CMP trace); the time shift which aligns the two is the time lag of the

peak in the cross-correlation function. The time shift Ty (from shot i and receiver

j ) can be written as follows:

Tij = S i + R j + C k + M kXf j

where Si is the shot static at station i\ R j is the receiver static at j; Ck is the

structural static at CMP k\ Mk is the residual normal moveout (RNMO) correction

at CMP k (the NMO hyperbola is approximated as a parabola); and X , j is the offset.

Si and R j are surface consistent (i. e. the shot correction is the same for all traces

from a particular shot, and likewise for the receiver correction). Ck and M* are

subsurface consistent (the same for the entire CMP gather). A large, overdetermined

set of equations for all of the time shifts is solved to decompose these shifts into the

various time correction terms.

The structural term is arbitrary since all traces in a CMP may be time shifted

by Ck without affecting the alignment of traces within the CMP. Therefore, Ck -my

be constrained to be zero (ibid.). The other parameters were selected empirically

based on two criteria: the quality of reflections on the stacked section, and the

stability of the static values (the solution is considered to be stable if the static cor

rections do not vary wildly between neighbouring stations and between iterations).

Best results were obtained when Mk wer constrained to be zero, and when the

shot and receiver statics were allowed to vary independently of one another. Since



the sources and the receivers were both located on the surface, one might expect

that there would be greater validity in constraining the source and receiver statics

to be equal; however, the vibrator and geophone arrays were not coincident, and

the quality of the stacked section is somewhat better without this constraint. Our

confidence in the solution is bolstered when one examines figure 3.30, which shows

the similarity of restilts when different sets of CMP gathers are used as input.

The maximum allowable static was chosen to be 8 ms. which is one half of a

wavelength for 60 Hz (approximately the highest frequency used in reprocessing).

When the residual statics were allowed to be larger (16 ms), the solution became

unstable for parts of the line. For the parts where the solution remained stable,

static values varied over approximately 8 ms, justifying this choice of a maximum

time shift. The CMP bin size was also found to be crucial for the stability of the

solution, with results becoming unstable when bins were more than 800 m wide.

The improvement in the stacked section after residual statics is remarkable (see

figure 3.31). F.cfiection quality is markedly improved both outside and within the

correlation window which was used to determine the statics. Combined with the

stability of the solution, this is strong evidence that the statics problem has been

attacked with great success.

The reprocessing or the high resolution line 14A by Milkereit of the Geological

Survey of Canada has revealed that there was an 8 ms time delay on that line

between the datasets recorded by the two DFS-V acquisition systems, each of which

recorded half of the recording spread. There was probably a similar delay for line

12A. Not being surface con-:stent, and being equal to the maximum allowable time

correction, this cannot be corrected using the residual static method. Unfortunately,

this face came to light too late to be corrected during processing.

It is clear that the determination of an accurate static correction solution was

crucial to the processing of the data. Recent developments in static methods show


Stat

ion

720

700

680

660

640

620

600

58015 -1 0 -5 0 5

Static Correction (m s)10

Figure 3.30 Plots of residual receiver static corre' tions determined using separate groups of CMP gathers. Each group contained 35 gathers, with an overlap between the two groups of 5. Note the similarity in the two solutions despite their being computed using different input data.


900

1000

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3.31

Slac

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secti

on

afte

r ap

plic

atio

n of

surf

ace-

cons

iste

nt

resi

dual

sta

tics.


potential for even greater data improvement in the future. A travel time tomo

graphic inversion method was used with success in reprocessing line 14A (Milkereit

[1990b]). Also, methods which use a random searching technique to select the set

of corrections which maximises the power of the stacked traces have been developed

and implemented with great success (Ronen and Claerbout [1985], Rothman [19S6],

and Dahl-Jensen [1989]).

\

3 .6 V elocity A n alysis

Due to the high velocities and long travel times in Lithoprobe data, the normal

moveout (NMO) is small for most of the data. This was illustrated using synthetic

CMP gathers containing a number of reflections, which were stacked using a suite

of velocity functions (figure 3.32). These results show that reflections below 2 s are

insensitive to velocity over a broad range, so only a cursory velocity analysis need

be made for the deep data. The same figure shows the need for a careful velocity

analysis above 2 s. Reprocessing of the data should result in much better velocity

control for the shallow data than during the preliminary processing.

There are two primary methods for the selection of stacking velocity: 1) the

examination of NMO corrected CMP gathers using a range of velocity functions,

with the aid of plots of coherency versus velocity and time (Yilmaz [1987]), and

2 ) the examination of stacked sections which were made using a suite of velocity

functions. Due to the poor pre-stack S/N, the second method is preferred.

Some surprising results were found. In the south half of the line, shallow (< 1 s)

dipping reflections stacked optimally at very high stacking velocities (7 to 7.5 km /s)

— see figure 3.33 . This is more likely a result of dip moveout effects rather than

true high seismic velocity since first arrivals indicate a velocity of approximately 6

km /s at the surface. A comparison of the dip moveout (DMO) and normal moveout

(NMO) equations shows that the increase in stacking velocity is one of the effects


VE

LO

CIT

Y O o o Oo If) o if)

<0

o<0

N<P

If)to

N<0

1 1 1 1

£ O o o Ow O If) o if)o M If) N

if) If) K) If)

O*0M

OoIf)

0.0 8

1.0

2 .0

3.0

4.0

Figure 3.32 Sensitivity to NMO velocity shown using synthetic CMP gathers. Velocities labelled at the top indicate the velocities at 0 s and a t 4 s respectively, with a linear increase between the two. The correct velocity function is 5750 to 6750 m/s.


300

400

500


Figu

re

3.33

(a)

Shall

ow

dipp

ing

refle

ctio

n en

ergy

sta

cked

at

6 k

m/s

(the

bedr

ock

velo

city

at

the

surf

ace)

.

