mag telluric

8/7/2019 Mag Telluric

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Magnetotellurics in

Frontier and ReconnaissanceExploration


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MT world-wide

Oil/gas

Minerals

Geothermal


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MT - Definition

Passive surface measurement of the earths naturalelectrical (E) and magnetic (H) fields

Measure changes in E and H w/time

Frequency range 10kHz to .001 Hz Used to derive the resistivity structure of the subsurface


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MT - History

First used for academic and geothermal Map plate boundaries, alteration, etc.

Use for petroleum starting ~1980

1980s: many in-house groups Shell, Amoco, Sohio, Arco, CGG

1990s: most work outsourced to

contractors and consultants


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Resistivity Contrasts

There must be a significant resistivity contrastwithin the depth of investigation for themethod to be useful

Contrast of 5:1 or greater Resolution depends on thickness and depth

of unit being mapped About 5% of depth e.g. the top of a horizon at

10000 can be mapped to +- 500


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Resistivity Values

1

10

100

1000

10000

Clay

Shale

Volc'clastics

Sandstone

Carbonate

Volc's

Igneous

R

e

s

i

st

i

v

i

t

y


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MT - Application - Oil/Gas

Reconnaissance or detail

High-resistivity (high-velocity) surface(volcanics, carbonates,

igneous)

Overthrust, fold belts, volcanics

Poor or no-record seismic OR Precede seismic, or integrate w/ seismic

Near-surface to >20 km


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MT - Detail vs. Recon Detail: prospect definition

spacing = .5 km on profiles

Recon: areal coverage spacing = 1-5 km on profiles or grids

Communication: GPS sync Acquisition rate: usu. 5-10 stas/day

MT in Nicaragua

by horseback MT helicoptersurvey in Montana


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MT - Source Field

High frequencies (>1 Hz) = Spherics thunderstorm activity world-wide

Low frequencies (


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ELECTROMAGNETIC SURVEYS

EXAMPLE USESEXAMPLE USES

Finding Mineral Prospects

Locating Contaminant

Plumes

Finding Buried Ordinance

Locating Utilities & Pipelines


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MT - Acquisition

Five channels at each station Ex Ey Hx Hy Hz

Two to five stations simultaneously

GPS sync between stations 24-hour recording/layout/pickup cycle

In-field processing and editing

Laying out a coil in Turkey;coils are used to measure themagnetic fields: Hx Hy Hz

GEOSYSTEM


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MT Acquisition System

Batteries

DigitalAcquisitionUnit

E-Lines

Coils


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MT Acquisition

Coils Hx Hy Hz

Amplifiers, digitizer, etc.

Electrodes

E-Lines Ex Ey

ComputerGPS antenna

yOne station set-up; 2-6 others simultaneously


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MT Data record

This is an actual time series record, showing (from top) Ex, Ey, Hx, and Hy varyingwith time.

Note the correlation between Ex and Hy, and between Ey and Hx. Hz is not shown.

Ex

Ey

Hx

Hy


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How resistivity is computed

Impedance tensor is measured at surface Compute apparent resistivity (and phase) as a

function of frequency

Two values computed, xy and yx, for the twoorthogonal pairs of E and H sensors in horizontal

directions Thus can interpret for strike and dip directions

a f Ex Hy=

1

5/

2


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Depth of Investigation

The depth of investigation is a result of thefrequency and resistivity of the subsurface

Lower frequency = deeper penetration

Higher resistivity = deeper penetration

Skin depth is an approximate estimate ofdepth of penetration at particular frequency

and resistivity Skin depth (in meters) =

where = resistivity and f = frequency500 / f


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MT: Current Systems

Similar to seismic advances since the1980s

24-bit A to D

GPS Synchronization

Unlimited no. of channels

Signal/robust processing Workstations w/ integration of other data 1d, 2d, 3d, modeling: fwd and inverse


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State of the Art MT Systems 1

Low weight (5kg); low power consmption(.6A)

Wide frequency range (DC to 30 KHz)

Wide dynamic range (120db, 24-bit A/D) = better S/N; less risk of saturation

Internal recording

(32MB flashcard, 1GB hard disk)

Recording schedule downloaded from PC

St t f th A t MT S t 2


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State of the Art MT Systems 2

GPS-synchronized ( 130ns accuracy) no cables or radios

2 to 8-channel units, all independent

High reliability (ISO9001 std), etc.

Fast set-up and deployment increased production

Operating from -40 to +75C; waterproof;

lightning protected Cable-link available for EMAP


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MT - Contractors

Geosystem (Italy, US, UK)

Phoenix (Canada)

Metronix (Germany)

Geodatos (Chile)

AOA (US - Marine)

Zonge (US) Geoinvest (Italy) MT + EM in Turkey

GEOSYSTE M


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MT Data Curves

2 1 0 -1 -2 -3

-1

0

1

2

3

4

Apparent Resistivity

LOG

RHO

(OHM-M)

LOG Frequency (Hz)RhoXY RhoYX

Apparent resistivityTwo curves, xy and yxQualitative view of

subsurface changesinresistivityUsed with phase datafor interpretation

Limestone

Clastics

Basement


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MT - Processing

Remote-reference

Coherency check on time series betweenstations; toss un-coherent data

Next: Edit data in time and frequencydomain

Remove noise from trains, lightning,

power stations, etc. Greatly improves data quality


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Robust Processing

Improve data qualityby time series editing

removal of outliers removal of coherent

noise

frequency domain

editing use of quiet remote

After

Before


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MT - Interpretation

PC workstation Editing, viewing of data and parameters

Data basing

1-D, 2-D, 3-D modeling: fwd and inverse Convert apparent resistivity vs. frequency to

true resistivity vs. depth

Colored x-sections and maps

Integration w/ geology, seismic, other data

Fast turnaround - can be done in the field


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MT - Statics Problems

Near-surface distortions to electric field created by resistivity variation at surface

channels, outcrop, etc.

