Fault interpretation using time slices

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Problem 12.9a

The series of time slices shown in Figure 12.9a may be faulted. Where are possible faults and what can you infer about them? Assume a velocity of 3000 m/s.

Figure 12.9a.  Sequence of time slices (after Brown, 1983).
Figure 12.9b.  Interpretation of Figure 12.9a showing postulated fault. The contour map (viii) is made by tracing the inner black area in (i) and its changes on (ii) to (vii).

Background

Time slices show the strike of features. Faults are suggested where reflections terminate systematically. A horizon slice that cuts through the 3-D volume along a picked horizon is often the best way to see stratigraphic features.

Solution

Assuming that the closure is due to an anticline, the only offset of the pattern that might suggest faulting is seen on slices (iii) to (v) in Figure 12.9a. This suggests an east-west down-to-the-south fault dipping to the north, i.e., a reverse fault, as suggested in Figure 12.9b. However, as drawn, the fault dip is only slightly greater than the bedding dip, which is geologically unreasonable. An alternative and more probable explanation is that it represents a re-entrant (valley) cutting into the structure roughly along the same alignment. A horizon slice should be created to corroborate a channel interpretation, since channels show up best on horizon slices.

Problem 12.9b

Locate possible faults on Figure 12.9c.

Solution

The very obvious offset of the contours (A on Figure 12.9d) almost certainly indicates a fault. The abrupt change of dip () probably indicates another fault. The location of these features becomes less clear toward the southern edge of this time slice. Study of other time slices or vertical sections would probably clarify the matter.

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