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VP 300 400 500

a0 .0 s

0.5

1.0

1.5Figure 3.33(b) Shallow dipping reflections stacked at 7 km /s, much higher than the bedrock velocity of 6 km /s . Note tha t reflections within the box are imaged better than in figures 3.31(a). This indicates tha t DMO effects are present in the data.


of dipping reflectors:

.2N M O :

R M S

D M O : t,i2c o r r2 x 2Cos26

* R M S

where t and t corr are the measured and corrected travel times respectively, x is the

offset, Vrm, is the root-mean-square velocity, and the reflector is dipping at an angle

0. Thus, the effective NMO velocity for a reflector of dip 6 is > Vr m s • The

dips in figure 3.33(b), 25° to 30°, imply stacking velocities of 6.6 to 6.9 km /s for

a bedrock velocity of 6 km/s. This is consistent with the stacking velocity used in

the figure (7.0 km/s).

The presence of a dipping reflector also causes the reflection point to move

updip as the offset is increased within a CMP gather, which results in reflection

point smearing (see figure 3.34). In order to remove this effect, one must apply

full pre-stack migration which is prohibitively computer intensive, or pre-stack par

tial migration, otherwise known as DMO removal. Considerable advances in the

efficiency of DMO removal routines have been made during the 1980s, increasing

their practicality. Unfortunately, however, no working DMO program was available

for use in the DISCO software package. The high stacking velocities are a strong

indication that DMO effects are present, suggesting that a DMO routine be made

available for the future processing.

3.7 V ariability o f R eflection E n erg y w ith O ffset

If reflector dips are small and there is little lateral velocity variation, then all

traces within a CMP gather should be sampling the same subsurface geology. Thus,

reflection energy should, be consistent across a CMF gather; however, in reality, there

is great variation (figure 3.35). Although the amplitude of the reflection energy


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with perm

ission of the

copyright ow

ner. Further


without

permission.

COMMONSHOT SHOT SHOT MID RECEIVER RECEIVER RECEIVER

POINT 2 3

F igure 3.34 Reflection point smearing for a clipping reflector. The 3 shot - receiver pairs belong to one CMP gather. As the <. flset is increased (from pair 1 to pair 3), the reflection point moves updip.

OFFSET (m)

500 1000

i0 .0 s

1.0

2.0

3.0

Figure 3.35 A portion of a CMP gather. Note the variation of reflection energy with offset.



appears to vary with offset, this could result to some extent when an AGC is applied

if the noise levels vary with time and offset. A controlled gain function should be

applied in order to describe the amplitude variation rigorously. Nevertheless, in

some areas of the figure, there is true amplitude variation (for example, where two

reflections are close in time and the amplitudes of the two vary independently with

offset). Furthermore, the phase of the reflections varies with offset, resulting in

different character and in different time shifts which cannot be described as a static

correction. Although these time shifts are small, they are significant, resulting in

much of the signal being attenuated through stacking. Post-stack reflection energy

is often comprised of energy from only a limited number of traces on CMP gathers.

There are three possible reasons fcr this variability of signal. 1) As a result of

complex interlayering of rock types in the vicinity of the reflection point, reflecting

energy may constructively or destructively interfere and scatter to varying degrees

as the angle of incidence (i.e., offset) is varied. This effect is more likely to play a

significant role for shallow data since the angle of incidence does not vary greatly

for deep data over the range of offsets used. 2) Due to the complexity of the geol

ogy, raypaths may vary considerably, with reflection energy from different reflection

points arriving at similar times. For example, undulations in the reflecting horizons

perpendicular t j the line may result in reflection energy being returned at similar

times lrom very different areas. This may result in time delays and interference be

tween energy travelling along the different paths. It is also possible that reflection

energy from very different geological units could arrive at similar times (i. e. out of

line reflections), which may result in complex interference patterns. 3) Relatively

localized, shallow velocity inhomogeneities could result in significant raypath bend

ing for different surface loc?fions, resulting in reflection point wander within a CMP

gather. Interaction between these three factors in differing degrees could produce

the variability of reflection energy which is seen in the data.

Since reflection energy is stacking coherently only over a limited range of offsets,



it was felt that forming stacked sections for different offset ranges would allow

one to select ranges along the line where most of the coherent signal is present.

However, differences were not significant when offset ranges of 800 metres were used,

indicating that the scale of the variation is smaller than this. An examination of a

number of CMP gathers revealed that the reflection character within each gather

varies rapidly dong the line. Thus, careful, detailed editing would be required

in order to result in an improvement after stacking. Such a procedure would be

practical only if automated.

In recent years, advances in automated trace editing procedures have been

made (e. g. Mayrand and Milkereit [1988]). Such procedures concentrate on iden

tifying traces which are dominated by noise or noise bursts, with this noise being

characterised by anomalous amplitude levels. In the future, it may be worthwhile to

concentrate editing procedures on selecting data which contain consistent coherent

signal rather than simply editing out noise. One possible method would be to apply

a coherency filter to the CMP gathers. This type of filter is generally, applied to

stacked data in order to pass only energy which is coherent over a minimum trace

window and range of dip. This filter could be applied before stacking, passing only

coherent energy with zero dip. Another possibility would be to apply a Ivarhunen -

Loeve filter, which also passes only coherent energy but is very computationally ex

pensive. Advances in the implementation of such methods could be of considerable

benefit to crustal seismic profiling.

3.8 Inclusion o f H igh Frequencies in th e P rocessin g

When the data were resampled to 4 ms (section 3.1), was any useful reflection

energy lost? Having determined a new processing scheme for the data, a portion

of the line was selected to test for the presence of high frequency reflection energy.

This portion of the line included shallow reflections, which would be most likely to


Chapter 3 Reprocessing of Line 12A 40

have retained high frequency components.

The data were spectrally equalised over the full bandwidth (20 to 120 Hz)

with the 2 ms sampling rate being retained. Only data with offsets up to 1340 m

(recorded by one DFS-V system) were processed, both in order to reduce processing

time and to reduce sensitivity to velocities. Otherwise, the same reprocessing flow

was followed. Filter panels of the resulting stacked section show that there is no

shallow (< 1 s) coherent reflection energy above SO Hz and none above 100 Hz

elsewhere in the section (figure 3.36). However, the application of NMO corrections

to pre-stack data results in a significant downward frequency shift for shallow data

(a phenomenon known as NMO stretch). The shallowest energy on the stacked

section is at 400 ms. At most, this 80 Hz stacked energy would be the result of 94

Hz energy on uncorrected pre-stack data. NMO stretch is not appreciable for the

reflection energy below 1 s.

