Environmental and Exploration Geophysics II

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Environmental and Exploration Geophysics II
Velocity Analysis and Pitfalls in Seismic
Interpretation
tom.h.wilson tom.wilson_at_mail.wvu.edu
Department of Geology and Geography West Virginia
University Morgantown, WV
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Game plan review

• Hand back problem 4.1 and discuss
• Exercise VI will is due this Wednesday
  Questions?
• Problems 4.4 and 4.8 are also due this
  Wednesday Questions?

Today

• Hand in Exercises IV-V today.
• Hand in your Mid Term report today.

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Velocity Analysis
4.1 Given that the near-offset trace is located
40 meters from the source. Successive traces are
spaced at 3 meter intervals yielding offsets
ranging from 40 meters to 73 meters on the outer
trace. Shot record shows reflection event with
near offset arrival time of approximately 80 ms.
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Multilayer rather than single layer?
You were asked to treat this as a single layer
reflection problem. But - one might argue that
there is a shallower reflection event that has
been overlooked. Can you see it?
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To simplify or not to simplify?
Anything else??
Of course it is tempting to simplify problems,
but just as with the refraction analysis, if we
miss a shallower layer, error is introduced into
our result.
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Dix Interval Velocity
Review
Start with definition of the RMS velocity
The Vis are interval velocities and the tis are
the two-way interval transit times.
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Let
the two-way travel time of the nth reflector
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hence
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Since
Vn is the interval velocity of the nth layer tn
in this case represents the two-way interval
transit time through the nth layer
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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.
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Problem 4.4
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.
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
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In t-x space we see hyperbolic moveout of the
three reflection events.
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Recall -
2
2
t - x
The variables t2 and x2 are linearly related.
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In t2-x2 space we have nice straight lines
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!
Then what?
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Review terms in the Dix Equation
See Berger et al. page 173
The
terms represent the velocities obtained from the
best fit lines. Remember these velocities are
actually NMO velocities.
the two-way travel time to the nth reflector
surface
the two-way interval transit time between the n
and n-1 reflectors
is the interval velocity for layer n, where layer
n is the layer between reflectors n and n-1
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Terminology review
Reflector 1
Layer 1
Layer 2
2
3
Layer 3
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Dix Interval Velocity
The interval velocity thats 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 dont 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
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Common Midpoint (CMP) Stack
The purpose of common midpoint is multi fold!
It enhances signal
It provides useful information about subsurface
physical properties
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Random Noise
Recall that the relative amplitude of the noise
is analogous to the distance traveled by the
random walker. We actually get somewhere when we
take steps at random! In the seismic analogue,
if N traces are summed together, the amplitude of
the resultant signal will be N times its original
value. The signal always arrives at the same time
and sums together constructively. The amplitude
of random noise on the other hand increases as
or just
Hence, the ratio of signal to noise is
where N is the number of traces summed together
or the number of traces in the CMP gather.
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Uncorrected and Unorrected CMP Gathers
Getting the velocities correct is critical to
attenuating noise and enhancing signal.
After NMO Correction
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All the traces in the NMO corrected CDP gather
are summed together to yield one trace in the
stacked seismic data set.
If the signal is summed together in phase we get
nice looking reflcetion events in the stacked
seismic section
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Coherent Noise
Stacking also helps attenuate coherent noises
Multiples

Refractions
Air waves
Ground Roll
Streamer cable motion
Scattered waves from off line
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Multiples are considered coherent noise or
unwanted signal
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Waterbottom and sub-bottom multiples
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Problem 4.8
or
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Dip moveout
In the
analysis (see pages 197 to 199), youll
run across the idea of dip moveout. This is
just the difference at some value of x from the
source of the arrival times at a distance x on
either side of the source.
In the text ? is used for dip rather than ?
Read over this material (pages 197 to 199) to
help guide you through the solution. Also see
table 4.7 and related discussions (pages 182-199).
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Pitfalls in seismic interpretation
Computer modeling
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Looking ahead

• Hand back problem 4.1 and discuss
• Exercise VI will is due this Wednesday
  Questions?
• Problems 4.4 and 4.8 are also due this
  Wednesday Questions?

Before you leave

• Hand in Exercises IV-V today.
• Hand in your Mid Term report today.

Questions?