environmental and exploration geophysics ii
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Environmental and Exploration Geophysics II. Common MidPoint (CMP) Records and Stacking. tom.h.wilson [email protected]. Department of Geology and Geography West Virginia University Morgantown, WV. Stack Trace. Pure signal. - PowerPoint PPT PresentationTRANSCRIPT
Tom Wilson, Department of Geology and Geography
Environmental and Exploration Geophysics II
Department of Geology and GeographyWest Virginia University
Morgantown, WV
Common MidPoint (CMP) Records Common MidPoint (CMP) Records and Stackingand Stacking
Tom Wilson, Department of Geology and Geography
If we sum all the noisy traces
together - sample by sample - we get the trace
plotted in the gap at right. This
summation of all 16 traces is
referred to as a stack trace.
Note that the stack trace
compares quite well with the pure
signal.
Stack Trace
Pure signal
Greenbrier
Huron
Onondaga
1
n
iji
a
Where i is the trace
number and j is a specific time
Tom Wilson, Department of Geology and Geography
+1-1
Noise comes in several forms - both coherent and random. Coherent noise may come in the form of some unwanted signal such as ground roll. A variety of processing and acquisition techniques have been developed to reduce the influence of coherent noise.
The basic nature of random noise can be described in the context of a random walk -
Random noise can come in the form of wind, rain, mining activities, local traffic, microseismicity ...
See Feynman Lectures on Physics, Volume 1.
Tom Wilson, Department of Geology and Geography
The random walk attempts to follow the progress one achieves by taking steps in the positive or negative
direction purely at random - to be determined, for example, by a coin toss.
+1-1
Tom Wilson, Department of Geology and Geography
Does the walker get anywhere?
Our intuition tells us that the walker should get nowhere and will simply wonder about their point of origin.
However, lets take a look at the problem form a more quantitative view.
It is easy to keep track of the average distance the walker departs from their starting position by following the behavior of the average of the square of the departure. We write the average of the square of the distance from the starting point after N steps as 2
ND
The average is taken over several repeated trials.
Tom Wilson, Department of Geology and Geography
21DAfter 1 step will always equal 1 ( the
average
of +12 or -12 is always 1. After two steps -
212
12
2 1or 1 DDD
which is 0 or 4 so that the average is 2. After N steps
1or 1 11 NNN DDD
121 12
12
1 NNN DDD
121 12
12
1 NNN DDD
Tom Wilson, Department of Geology and Geography
121 12
12
1 NNN DDD
121 12
12
1 NNN DDD
Averaged over several attempts to get home the wayward wonderer gets on average to a distance squared
121
2 NN DD
from the starting point.
Since 2
1D =1, it follows that
NDN 2
and therefore that NDN
Tom Wilson, Department of Geology and Geography
The results of three sets of random coin toss experiments
See Feynman Lectures on Physics, Volume 1.
Tom Wilson, Department of Geology and Geography
The implications of this simple problem to our study of seismic methods relates to the result obtained through stacking of the traces in the common midpoint gather.
The random noise present in each trace of the gather (plotted at left) has been partly but not entirely eliminated in the stack trace.
Just as in the case of the random walk, the noise appearing in repeated recordings at the same travel time, although random, does not completely cancel out
Tom Wilson, Department of Geology and Geography
The relative amplitude of the noise - analogous to the distance traveled by our random walker- does not drop to zero but decreases in amplitude relative to the signal.
If N traces are summed together, the amplitude of the resultant signal will be N times its original value since the signal always arrives at the same time and sums together constructively.
The amplitude of noise on the other hand because it is a random process increases as
N
Hence, the ratio of signal to noise is N
Nor just
where N is the number of traces summed together or the number of traces in the CMP gather.
N
Tom Wilson, Department of Geology and Geography
In the example at left, the common midpoint gather consists of 16 independent recordings of the same reflection point.
The signal-to-noise ratio in the stack trace has increased by a factor of 16 or 4.
The number of traces that are summed together in the stack trace is referred to as its fold – i.e. 16 fold.
Tom Wilson, Department of Geology and Geography
If you had a 20 fold dataset and wished to improve its signal-to-noise ratio by a factor of 2, what fold data would be required?
Square root of 20 = 4.472
Square root of N(?) = 8.94
What’s N
To double the signal to noise ratio we must quadruple the fold
Tom Wilson, Department of Geology and Geography
The reliability of the output stack trace is critically dependant on the accuracy of the correction velocity.