E ither frequency copmponents above 100 Hz are attenuated and scattered to

such an extent that the reflection energy has a very low S/N, or the processing

method is inadequate to image the high frequencies. For example, high frequency

reflections are extremely sensitive to inaccuracies in static corrections and NMO

velocities, and to presence of dip. The determination of a static correction solution

using more recent advanced methods, the removal of DMO effects, and the resulting

better control on NMO velocities may result in higher frequency reflection informa

tion. W hatever the reason, a substantial amouni of the source effort (20% of the

sweep time) did not contribute significantly to the imaging of reflections. In future

surveys, the lowering of the upper sweep limit to 100 Hz should be considered.

3 .9 S tacked S ection s

The data were initially processed to obtain a preliminary stack (plate 1) using

the flowchart shown in figure 3.37 . The data were reprocessed to give a new


Reproduced

with perm

ission of the

copyright ow

ner. Further


without

permission.

20 - 35 HZ 35 - 50 HZ

Figure 3.36 Filter panels of a stacked section where frequencies up to 120 Hz have been retained and equalised. Note tha t there is little reflection energy above 80 Hz. Bandwidths used in the spectral equalisation were 20-30 (0.5 weight), 35—45, 50-60, 65-80, 85-100, 105-120 (each 1.0 weight).


Figu

re

3.36

(Con

tinue

d)

100

- 12

0 H

Z


Figu

re

3.36

(Con

tinue

d)


stacked section (plate 2) using the flowchart shown in figure 3.38 . A post-stack

coherency enhancement filter was applied to both sections. The section in plate

2 was then migrated using a finite difference method to give the section in plate

3. The improvements after reprocessing are substantial, and will be examined in

chapter 4.


STACK OF ADJACENT SHOT GATHERS

AUTOMATIC GAIN CONTROL

CROOKED LIN E GEOMETRY

REFRACTION STATIC CORRECTIONS

C M P SORT

VELOCITY ANALYSIS

N M 0 CORRECTION

FIR ST BR]EAK MUTE

NON SURFACE - CONSISTENT RESIDUAL STATIC CORRECTION

Figure 3.37 Preliminary processing flowchart.

STAC


%'*

*3

RESAMPLE DATA FROM 2 m s TO 4 m s

NEW CROOKED LINE GEOMETRY

SURFACE - CONSISTENT RESIDUAL STATIC CORRECTIONS

STACK

C M P SORT

REAPPLY MUTES

FIRST BRETK PJCKING

SPECTRAL EQUALISATION

AUTOMATIC GAIN CONTROL

REFRACTION STATIC CORRECTIONS

MUTE OF GROUND ROLL AND FIRST BREAKS

VELOCITY ANALYSIS TO COMPENSATE FOR DIP MOVEOUT

F igure 3.38 Reprocessing flowchart.


Chapter 4

Conclusions

4.1 S u g g estio n s

The quality of the stacked section has been greatly improved after repro

cessing (see plates 1 and 2). The most crucial processes were spectral equalisation

and the determination of a complete static solution (refraction and surface consis

tent residual statics). Spectral equalisation was essential to the imaging of shallow

reflections as was shown in section 3.1 . A comparison of data below 3 s in plates 1

and 2 shows the marked improvement after the new static solution. These processes

must be applied to obtain an adequate signal to noise ratio before further testing

is done, e. g. velocity analysis, or stacks using limited offset ranges. Although the

improvement in plate 2 is notable, further improvement is possible. Throughout

chapter 3, inadequacies in the processing steps and suggestions for improvement

were stated, and are summarised here.

The two m ajor forms of coherent noise — ground roll and vibrator noise —

were not attenuated optimally. 1) The ground roll was surgically muted, thereby

also removing any reflection energy which may be present. 2) The vibrator noise

was partially attenuated through spectral equalisation; however, the S/N was still

too low on many traces to contribute reflection energy to the stack.

Velocity filtering showed some promise in attenuating the ground roll. An

improved routine may aid in extending some of the shallow reflections even closer

to the surface. Other possibilities involve the muting of this noise in the r-p domain,

and removing this noise using a multiple trace median filter (a technique used to

- 4 2 -


Chapter 4 Conclusions 43

remove coherent energy in vertical seismic profiling data). Being a ghost image of

the ground roll, vibrator noise could also be removed through median filtering.

The problem of vibrator noise is common to much seismic data, both deep

crustal and exploration. Therefore, prevention, not merely attenuation, of this noise

would be of great benefit. However, the causes of this noise must be determined

first; to this end, steps should be taken in future surveys to record uncorrelated

data in an affected area for detailed examination.

The importance of an accurate static correction solution has been emphasized

in section 3.5 . Although data quality was greatly improved, further improvement

may still be possible. First of all, the 8 ms time shift between data recorded by the

two DFS-V systems is not surface consistent, and so should be removed before a

static solution is determined. Furthermore, the use of newer, more advanced static

correction methods, such as travel time tomographic inversion (Milkereit [1990b])

or random search optimisation (Dahl-Jensen [1989]), holds promise for future pro

cessing.

In thu southern half of line 12A, shallow reflections dipping at up to 30° on the

unmigrated section (assuming an average velocity of 6 km /s) were found to stack

optimally at abnormally high velocities, indicating that dip moveout effects are

significant. These shallow reflections are crucial to the interpretation of the section,

linking the surface geology with the subhcrizontal structures at depth. Thus, a

DMO removal routine should be made available for future processing of data from

the AGB.

Reflection energy varies strongly both in phase and amplitude character within

a CMP gather so that few traces contribute reflection energy to the stacked trace.

The development of an efficient method of passing only consistent energy with zero

dip in the NMO and DMO corrected CMP gathers (e. g. coherency enhancement,

Karhunen - Loeve filtering) could benefit the processing of crustal data.



Finally, no reflection energy above 100 Hz contributed to coherent reflection

images after stacking using the processing methods available for this thesis. How

ever, it is not clear whether this is a physical limitation due to strong attenuation in

the shallow bedrock and overburden, or whether it is simply a result of inadequate

processing methods.

