Programmed gain control
Various types of gain criteria are used in practice. Based on a desired criterion, a gain function g(t) is derived from the data and multiplied with trace amplitudes at each time sample. This is illustrated in Figure 1.4-8. The gain function is specified or estimated at the time samples indicated by the dots and interpolated between these samples.
Programmed gain control (PGC) is the simplest type of gain. Referring to part of a stacked section in Figure 1.4-9, a gain function can be defined by interpolating between some scalar values specified at particular time samples. Larger scalar values naturally would be assigned at late times. In Figure 1.4-9, the applied PGC factors are indicated by the pairs of numbers corresponding to 0 and 6 s. While the input panel indicates strong amplitudes at shallow times, the section scaled by the PGC factors (1,4) indicates a balanced amplitude level from top to bottom.
Figure 1.4-8 Gain is a time-variant scaling defined by a function, g(t). Based on some criteria, this function is defined at the time samples (shown by solid circles) that are usually at the center of specified time gates along the trace as indicated by 1, 2, 3, and 4. Gain application simply involves multiplying g(t) by the input trace amplitudes.
Figure 1.4-9 A portion of a CMP stack before and after application of two different PGC functions. The scale factors used in constructing the gain functions are indicated by the pairs of numbers on top of the gained sections; the first scalar in the pair corresponds to time t = 0, and the second scalar corresponds to time t = 6 s.
Rather than picking the scalars in a qualitative manner, the envelope of the ungained trace can be computed and smoothed. The envelope, which is the curve drawn by smoothly connecting the adjacent peaks (or troughs) along the trace, is a reliable attribute that describes amplitude decay rate. The PGC function then is the inverse of the trace envelope. A single PGC function is applied to all traces in a gather or stacked section to preserve the relative amplitude variations in the lateral direction.