Tom Wilson, Department of Geology and Geography
Accurate correction ensures that the same part of adjacent waveforms are summed together in phase.
Average Amplitude
Stack = Summation
Tom Wilson, Department of Geology and Geography
then the reflection response will be “smeared out” in the stack trace through destructive interference
between traces in the sum.
If the stacking velocities are incorrect ….
Tom Wilson, Department of Geology and Geography
The two-term approximation to the multilayer reflection response is hyperbolic. The velocity in this expression is a root-mean-square velocity.
Are they also hyperbolic?
The real world: multilayer reflections
Tom Wilson, Department of Geology and Geography
The sum of squared velocity is weighted by the two-way interval transit times ti through each
layer.
A series of infinite terms – but we just ignore a bunch of them
Tom Wilson, Department of Geology and Geography
The approximation is hyperbolic, whereas the actual is not. The disagreement becomes significant at longer offsets, where the actual reflection arrivals often come in earlier that those predicted by the hyperbolic approximation.
Tom Wilson, Department of Geology and Geography
The reason for this becomes obvious when you think of the earth as consisting of layers of increasing velocity. At larger and larger incidence
angle you are likely to come in at near critical angles and then will travel significant distances at higher than average (or RMS) velocity.
Greenbrier Limestone
Big Injun
Refraction into high velocity layers brings the events in along paths that don’t have hyperbolic moveout.
Tom Wilson, Department of Geology and Geography
VRMS VAV and VNMO are different. VNMO does not equal VRMS. Each of these 3 velocities has different geometrical
significance.
Tom Wilson, Department of Geology and Geography
The VNMO is derived form the slope of the regression line fit to the actual arrivals.
In actuality the moveout velocity varies with offset.
The RMS velocity corresponds to the square root of the reciprocal of the slope of the t2-x2 curve for relatively short offsets.
Tom Wilson, Department of Geology and Geography
The general relationship between the average, RMS and NMO velocities is shown at right.
Tom Wilson, Department of Geology and Geography
Geometrically the average velocity characterizes travel along the normal incidence path.
The RMS velocity describes travel times through a single layer having the RMS velocity. It ignores refraction across individual layers.
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Water Bottom Reflection
Reflection from Geologic intervalWater Bottom
Multiple
Normal Incidence Time Section ….
Tom Wilson, Department of Geology and Geography
DEPTH
Interbed Multiples
Tom Wilson, Department of Geology and Geography
Interbed Multiples
Tom Wilson, Department of Geology and Geography
The Power of Stack extends to multiple attenuation
Tom Wilson, Department of Geology and Geography
Velocities associated with primary reflections are higher than those associated with multiples. The primaries are flattened
out while residual moveout remains with the multiple reflection event.
The NMO Corrected CDP gather
Tom Wilson, Department of Geology and Geography
Multiple attenuation
Multiple Multiple
Primary Reflections
Tom Wilson, Department of Geology and Geography
Buried graben or multiple
Examples of multiples in marine seismic data
Tom Wilson, Department of Geology and Geography
Multiples are considered “coherent” noise or unwanted signal
Tom Wilson, Department of Geology and Geography
Interbed multiples or Stacked pay zones
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Waterbottom and sub-bottom multiples
Tom Wilson, Department of Geology and Geography
Other forms of coherent “noise” will also be attenuated by the stacking process.
The displays at right are passive recordings (no source) of the background noise.
The hyperbolae you see are associated with the movement of an auger along a panel face of a longwall mine.
Tom Wilson, Department of Geology and Geography
Offset (m) Reflection1 Reflection2 Reflection3
x t1 (ms) t2 (ms) t3 (ms)
3 21.4 62.3 79.4
6 25 62.4 79.5
9 30.1 62.6 79.6
12 36.1 62.9 79.9
15 42.5 63.2 80.1
18 49.2 63.6 80.5
21 56.2 64.1 80.9
24 63.3 64.7 81.3
27 70.4 65.4 81.8
30 77.6 66.1 82.4
33 84.9 66.9 83
36 92.2 67.7 83.7
Table 1 (right) lists reflection arrival times for three reflection events observed in a common midpoint gather. The offsets range from 3 to 36 meters with a geophone spacing of 3 meters.