4 .2 E va lu ation and In terp reta tion o f th e R ep ro cessed S ection

The preliminary and reprocessed stacked sections are shown as plates 1 and 2

respectively with the migrated, reprocessed section shown as plate 3. The improve

ment in data quality after reprocessing is striking. The preliminary section has

been interpreted by Green et al. [1990] and Jackson et al. [1990]. In the following

pages, the improvements after reprocessing will be evaluated, with some geological

interpretation being made based on these two citations.

Firstly, the prim ary goal — to image reflections in the upper I s — has been

achieved. In the southern half of the line (VP 100 to 900), much coherent, north

dipping reflection energy is visible (e. g. reflections A, B, and C). This will aid in

extending the known surface geology through the upper several kilometres of the

crust during interpretation. In the north part of the line (VP 900 to 1600), the

improvement in the shallow data is not so great as in the south. Most notably, no

dipping reflections are imaged clearly. The weaker shallow reflectivity in the north

may be due to the complex internal stratigraphy within the Blake River Group

(Dimroth et al. [1983]), which the northern portion of the line traverses. However,

this could also be due to higher noise levels. True am plitude processing must be

done in order to determine if this change in amplitude levels is real.

In addition to the notable improvement in the upper 1 s, data quality in much

of the rest of the seismic section has also been improved. Possible exceptions to

this are D and E, where there is a reduction in reflection energy and coherence



after reprocessing. The stacking velocity was picked by maximising the coherence

of reflection D, suggesting that the decline in quality is a result of inaccurate static

corrections rather than a poor choice of stacking velocity. The residual static so

lution was determined using a correlation window centred on reflections F and G.

Reflection D is imaged well where data centred on reflection G were used as input to

the residual, static routine; however, where the input data were centred on the more

steeply dipping tail of reflection F, there is a reduction in the quality of reflection

D.

The south half of line I2A is characterised by north dipping reflections in

the upper 5 s. After reprocessing, some of these shallow reflections can be traced

very near to the surface. For example, reflection A, dipping at roughly 25° on the

unmigrated section (assuming a velocity of 6 km /s) can be traced to 0.4 s, and

projects to the vicinity of the Crosby Sill, an ultramafic intrusion. This reflection

may be attributed to this intrusion, a nearby deformation zone, or the contact

between the Kinojevis and Blake River Groups (Jackson et al. [1990]). Reflection

B, dipping at 30°, projects to within the Kinojevis Group at the surface. Reflection

C is more gently dipping (20°), and projects to near the Larder Lake - Cadillac

Deformation Zone at the surface (although it is most likely not an image of the

deformation zone, which is commonly believed to be sub-vertical).

The most prominent mid-crustal reflection, F, has been interpreted as orig

inating from the base of the greenstone volcanic material (Jackson et al. [1990],

Green et al. [1990]). Whereas on the preliminary section, reflection F is laterally

intermittent, after reprocessing it forms a very coherent, more laterally continuous

reflection package. Furthermore, it is considerably thicker, which could either be

the result of extensive interlayering, or of cross line undulations in the reflecting

horizon. After migration, the geometry of F and G is consistent with a synclinal

structure for the base of the greenstone volcanic material.



The material below F and above 5 s may be comparable to the Wawa Domal

Gneiss Terrain in the KSZ, but Jackson et al. [1990] prefer the interpretation that

it is the Pontiac Metasedimentary Belt and/or its basement, over which the AGB

has been thrust from the north.

Below 5 s, there is a large amount of subhorizontal reflection energy along

with dipping diffraction energy. Reprocessing has resulted in improvement of S/N,

particularly from VPs 100 to 600, and 1200 to 1600. There is a marked increase

in lateral continuity, suggesting that there is more laterally extensive layering in

the lower part of the section than can be surmised from th,* preliminary section.

However, the presence of the diffraction energy suggests that there is still a large

amount of faulting and/or truncation of reflective layers. Based on the crustal model

developed from the studies of the KSZ (Percival et al. [19S9]). this material has been

interpreted to consist of layered mafic/felsic granulites and layered anorthosites

(Green et al. [1990]).

The reprocessing of the data produced very good results. The improved imaging

allows the surface geology to be extended to depth with greater confidence, and

should allow a more detailed interpretation of reflection zones at depth. It is clear

tha t substantial effort should be expended in processing the data which will be

acquired for the Abitibi - Grenville Transect, and the results illustrate the value of

detailed processing of deep crusted seismic data from Archaean cratons in general.


Appendix

DISCO Job Decks

The job decks used in the reprocessing of the data are shown in the follow

ing pages. For information on the various parameters or on the operation of the

processes, see the DISCO User’s Manual. Where implementation was not straight

forward, a discussion is included.

A .l Surface G eo m etry D efin itio n

♦JOB ABITIBI 12A JAN SURFACE GEOMETRY♦CALL DUMIN♦♦*♦♦♦ D efin e surface lo c a t io n s and e le v a tio n s o f s ta t io n s♦♦♦CALL LINE STATIONSLOCN 101 LXY 59603S 5328926 293LOCN 102 LXY 596038 5328952 295LOCN 103 LXY 596041 5328972 296*♦ e tc .♦♦♦♦ D efine th e recording spreads♦♦♦♦ No gap p attern ( fo r r o l l -o n and r o l l - o f f )♦CALL PATTERN 240 YESPSTAT 11 1240 240♦♦♦♦ 17 s ta t io n gap, a t 181 to 197♦CALL PATTERN 17 YESPSTAT 1891 1180 180

- 4 7 -


Appendix DISCO Job Decks 48

181 198240 257* *

♦♦ A ssign th e lo c a t io n s of th e shots♦♦♦CALL SOURCE 1 240SHOT 1 101 240 101THRU 66 1 0SHOT 67 168 240 101THRU 70 1 0SHOT 71 177 240 101THRU 183 1 0SHOTTHRU

1841462

2901

17

SHOT 1463 1569 240 1397THRU♦END

1530 1 0

A .2 S u b -su rface G eom etry D efin ition

One trace with an offset of 750 metres from every fourth shot was selected.

The mid point locations for each of these traces were linearly interpolated to define

the CMP line which would be used in sorting the data.

A .3 T race E d its

Traces with low S/N were chosen manually during preliminary processing from

plots of adjacently stacked shot gathers. These same trace edits were used during

the reprocessing.



