Interpretation of a depth-migrated section

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Problem

What features can be seen in Figure 10.16a?

Background

Problem 10.14 showed the lateral shift of a diffraction curve because of velocity changes in the horizontal direction. Depth migration is a way to remedy this; it involves raytracing through a velocity model that incorporates the horizontal velocity changes. Figure 10.16b shows the velocity model and tracing of image rays (see problem 10.14) for this section. The bending of image rays corrects for the horizontal errors of placement because of the horizontal velocity changes.

Note that depth migration differs from time-to-depth conversion, which does not correct for horizontal velocity changes.

Figure 10.16a.  Depth-migrated seismic section (from Hatton et al., 1981).
Figure 10.16b.  Tracing of image rays through the velocity model.
Figure 10.16c.  Location of features discussed in the text.

Solution

Figures 10.16a,c have about a 2:1 vertical exaggeration and “timing lines” occur at 150-m intervals. Several unconformities can be seen, some showing angularities below and some above them. The most prominent geologic feature is the angular unconformity (Figure 10.16c), which is a strong reflection to the right of 3 km, but it appears weaker and changes character left of 3 km where the depth model (Figure 10.16b) does not show a velocity contrast. Other unconformities can be seen below . Because truncates reflections below it so sharply, we infer that it is erosional, although generally it is not obvious whether unconformities are errosional or nondepositional.

Note the more-or-less uniform leftward thickening of (the unit above ), the thickening not seeming to be related to the sharp folding between 600 m and 2300 m except for the lowest portion of which shows the folding in highly attenuated form. Hence, fold mainly occurred before , but some folding continued into the lower portion of .

is another unconformity; the velocity difference at to the left of 4.3 km is 270 m/s where is present and 630 m/s where is absent. The small portion of above the right-hand syncline bears no obvious correlation with the main body of , so we do not know how they may be related. We see onlap onto and truncation below . and merge at 3.2 km; may have been eroded off to the right of 3.2 km. The pieces of reflection labeled are multiples of the sea-floor reflection, as is evident on a time section but not obvious on the depth section.

The strong event seems rather strange; it cannot be a multiple. It appears to cut across the bedding, especially from 3.0 to 5.0 km, and the folding around 5.2 and 6.6 km is more intense, both above it and below it. It truncates reflections below it, and reflections above it appear to downlap onto it, so it seems to be an unconformity, but the higher intensity of folding above it seems very odd. It might be an out-of-the-plane reflection or perhaps a fault.

There may be a reverse fault at 5.0 km at about 1.2 km depth. The strong reflection also has some of the same problems as and, in addition, it appears to be downthrown to the right at , whereas other events seem to be downthrown to the left. The velocity model has another reverse fault at . The strong event generally parallels , but not exactly, as the section between and thickens and thins.

Thus we see a number of problems with interpreting this section. We would like to do palinspastic reconstruction, that is, flattening successive reflections, as an aid to understanding it. While we do not have the data to do this, we suspect that doing so would show up more inconsistencies and not resolve all of the problems cited above.

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