Recognition of a reef

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Problem

What kind of feature shows in Figure 10.11a about 75% of the way across from the left end of the section at about 2.5 s? What characteristics help to identify it?

Background

Reefs grow under proper conditions of water temperature, depth, and water clarity. They consist of corals or other marine animals with calcareous parts. Reef growth stops when conditions are no longer suitable, especially when not enough sunlight reaches them. Generally reefs grow sufficiently rapidly that they can keep up with subsidence, but sediment suspended in the water or temperatures that are too low kill them.

Figure 10.11a.  Portion of zero-phase seismic section (courtesy of Grant Geophysical).
Figure 10.11b.  Criteria for reef identification.

Figure 10.11b illustrates some of the criteria useful in recognizing reefs on a seismic section. The top of a reef may have a strong velocity contrast with the overlying sediments and hence produce a strong reflection (a). The reef itself is usually devoid of reflections (b,d). There may be diffractions where surrounding beds are truncated at the reef (c). Back-reef and fore-reef deposition are often distinctly different (e). Differential compaction frequently produces drape over a reef (f). Differences between velocity in the reef and surrounding sediments often produces velocity anomalies below the reef, usually a pull-up (g), but occasionally a push-down (h). The venue of a reef is often a structural high (j), a shelf-edge, a previous reef, or a hinge-line (i).

Figure 10.11c.  Location of the reef.

Solution

This section appears to be nearly zero-phase as many reflections are indicated by single strong peaks that are probably caused by impedance contrasts.

This feature has a number of the characteristics that we associate with reefs. It is located over a hinge line, as can be seen by the change of slope of the underlying reflections; a straight edge can be laid on the reflections to emphasize the change of dip. There is drape over the shaded feature and onlap onto the drape. The reflection pattern also changes on opposite sides of the feature. While there is no obvious velocity anomaly here and the feature in Figure 10.11c is poorly outlined, identifying it as a possible reef seems reasonable. The leftward extension of the moderately strong reflection that reaches the right side of the section just above 2.8 s may mark the base of the reef.

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