Conduct velocity analysis of these three reflection events to determine their NMO velocity. Using that information, determine the interval velocities of each layer and their thickness.
Tom Wilson, Department of Geology and Geography
0 5 10 15 20 25 30 35 40
Source Receiver Offset (meters)
0
20
40
60
80
100
Arr
ival
Tim
e (m
s)
Note hyperbolic moveout of the three reflection events.
Tom Wilson, Department of Geology and Geography
22 2
0 2rms
xt t
V
Recall -
The variables t2 and x2 are linearly
related.20
2
is the intercept &
1 is the slope
rms
t
V
Tom Wilson, Department of Geology and Geography
0
2000
4000
6000
8000
10000
Tim
e2
0 200 400 600 800 1000 1200 1400
X2
Estimates of RMS velocities can be determined from the slopes of regression lines fitted to the t2-x2 responses.
Keep in mind that the fitted velocity is actually an NMO velocity!
Tom Wilson, Department of Geology and Geography
Start with definition of the RMS velocity
n
ii
n
iii
nRMS
t
tVV
1
1
2
The Vis are interval velocities and the tis are the two-way interval transit times.
Tom Wilson, Department of Geology and Geography
n
ii
n
iii
nRMS
t
tVV
1
1
2
2
n
iintt
10Let
the two-way travel time of the nth reflector
Tom Wilson, Department of Geology and Geography
n
iiinnRMS tVtV
1
20
2
1
1
220
2n
iiinnnnRMS tVtVtV
hence
Tom Wilson, Department of Geology and Geography
102
1022
nnRMSnnRMSnn tVtVtV
1
1
210
21
n
iiinnRMS tVtVSince
102
12
02
nnRMSnnnnRMS tVtVtV
Vn is the interval velocity of the nth layer
tn in this case represents the two-way interval transit time through the nth layer
Tom Wilson, Department of Geology and Geography
102
1022
nnRMSnnRMSnn tVtVtV
n
nnRMSnnRMSn t
tVtVV 10
210
22
Hence, the interval velocities of individual layers can be determined from the RMS velocities, the 2-way zero -offset reflection arrival times and interval transit times.
Tom Wilson, Department of Geology and Geography
n
nnRMSnnRMSn t
tVtVV 10
210
22
n
iitt
n1
0 the two-way travel time to the nth reflector surface
100 nnnn tttt the two-way interval transit time between the n and n-1 reflectors
nRMSVThe terms represent the velocities obtained from the best fit lines. Remember these velocities are
actually NMO velocities.
nV is the interval velocity for layer n, where layer n is the layer between reflectors n and
n-1
See Berger et al. page 173
Tom Wilson, Department of Geology and Geography
The interval velocity that’s derived from the RMS velocities of the reflections from the top and base of a layer is referred to
as the Dix interval velocity.
However, keep in mind that we really don’t know what the RMS velocity is.
The NMO velocity is estimated from the t2-x2 regression line for each reflection
event and that NMO velocity is assumed to “represent” an RMS velocity.
You put these ideas into application when solving problems 4.4 and 4.8
Tom Wilson, Department of Geology and Geography
•Multiples
•Refractions
•Air waves
•Ground Roll
•Streamer cable motion
•Scattered waves from off line
Stacking helps attenuate random and coherent noises
Tom Wilson, Department of Geology and Geography
Back to your computer projects. Please review the Summary Activities and Report Handout. We are at the
mid point in the semester and time will run out quickly.
Questions about gridding and math on two maps topics
covered in the last lecture?
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Without Extrapolation
Tom Wilson, Department of Geology and Geography
With Extrapolation
Tom Wilson, Department of Geology and Geography
Increased projection distance
Tom Wilson, Department of Geology and Geography
Time Surface
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Wells > Edit > Time Depth Charts
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Formation top approach (times derived from TD chart and formation top depths from well file)
Tom Wilson, Department of Geology and Geography
Time horizon
Tom Wilson, Department of Geology and Geography
Conversion to depth
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
• Problem 4.1 is due today
• Problems 4.4 and 4.8 are due 1st Wednesday following Spring Break.
• Look over Exercises IV-V. These will be due 1st Monday after Spring Break.
• Exercise VI will be due Wednesday after Spring Break
• For the remainder of today – Conversion to depth and project work.
• Mid Term Reports are due the Monday March 23rd.