A .4 W rite G eo m etry In fo rm a tio n in to T race H ead ers

** Omit dead sh ots **♦CALL EDIT FFID SEQNOALL OMIT RANGE0 3 29 30 56 58 82110 111 127 129 141 142 168196 197 226 227 257 258 288319 320 343 351 381 382 412443 444 474 476 506 507 537568 569 599 600 615 617 632656 662 690 691 713 714 722753 7F1 784 785 816 817 832849 850 880 887 914 915 942946 947 977 978 1009 1010 10221040 1041 1072 1074 1103 1104 11251159 1160 1189 1196 1226 1227 12571288 1289 1320 1321 1352 1353 13691403 1404 1433 1446 1475 1476 15071539 1540 1571 1572 1603 1604 16191652 1653 1680 1681 1708 1709♦CALL EDIT FFID SEQNOALL OMIT NORANGE133 827 1044**** Write shot number in to tra ce headers**♦CALL HEADPUT SHOT STORE INTEGERINPUT FFID ALL** f i l e shotDATA 4 1DATA 28 25DATA 31 26DATA 55 50DATA 59 51DATA 81 73DATA 84 74DATA 109 99DATA 112 100DATA 126 114

83169289413538633723833943102611321258137715081627



DATA 130 115DATA 132 117DATA 134 118DATA 140 ' 124DATA 143 125DATA 167 149DATA 170 150DATA 195 175DATA 198 176DATA 225 203DATA 228 204DATA 256 232DATA 259 233DATA 287 261DATA 290 262DATA 318 290DATA 321 291DATA 342 312DATA 352 313**** For th e north h a lf o f th e spread:DATA 383 344DATA 386 345**** For th e south h a lf o f th e spread:DATA 380 341DATA 383 342****DATA 411 370DATA 414 371DATA 442 399DATA 445 400DATA 473 428DATA 477 429DATA 505 457DATA 508 458DATA 536 486DATA 539 487DATA 567 515DATA 570 516DATA 598 544



DATA 601 545DATA 614 558DATA 618 559DATA 631 572DATA 634 573DATA 655 594DATA 663 595DATA 689 621DATA 692 622DATA 712 642DATA 715 643DATA 721 649DATA 724 650DATA 752 678DATA 755 679DATA 783 707DATA 786 708DATA 815 737DATA 818 738DATA 826 746DATA 828 747DATA 831 750DATA 834 751DATA 848 765DATA 851 766DATA 879 794DATA 888 795DATA 913 820DATA 916 821DATA 941 846DATA 944 847DATA 945 848DATA 948 849DATA 976 877DATA 979 878DATA 1008 907DATA 1011 908DATA 1021 918DATA 1027 919DATA 1039 931DATA 1042 932DATA 1043 933

Reproduced with permission of the copyright owner. Further rep ro d u c tio n prohibited without permission.


DATA 1045 934DATA 1071 960DATA 1075 961DATA 1102 988DATA 1105 989DATA 1124 1008DATA 1133 1009DATA 1158 1034DATA 1161 1035DATA 1188 1062DATA 1197 1063DATA 1225 1091DATA 1228 1092DATA 1256 1120DATA 1259 1121DATA 1287 1149DATA 1290 1150DATA 1319 1179DATA 1322 1180DATA 1351 1209DATA 1354 1210**** For th e north h a lf o f th e spread: DATA 1368 1224**** For th e south h a lf o f th e spread:DATA 1363 1219DATA 1364 1221DATA 1367 1224****DATA 1378 1225DATA 1402 1249DATA 1405 1250DATA 1432 1277DATA 1447 1278DATA 1474 1305DATA 1477 1306DATA 1506 1335DATA 1509 1336DATA 1538 1365DATA 1541 1366


Appendix DISCO Job Decks__________ 53

♦ ♦♦♦ For the north h a lf o f th e spread:DATA 1544 1369DATA 1546 1370♦ ♦♦♦ For th e south h a lf o f th e spread:DATA 1545 1370DATA 1547 1371♦ ♦* *

DATA 1570 1394DATA 1573 1395DATA 1602 1424DATA 1605 1425DATA 1618 1438DATA 1628 1439DATA 1651 1462DATA 1654 1463DATA 1679 1488DATA 1682 1489DATA 1707 1514DATA 1710 1515DATA 1725 1530♦ ♦

♦♦ For th e north h a lf o f th e spread ONLY, add 120 to th e channel ♦♦ number (to make channels 121 to 240)♦ ♦

’‘CALL HEADPUT CHAN ACCUM INTEGER INPUT FFID ALLDATA 1 120DATA 2000 120

♦♦ W rite geometry in form ation in to tr a c e headers ♦♦♦CALL PROFILE 15 400



A .5 F irst A rrival P icks

ABITIBI 12A JAN♦JOB * *

** Read only north h a lf o f th e spread ♦ ♦

240

FTRS7 ARRIVAL PICKING

♦CALL GIN 1000 2DENSITY 6250REEL LF0811 126 161♦♦♦♦ P ick only fo r every s ix th♦♦♦CALL EDIT SHOT SEQN0ALL OMIT RANGE121 125 127 131145 149 151 155

133157

FFID I NCR SEGY

137161

139 143

♦♦ W rite shot numbers, channel numbers, and geometry inform ation in to ♦♦ tr a c e headers (se e s e c t io n A.4)**♦♦ Do not p ick f i r s t a r r iv a ls fo r n earest 500 m etres ♦ ♦

OFFSET OMIT

FFID2000

♦CALL EDITSEL 10 500♦♦♦ ♦

♦CALL FIRSTA SHOTESTIMAT 219♦♦ x l t l x20 0 5000TECH1 40 .5DISK PK.LINE♦END

RANGE

OFFSET 100

t2834



A .6 R efraction S tatics

♦JOB♦CALL♦♦♦♦♦CALLSHIFTS♦ ♦

♦♦

VONEVZERO101♦ ♦

FIRSTAPRINT♦END

ABITIBIDUMIN

12A JAN REFRACTION STATICS

STATICG0

v(low )5300

1600

maxit e r3

0 0smooth

v(h igh) p o in ts6800 50

dtsmooth5

max timeerror15 STAT.LN

x(min)500

x(max)1340 PK.LINE

A .7 R esa m p le

♦JOB ABITIBI 12A JAN♦CALL GIN 8000 2DENSITY 6250REEL LF0809 4 79REEL LF0811 80 161REEL LF0813 162 237REEL LF0815 238 314REEL LF0817 315 400REEL LF0819 401 442REEL LF0821 445 520REEL LF0823 521 597REEL LF0825 598 684REEL LF0827 685 764REEL LF0829 765 844

