Improvement due to amplitude preservation

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Problem 10.1

Figures 10.1a and 10.1b show the same data except that Figure 10.1a is a “relative amplitude preserved” section plotted at reduced gain, and with a well log inserted. What conclusions can be drawn from Figure 10.1a that are less evident in Figure 10.1b?

Background

In a sand-shale section, deflections to the left on an SP log (see Telford et al., 1990, chapter 11 for a description of well logs) often indicate sand; a gamma-ray log often appears much the same. Sonic logs usually display slowness (or specific transit time, the reciprocal of velocity), with slowness increasing to the left (velocity increasing to the right).

Ideally we would like to have trace amplitudes depend solely upon the reflection coefficients at the various interfaces. Amplitude preservation attempts to achieve this goal by correcting for extraneous effects including spreading, absorption, nonlinear effects of the recording system, and so on. While we cannot fully compensate for all of the factors that affect amplitude because there are always too many unknowns, we try to maintain the same gain along the seismic line so that lateral changes in reflectivity will be visible. This is particularly important in areas where hydrocarbon accumulations produce significant amplitude changes (often increases or “bright spots” (see problem 10.17). In this area of the U.S. Gulf Coast a common display technique was to decrease the system gain so that only high-amplitude reflections stood out clearly.

Solution

Decreasing the gain emphasizes the highest amplitudes and hydrocarbon accumulations where bright-spot conditions exist. On the other hand, the reduced gain makes it much more difficult to see less prominent reflections and this makes structural interpretation (such as evidences of faulting) more difficult. Relative amplitude-preserved (RAP) sections are generally used as a supplement to, rather than replacement for, regular displays.

Figure 10.1a.  Preserved-amplitude display at lowered gain with well logs inserted.

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