Transcript No Slide Title

Environmental and Exploration Geophysics II
Velocity Analysis and Pitfalls in Seismic Interpretation
tom.h.wilson
[email protected]
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography
• 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?
•Hand in Exercises IV-V today.
•Hand in your Mid Term report today.
Tom Wilson, Department of Geology and Geography
4.1 Given that the nearoffset 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.
Tom Wilson, Department of Geology and Geography
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?
Tom Wilson, Department of Geology and Geography
Anything else??
12000
y = 0.4075x + 7693.4
R2 = 0.9698
10000
R2
8000
R3
R1
6000
Linear (R2)
Linear (R3)
4000
Linear (R1)
2000
0
0
1000
2000
3000
4000
5000
6000
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.
Tom Wilson, Department of Geology and Geography
Start with definition of the RMS velocity
n
VRMS 
n
2
V
 i ti
i 1
n
 ti
i 1
The Vis are interval velocities and the tis are
the two-way interval transit times.
Tom Wilson, Department of Geology and Geography
n
2
V
 i ti
2
i 1
VRMS

n
n
 ti
i 1
Let
t0 n
n 
   ti 
 i 1 
the two-way travel time of the nth reflector
Tom Wilson, Department of Geology and Geography
hence
2
VRMS
t
n 0n
2
VRMS
t
n 0n
Tom Wilson, Department of Geology and Geography
n
 Vi 2ti
i 1
 Vn2tn
n 1
 Vi 2ti
i 1
Since
2
VRMS
t
n 1 0 n 1
n 1
 Vi 2ti
i 1
2
2
2
VRMS
t

V
t

V
n n
RMSn 1t0 n 1
n 0n
2
2
Vn2tn  VRMS
t

V
RMSn 1t0 n 1
n 0n
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
2
2
Vn2tn  VRMS
t

V
RMSn 1t0 n 1
n 0n
Vn2 
2
2
VRMS
t

V
RMSn 1t0 n 1
n 0n
tn
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
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
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
In t-x space we see hyperbolic moveout
of the three reflection events.
Source Receiver Offset (meters)
0
0
Arrival Time (ms)
20
40
60
80
100
Tom Wilson, Department of Geology and Geography
5
10
15
20
25
30
35
40
Recall 2
x
t t  2
Vrms
2
2
0
The variables t2 and x2
are linearly related.
2
0
t is the intercept &
1
is the slope
2
Vrms
Tom Wilson, Department of Geology and Geography
In t2-x2 space we have nice straight lines
X2
0
200
400
0
2000
Time
2
4000
6000
8000
10000
Tom Wilson, Department of Geology and Geography
600
800
1000
1200
1400
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!
See Berger et al. page 173
Vn2 
The
VRMSn
2
2
VRMS
t

V
RMSn 1t0 n 1
n 0n
tn
terms represent the velocities
obtained from the best fit lines.
Remember these velocities are
actually NMO velocities.
 n 
t0n    ti  the two-way travel time to the nth reflector surface
 i 1 
tn  tn  t0n  t0n1 
Vn
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
Tom Wilson, Department of Geology and Geography
Terminology review
X2
0
Reflector 1
0
200
400
600
800
1000
1200
1400
Layer 1
2000
Layer 2
2
Time
2
4000
3
 n 
t 0 n    ti 
 i 1 
6000
8000
10000
Tom Wilson, Department of Geology and Geography
t3  t3  t03  t02 
tn  tn  t0n  t0n1 
Layer 3
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 t2x2 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
Tom Wilson, Department of Geology and Geography
The purpose of common midpoint is multi fold!
It enhances signal
It suppresses noise
Both coherent
and incoherent
It provides useful
information about subsurface
physical properties
Tom Wilson, Department of Geology and Geography
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
N
Hence, the ratio of signal to noise is
N or just
N
N
where N is the number of traces summed
together or the number of traces in the CMP
gather.
Tom Wilson, Department of Geology and Geography
Getting the velocities
correct is critical to
attenuating noise and
enhancing signal.
After NMO
Correction
Tom Wilson, Department of Geology and Geography
All the traces in the NMO corrected CDP gather are summed
together to yield one trace in the stacked seismic data set.

Tom Wilson, Department of Geology and Geography
If the signal
is summed
together in
phase we
get nice
looking
reflcetion
events in
the stacked
seismic
section
Stacking also helps attenuate coherent noises
•
•
•
•
•
•
Tom Wilson, Department of Geology and Geography
Multiples
Refractions
Air waves
Ground Roll
Streamer cable motion
Scattered waves from off line
Multiples are considered “coherent” noise or unwanted signal
Tom Wilson, Department of Geology and Geography
Waterbottom and sub-bottom multiples
Tom Wilson, Department of Geology and Geography
t x
2
tmin
t0
2
600
or
t0
offset
-40
-20
0
0
500
400
5
300
arrival time
T-Square
tapex
200
10
15
100
20
0
0
200
400
600
800
X-Square
Tom Wilson, Department of Geology and Geography
1000
1200
1400
25
20
40
In the t 2  x 2 analysis (see pages 197 to 199), you’ll
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.
TDMO  t x  t x
2 x sin 

V
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).
Tom Wilson, Department of Geology and Geography
Pitfalls in seismic interpretation
Tom Wilson, Department of Geology and Geography
• 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?
•Hand in Exercises IV-V today.
•Hand in your Mid Term report today.
Tom Wilson, Department of Geology and Geography