RESAMPLE120 FFID INCR SEGY

♦ ♦

♦♦ Write shot numbers, channel numbers, and geometry in form ation in to ♦♦ tr a c e headers ♦ ♦

♦CALL RESAMP 4♦CALL TAP0UT 6250 RSM1NREEL L00773



REEL L00772REEL L00771REEL L00770REEL L00769REEL L00768REEL L00767REEL L00766REEL L00765REEL L00764♦END

Note that the data from the two DFS-V recording systems were resampled

separately, and written to two tape sets (north and south halves). When being

sorted to CMP gathers, these two datasets Eire merged.

A .8 CMP Sort

♦JOB ABITIBI 12A JAN CMP SORT♦CALL TAPIN♦♦ north h a lfSET RSM1N♦♦ south h a lfSET RSM1S♦♦

♦CALL SORT 1000 10000MAJOR CDPMINOR OFFSET♦ ♦

♦CALL TAP0UT 6250 S0RT1REEL L00450REEL L00451REEL L00455REEL L00739REEL L00740REEL L00742REEL L00743REEL L00745REEL L00761REEL L00762REEL L00763



REEL L00746REEL L00748REEL L00759REEL L00760♦END

A .9 Spectral Equalisation

♦JOB ABITIBI 12A JAN SPEQ ♦CALL DSKRD CDP.601.630 ♦ ♦

♦♦ Apply r e fr a c tio n s t a t ic s ♦ ♦

♦CALL HEADPUT REC-RFSTATTRI REC-STATSTAT-LN STATICG STATION♦CALL HEADPUT SHT-RFSTATTRI SHT-STATSTAT.LN STATICG STATION♦CALL STATICMULTISHT-RFSTAPPLYREC-RFSTAPPLY♦ ♦

♦♦ F ir s t a r r iv a l mute ♦♦

♦CALL MUTE CDP OFFSET 20 INT INTON 259♦ ♦ X t X t X t X

200 350 800 420 801 210 20002300 470 2900 700 4100 1160♦♦♦♦ Ground r o l l mute♦♦♦CALL MUTE CDP OFFSET 20 INT INTSURG 259♦ ♦ X t ( i ) t ( f )799 0 50800 225 4201000 278 4751400 385 5652000 570 7002200 630 767

t360



2900 800 1000♦♦♦CALL SPEQ 150FGATES15 20 30 3530 35 45 5045 50 60 65SPEC0UT25 .5 40 1♦♦♦CALL FILTER CDP

100

NOTCH 60 ♦ ♦

♦♦ Reapply mutes ♦ ♦

♦CALLON2002300♦CALLSURG799800 1000 1400 2000 2200 2900 * *

♦CALL♦END

MUTE259350470MUTE2590

225278385570630800

CDP

8002900CDP

504204755657007671000

OFFSET 20 INT INT

420 801 210 2000 360700 4100 1160OFFSET 20 INT INT

DSKWRT CDP-601.630-SPQ

The implementation, of this routine was found to be extremely problematic.

Being very computationally intensive, the data had to be broken into subsets of

less than 50 CMP gathers for processing. Although using the array processor can

significantly reduce the processing time, the array processor frequently failed while

performing spectral equalisation, making its use virtually impossible. Thus, pro

cessing time had to be reduced by using disk input and output (reading the input

data from tapes and writing output to tapes separately from the spectral equali

sation process). Nevertheless, the total amount of processing time was extremely



large — it took two months to complete the processing. Since spectral equalisation

was found to be so crucial in the processing of the shallow data, it is necessary that

the problem with the array processor be solved in the near future.

A .10 Surface - C onsistent R esidual Statics

♦JOB ABITIBI 12A JAN♦CALL TAPIN SET S0RT1♦ ♦

♦♦ Prelim inary V e lo c it ie s ♦ ♦

SURF CONS RES STATICS

♦CALL DEFINE CDPHANDVEL 1♦♦ t V t V

0 5750 4000 6750♦CALL NM0 PRELIM♦♦♦♦ S e le c t and f la t t e n a data♦♦♦CALL PREPARE CDPGATE 0♦♦ cmp t t ( i ) t ( f )41 2200 2000 240070 2200 2000 2400100 2280 2080 2480170 2364 2164 2564226 2500 2300 2700270 2680 2480 2880345 2820 2620 3020390 2940 2740 3140535 3420 3220 3620658 3980 3780 4180760 4640 4440 4840809 4940 4740 5140882 4940 4740 5140914 5060 4860 5260967 5060 4860 5260♦♦♦♦

INV

t8000

311

PRELIM

v7000

345



♦♦ nchan maxshft itermax name♦CALL STATICR 110 8 3 SCLN10FSMASH 3 3 3♦♦ cm p(i) cmp(f) s h o t ( i ) s h o t ( f ) r e c ( i ) r e c ( f )GE0M 311 345 455 660 515 710♦END

The residual statics routine was found to have memory limitations, which ne

cessitated that the input data be broken into a large number of small datasets. The

process was applied to groups of 35 CMP gathers each (with an overlap of 5 between

groups) using a correlation window of 400 ms length. Although the process did not

fail if larger input datasets were used, the results were of significantly lower quality;

future processing of other datasets must involve testing to determine the optimal

size of input dataset which can be used.

