Estimating lithology from stacking velocity

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Problem 5.18a

A velocity analysis at SP 100 of Figure 5.18a yields the plot shown in Figure 5.18b. Pick stacking velocity versus time pairs and calculate interval velocities.

Figure 5.18a.  Unmigrated seismic section (courtesy Grant Geophysical).

Solution

Velocity-time pairs are listed in Table 5.18a. Depth and interval velocities . In Table 5.18a, values are for the intervals above the reflection picks.

Table 5.17a. Time-velocity pairs.
(s) (m/s) (m) (m/s)
0.22 2450 270 2450
0.38 2650 495 2800
0.58 2800 800 3050
0.72 2750 990 2715
1.12 3150 1765 3875
1.35 3600 2450 4890
1.58 4100 3280 7215
2.22 4550 5050 5530
Figure 5.18b.  A velocity analysis.

The quality of Figure 5.18b does not permit accurate picking, but the NMO correction is fairly tolerant of errors. We did not tabulate time-velocity pairs for the events at (0.97, 2600), (1.48, 3000), (1.79, 3050), and (1.90, 3000), because we thought these events were multiples. Interpreters usually ignore stacking velocity values lower than those at shallower depths. Structure, faulting, and other features can distort velocity analyses.

Problem 5.18b

What can you tell about the lithology from this?

Solution

The interval velocity values are plotted on Figure 5.18c with a smooth curve for Tertiary clastics plotted as a reference. The interval-velocity curve is everywhere above the reference, and above 1 km it is roughly parallel to but higher than the reference. The section above 1.8 km is probably mainly clastics whose velocity is increased, perhaps by age, cementation, uplift, or the presence of carbonates. The higher velocities below 1.8 km suggest carbonates. The velocity from 2.5 to 3.3 km is unreasonably high. While overlying reflections are mostly strong and continuous, data quality deteriorates here.

Problem 5.18c

If the section that is present in the syncline but is missing over the anticline consists of poorly consolidated rocks, what values would you expect for a velocity analysis at SP 45?

Figure 5.18c.  Interval velocity versus depth.

Solution

Poorly consolidated rocks will have lower velocities than at SP 100 and this will lower the measured stacking velocities. Interval velocities between the same reflections, which will be deeper at SP 45, will probably be slightly higher than for the same intervals at SP 100.

Problem 5.18d

Note the downdip thinning of the section to the left of SP 100 between 0.75 to 1.25 s. Suggest an explanation.

Solution

If the interval thickness remains constant, traveltime through it will decrease with increasing depth because the rocks have higher velocities as they are buried deeper and thus are under greater pressure.

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