Cause static shift in data DC jump at all freqs along a curve

Best correction = TDEM

Acquire EM data at station center Interpret for near-surface section

Incorporate into MT data and shift MT curve

Advantages and disadvantages of AMT/MT


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g gfor petroleum exploration:

Great depth of penetration

(10's of kms)

Provides information in non-seismic or poor seismic areas

No transmitter required

Light-weight equipment --veryportable

Good production rate (2 - 5 km/day)

Better resolution than grav/mag

Well-developed interpretationprocedure

Fast interpretation Little impact on environment

Can access almost anywhere

Coupling with lateral conductors(e.g. sea) also has to be considered

Natural signal can be irregular, andindustrial noise a potential problem

Resolution less than seismic

Data processing and interpretationare complex

Static shift of apparent resistivitycurves sometimes significant

Inversion techniques rely onsmooth models, tougher to

interpret in complex areas

Pros Cons


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PNG Seismic

Exploration in Papua New Guinea fold beltdifficult due to steep dips, remote

location, karstified limestone surface

Surface limestone = 1-3 km thick

Seismic costs = $100k/km+ for 2-D

Most data poor to no-record

Alternative = MT, surface geologymapping, Sr isotope dating of limestone


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Egele MT, PNG

MT predictedbase Darai Ls

w/in 7%

Drilled by Mobil

Exploration =only surfacegeology and MT

Well

Basement

Limestone

2000

0

clastics


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Irou, PNG

MT Geology based on MT and dips

VE=1:1

LIMESTONE

LIMESTONE

clastics


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PNG Overthrust

|11 |

10

|9 |

8

|7 |

6

|5 |

4

|3 |

2

|1

-5000

0

FEET

0 5000 10000 15000

2

3

5

10

20

50

100

200

500

100

200

Ohm-M

clastics

limeston

limeston

Limestone thrustover very lowresistivity clastics

Map depth to base

of hanging wall ls Map depth to top of

footwall ls

Target is folded ss

in hanging wallsection

MT in N Africa DOLOMITE ANHYDRITE


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MT in N. Africa

HIGH RESISTIVITY

METAMORPHIC

BASEMENT

DOLOMITE+ANHYDRITE:

POTENTIAL RESERVOIR UNIT

LOW RESISTIVITY

SHALE

BASALT &

SAND DUNES

GEOSYSTEM

VERTICAL EXAGGERATION=2.0.

Hi

Lo

Resistivity

MT in N Africa


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DOLOMITE+ANHYDRITE:

RESERVOIR STRUCTURE

GEOSYSTEM

CROSS-SECTION FROM 2D INVERSION SHOWING (a) RESERVOIR STRUCTURE

(b) STRUCTURE AT TOP OF BASEMENT. VERTICAL EXAGGERATION=2.5.

MT in N. Africa

Hi

Lo

Resistivity

G l d


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Greenland

Nuusaaq peninsulaWest coastVolcanics at surfaceRough terrainBasin structure unknown

Recon MT linesOffshore seismic has

basement at >7 kmSome strat holes

-10000

-9000

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

Elevation(ME

TERS)

SW NE

104105

106107

108109

110111

112

METERS

05000 10000 15000 20000 25000

Ohm-M

AIR

1

10

2030

50

70

100

200

500

1000

Basement (approx.)

Lo -rho clastics

Volcanics

Higher-rho clastics

Salt Problem


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Salt Problem

Tr Salt

OligocenePliocene

Jurassic Carbonatesand older

WellTop of salt 1800m

GEOSYSTEM

T k


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Turkey

Poor seismic

MT shows overthrust

Ties with good seismicreflectors (white lines)

Supported by drilling

Target is Mardincarbonate

Next slide shows sharp-

boundary inversion withinterpretation and seismic

picks

GEOSYSTEM


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Granite Overthrust - Wyoming

Unexplored - no

seismic, no wellsMT shows structure -amount of subthrust

Fast acquisition andinterpretation

| | | || |

30000

20000

10000

0

De

pth

(F

EET)

Ohm-M

2

5

10

20

50

100

200

500

1000

|

1 2

|

3 4

|

56

|

8 9

|

1011

|

12 13

|

1415

|

16

FEET

0 5000 10000 15000 20000 25000 30000 35000

Precambrian

|Tertiary

T/K/J

Granite overthrust

Tr and older

l

crbp


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Columbia Plateau WA

MILES0 20 40 60 80

| | | | | | | | | | | | | | | | | |1 2 3

4 5

6 7

8 9

1011

1213

1415

1617

18

20000

0

50

100

500

7

Depth

inFeet

West East

|

0 0 1|0 0 2

|

0 0 3 |0 0 4

|

0 0 5 |0 0 6

|

0 0 7 |0 0 8

|

0 0 9 |0 1 0

|

0 1 1 |0 1 2

|

0 1 3 |0 1 4

|

0 1 5|0 1 6

|

0 1 7 |0 1 8

O hm - M

0 .1

0 .2

0 .5

1

2

7

1 0

5 0

5 0

1 0 0

2 0 0

1 0 0

1 0 0 0

2 0 0 0

Volcanic-coveredprovince

Covers 35000 sq

miles

Where is the basin?

x Flood basalts (Miocene) up to 20000

feet thickx Clastic section up to 20000 feet thick

beneath basalts

x Non-seismic area

Basalt

Basement

clastics


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SHALLOW STRATIGRAPHY

USING RESISTIVITY


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PLUME MAPPING WITH

RESISTIVITY


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