A .11 NM O Correction and Stack

♦JOB ABITIBI 12A JAN STACK♦ ♦

♦♦ Read s p e c tr a lly balanced CMP gathers ♦♦

♦CALL TAPINSET SPEQ♦ ♦

♦♦ Stack on ly data w ith o f f s e t between 200 and 2000 metres * *

♦CALL EDIT CDP OFFSETSEL 1 967 OMIT RANGE0 199 2001 9999♦ ♦♦♦ R efined v e lo c i t i e s **♦CALL DEFINE CDP INV REFINE♦♦ t V t V t V t V

HANDVEL 10 7000 1000 7250 2000 6750 8000 7000HANDVEL 2200 7000 1000 7250 2000 6750 8000 7000



HANDVEL 2400 7800 1000 7500HANDVEL 2800 7800 1000 7500HANDVEL 4700 7000 1000 7250HANDVEL 5200 6250 2000 6750HANDVEL 7600 6250 2000 6250HANDVEL 9670 6250 2000 6250♦CALL NMO REFINE♦ ♦

♦♦ Apply r e s id u a l s t a t i c s ♦ ♦

♦IFCDP 1 43HEADPUT SC.STAT STORESHOT SCLNOF SHOTHEADPUT SC.STAT ACCUMREC-STATSCLNOF REC

RANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI.♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALL

CDP 44 73HEADPUT SC.STAT STORE SHOT SCLN1F SHOT HEADPUT SC.STAT ACCUM REC-STATSCLNIF REC

CDP 74 103HEADPUT SC.STAT STORE SHOT SCLN2F SHOT HEADPUT SC-STAT ACCUM REC-STATSCLN2F REC

CDP 104 133HEADPUT SC.STAT STORE SHOT SCLN3F SHOT HEADPUT SC.STAT ACCUM

2000

2000

2000

8000

4000

4000

6750

6750

6750

7000

6750

6750

8000

8000

8000

8000

8000

SHOT

STATION

SHOT

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SHOT

STATION

SHOT

7000

7000

7000

7000

7000



ATTRI♦RESET♦ IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALL

REC-STATSCLN3F REC



CDP 194 223HEADPUT SC-STAT STORE SHOT SCLN6F SHOT HEADPUT SC-STAT ACCUM REC-STATSCLN6F REC



CDP 284 313HEADPUT SC-STAT STORE SHOT SCLN9F SHOT HEADPUT SC-STAT ACCUM

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT



ATTRI REC-STATSCLN9F REC♦RESET♦IFRANGE CDP 314 343♦CALL HEADPUT SC.STAT STOREATTRI SHOT SCLNIOF SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLNIOF REC ♦RESET ♦ IFRANGE CDP 344 373♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN11F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN1IF REC♦RESET ♦IFRANGE CDP 374 403♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN12F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN12F REC ♦RESET ♦IFRANGE CDP 404 433♦CALL HEADPUT SC.STAT STOR-ATTRI SHOT SCLN13F SHOT♦CALL HEADPUT SC.STAT ACCUMATTRI REC-STATSCLN13F REC ♦RESET ♦IFRANGE CDP 434 463♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN14F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN14F REC ♦RESET ♦IFRANGE CDP 464 493♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN15F SHOT♦CALL HEADPUT SC-STAT ACCUM

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

R e p ro d u c e d with p e rm iss ion of th e copyright owner. F u r th e r reproduction prohibited without perm iss ion .


ATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALLATTRI♦RESET♦IFRANGE♦CALLATTRI♦CALL

REC-STATSCLN15F REC

CDP 494 523HEADPUT SC.STAT STORE SHOT SCLN16F SHOT HEADPUT SC.STAT ACCUM REC-STATSCLN16F REC





CDP 644 673HEADPUT SC.STAT STORE SHOT SCLN21F SHOT HEADPUT SC.STAT ACCUM

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT



ATTRI REC-STATSCLN21F REC♦RESET♦IFRANGE CDP 674 703♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN22F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN22F REC ♦RESET ♦IFRANGE CDP 704 733♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN23F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN23F REC ♦RESET ♦IFRANGE CDP 734 763♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN24F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN24F REC ♦RESET ♦IFRANGE CDP 764 793♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN25F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN25F REC ♦RESET ♦IFRANGE CDP 794 823♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN26F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN26F REC ♦RESET ♦IFRANGE CDP 824 853♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN27F SHOT♦CALL HEADPUT SC-STAT ACCUM

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission


ATTRI REC-STATSCLN27F REC*RESET♦IFRANGE CDP 854 883♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN28F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN28F REC ♦RESET ♦IFRANGE CDP 884 913♦CALL HEADPUT SC.STAT STOREATTRI SHOT SCLN29F SHOT♦CALL HEADPUT SC.STAT ACCUMATTRI REC-STATSCLN29F REC ♦RESET ♦IFRANGE CDP 914 943♦CALL HEADPUT SC .ST AT STOREATTRI SHOT SCLN30F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN30F REC ♦RESET ♦IFRANGE CDP 944 967♦CALL HEADPUT SC-STAT STOREATTRI SHOT SCLN31F SHOT♦CALL HEADPUT SC-STAT ACCUMATTRI REC-STATSCLN31F REC♦RESET♦CALL STATIC SC-STAT♦ ♦

♦ ♦

♦CALL STACK 800NORM♦CALL DSKWRT STACK♦END

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION

SHOT

STATION


Appendix DISCO Job Decks 6J_

A .12 C oh eren cy F ilter

♦ JOB ABITIBI 12A JAN♦CALL DSKRD STACK**♦CALL AGC 2000♦CALL FILTER CDPKEYDEFBAND15

1BP20 60 70

♦♦♦♦ W rite su rface (VP) lo c a t io n s♦♦♦CALL HEADPUT VP STOREINPUT CDP ALL SEqNODATA 1 1DATA 967 1♦CALL HEADPUT VP STOREINPUT CDP NONE SEQNODATA 31 150DATA 63 200DATA 93 250DATA 121 300DATA 154 350DATA 188 400DATA 220 450DATA 246 500DATA 278 550DATA 307 600DATA 339 650DATA 367 700DATA 399 750DATA 429 800DATA 459 850DATA 492 900DATA 525 950DATA 558 1000DATA 589 1050DATA 622 1100DATA 656 1150DATA 689 1200DATA 722 1250

