Environmental and Exploration Geophysics II

Static Anomalies Velocity Pitfall/Interpretation

tom.h.wilson

[email protected]

Department of Geology and Geography West Virginia University Morgantown, WV

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography Due Wednesday, April 7 th Due Monday, April 12 th Due Wednesday, April 14 th

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Surface elevation Datum elevation is above surface fold Stacking Velocities

Tom Wilson, Department of Geology and Geography

Velocity Pitfall Problem/Interpretation: handed out in class last week C B A A’ B’ C’ Tom Wilson, Department of Geology and Geography

Homework assignment (see handout)

Seismic section (Figure 1 of your handout) showing the arrival times of three reflection events. C B A A’ B’ C’ Think of these records as normal incidence seismograms Tom Wilson, Department of Geology and Geography

In the travel time plot, the reflectors are offset by an almost vertical normal fault. Reflectors A, B, and C lie on the high side, and A’, B’, and C’ lie on the low, or downthrown, side of the fault at right. Note that the interval transit times, A to B and B to C on the upthrown block are greater than the interval transit times A' to B' and B' to C' on the down-thrown block. C B A A’ B’ C’ Tom Wilson, Department of Geology and Geography

With some Noise Note that the fault plane is drawn through the diffraction apexes. Tom Wilson, Department of Geology and Geography

Recall that normal incidence records are the result of the stacking process. Each record from any point on this line will be a sum of records (after NMO correction) all of which shared a common midpoint but had different source receiver offsets. A normal incidence or coincident source-receiver section is also often referred to as a stack section.

s e c o n d s Tom Wilson, Department of Geology and Geography

Stack sections are compiled from a summation of the traces in a CMP gather after the reflection arrival times have been corrected for moveout. The section below corresponds to the CMP gather located at 2500 feet along the profile (Figure 2) & the arrival times have not been corrected for moveout.

A B C Tom Wilson, Department of Geology and Geography Figure 2 of problem handout

The CMP Gather B C 2500’ A Figure 2 of problem handout Tom Wilson, Department of Geology and Geography A’ B’ C’

2,500 feet is the location of the midpoint for the traces in the gather. The first trace at 2,750 feet lies 500 feet from the source. The second trace lies at 2,950 feet.

2950’ Tom Wilson, Department of Geology and Geography Figure 2 of problem handout

Since the midpoint at 2,500 feet is shared by all traces in the gather, the shots must be at equal distances to the left of the midpoint. Where would the corresponding shot point be located?

’ 2950 Tom Wilson, Department of Geology and Geography Figure 2 of problem handout

What is the source receiver offset for the record located at 2950’?

’ 2950 Tom Wilson, Department of Geology and Geography Figure 2 of problem handout

The offset of the first receiver is 500’, the second 900’ the third at 1300’ and so on, increasing by 400’ from one receiver to the next one out. 500’ 4100’ Tom Wilson, Department of Geology and Geography Figure 3 of problem handout

The same relationships hold for the CMP gather shown in Figure 3. The source-receiver offset for the first trace in the gather is 500’, 900’ for the second and so on.

500’ 4100’ Tom Wilson, Department of Geology and Geography Figure 3 of problem handout A’ B’ C’

Develop a table for each CMP gather measure and tabulate the arrival times at each offset.

X 500 900 1300 1700 2100 2500 2900 3300 3700 4100 t1 t2 t3 Tom Wilson, Department of Geology and Geography

Square the source-receiver offset distances and arrival times.

Fit regression lines to the resulting x individual V NMO 2 -t 2 data points and determine velocities. Keep in mind that these are NMO velocities. You will use them as RMS velocities to estimate the interval velocities of each layer, but keep in mind the distinction between the V NMO and V RMS . The V RMS is a theoretical value that we approximate using the V NMO velocity. t1 2 t2 2 t3 2 X 2 500 2 900 2 1300 2 1700 2 2100 2 2500 2 2900 2 3300 2 3700 2 1.681 x 10 7 Tom Wilson, Department of Geology and Geography

• Compute V int s for the three intervals on the high side of the fault (A, AB, and BC) and the low side (A’, A’B’ and B’C’) • Compute the thickness of each interval (high side and low side).

• Discuss your results in a geologic context. A B C A’ B’ C’ Tom Wilson, Department of Geology and Geography

Potential pitfalls in Time Slice Interpretation Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography Deep sea fan? Turbidite?

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

View of channeled horizon surface

Tom Wilson, Department of Geology and Geography

Flattened volume with horizon slice on base

Tom Wilson, Department of Geology and Geography

View of channeled horizon surface

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Scour during a low-stand?

Tom Wilson, Department of Geology and Geography

Generate flattened volume & horizon slice Middle zone

Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

Structure on mid zone

Due Wednesday, April 7 th Be sure and bring questions to class about problem 4.14 and the velocity analysis problem this Wednesday!

Due Monday, April 12 th Due Wednesday, April 14 th Tom Wilson, Department of Geology and Geography