COHERENCY FILTER

INTEGER 1

INTEGER 1

967

967



DATA 754 1300DATA 787 1350DATA 820 1400DATA 852 1450DATA 883 1500DATA 913 1550DATA 945 1600♦ ♦

♦♦ Coherency f i l t e r ♦ ♦

♦♦ xgate x in c tg a te t in e mindip♦CALL SIGNAL 19 9 100 60 -200♦♦d ip in c th resh f i l t e r15 0 .15 0♦♦ Add back 257. o f th e non co h e r e n c y -filter e d data♦CALL DIGISTK 0.75**♦CALL DSKWRT STACK.C0H♦END

A .13 P lo t

♦JOB ABITIBI 12A JAN PLOT FINAL SECTION♦CALL DSKRD STACK.C0H♦♦♦♦ t p i ip s♦CALL SECPL0T VA 30 3LABEL VP 100GAIN 4SETAMP PEAKPL0T0PT /NAME=P1♦♦ above sc a le width height♦CALL SIDELBL 8 30PL0T0PT /NAME=P2/P0S= (AFTER,PI)SPACE 1 .FREE B0X1SPACE 1 .ARROW NORTHSPACE .5FREE B0X2 1 7.SPACE .5

maxdip200

b ia s- .3


Appendix DISCO Job Decks_____________________________________________ 69

FREE B0X3 1 7 .SPACE .5FTEXT B0X1 .5 2LITHOPROBEFTEXT B0X1 .3 1(9KSZ TRANSECT LINE 12AFTEXT B0X2 .3 2DATA ACQUISITION FTEXT B0X2 . IS 1<0ACQUIRED BY : VERITAS GEOPHYSICAL ®® <2SOURCE: ®® 2 MERTZ VIBRATORS <0G 20 - 120 HZ SWEEP (9<9 8 S SWEEP LENGTH ®<9 8 SWEEPS PER VP <9(9 20 METRE VP SPACING Q<9 <9RECEIVERS: 0(9 OYO 14 HZ GEOPHONES <9<9 12 PHONES PER GROUP <9<0 20 METRE RECEIVER SPACING <9<2 240 CHANNELS <2<9 SPLIT SPREAD (EXCEPT ROLL-ON/OFF) <2® 180 CHANNELS (SOUTH), 60 CHANNELS (NORTH) ®® 19 STATION GAP FOR VP ®a aRECORDING: ®0 TWO DFS-V SYSTEMS aa 16 S LISTEN TIME ®a 8 S CORRELATED RECORDS aa 2 MS SAMPLE INTERVAL aaFTEXT B0X3 .3 2PROCESSING SEQUENCE FTEXT B0X3 .15 1aPROCESSED AT THE LITHOPROBE SEISMIC PROCESSING FACILITY aUSING THE CYBER/DISCO SYSTEM BY JAN KOZEL (UNIVERSITY OF TORONTO)®



« G

1 . CROOKED LINE GEOMETRY cIBIB2.

30 X 800 M CMP BINS «

TRACE EDIT «(B DETERMINED MANUALLY FROM STACKED SHOT GATHERS USED IN «IB/A

PRELIMINARY PROCESSING «

3. FIRST ARRIVAL PICKS IBO/A

DETERMINED USING THE DISCO AUTOMATIC PICK ROUTINE IB<D

4. RESAMPLE TO 4 MS IBIBA

ANTIALIAS FILTER: 100 - 125 HZ ROLL-OFF IB

5. REFRACTION STATIC CORRECTION IBIBA

1 LAYER (1600 M/S) OF VARIABLE THICKNESS OVER BEDROCK IB<86. MUTE OF FIRST ARRIVALS IBIB 0 M 350 MS Q

IB 200 M 350 MS IB0 800 M 420 MS IBIB 801 M 210 MS <BIB 2000 M 360 MS IB(B 2300 M 470 MS IBIB 2900 M 700 MS IBIBA

4100 M 1160 MS IBVS

7. SURGICAL MUTE OF GROUND ROLL IB(B 800 M (225 , 420) MS IBIB 1000 M (278, 475) MS IBIS 1400 M (385, 565) MS IBIB 2000 M (570, 700) MS IBIB 2200 M (630, 767) MS IB<SA

2900 M (800, 1000) MS e<8

8. SPECTRAL EQUALISATION FOR 0 S TO 3 S IB(B (2 0 ,3 0 ) (3 5 ,4 5 ) (50 ,60 ) HZ BANDWIDTHS IBIB 0 .5 1 .0 1 .0 WEIGHTS IB(BA

EQUALISED USING 150 MS AGC IB(89.IB

AGC FOR 3 S TO 8 S (750 MS WINDOW) e



10. 60 HZ NOTCH FILTER ISID11. REAPPLICATION OF FIRST ARRIVAL AND GROUND ROLL MUTE ISIS12. COMMON MID POINT SORT IS(013. SURFACE CONSISTENT RESIDUAL STATICS (SID MAXIMUM CORRECTION 8 MS IS10 400 MS CORRELATION WINDOW: IS(0 VP TIMES (MS) ISID 160 2000 2400 ID(0 210 2000 2400 ISIS 260 2080 2480 (S<0 375 2164 2564 ISIS 460 2300 2700 (3(0 540 2480 2880 IS(0 660 2620 3020 ISID 740 2740 3140 (SIS 960 3220 3620 ISIS 1150 3780 4180 IBIS 1310 4440 4840 (S(S 1380 4740 5140 IS<S 1500 4740 5140 isIS 1550 4860 5260 isIS 1636 4860 5260 ISIS14. VELOCITY ANALYSIS AND NMO REMOVAL 0Q15. STACK isIS16. AGC (2000 MS WINDOW) ISIS17. BANDPASS FILTER isIS 15/20 - 60/70 HZ ISIS18. COHERENCY FILTER isIS 19 TRACE WINDOW 9 TRACE INCREMENT ISIS 100 MS TIME WINDOW 60 MS INCREMENT ISIS19. ADD BACK UNFILTERED STACK ISIS 75 IS 25 (S20. PLOT IS



<8 VARIABLE AREA (30 */. NEGATIVE BIAS) <89 30 TRACES / INCH <DID 3 .0 INCHES / SECOND <0IS♦CALL TOPLBL 3 30.INPUT VELDEFN VELDEFN 1990DISPLAY 1 967STRIP .5 2 TVBOXTVBOX TV TOP NOANNOTPLOTOPT /NAME=P3/P0S=(ABOVE,PI)♦END


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