Page 290 - Acquisition and Processing of Marine Seismic Data

5.6 GAIN RECOVERY 281

FIG. 5.41 AGC gain corrections for different AGC scalar calculation methods on a marine shot gather. (A) Filtered shot
with no gain, and its AGC corrected versions using 500 ms operator length calculated by using (B) mean, (C) RMS, and
(D) median values of the amplitudes samples within the AGC window.

Fig. 5.43 illustrates the effect of AGC operator termed the shadow zone. In the case of using
length on the efficiency of the application. AGC AGC scalar applications on the central samples
operation is much more effective for smaller of the AGC window, there will be a low ampli-
operator lengths, and the amplitudes of both in tude “quiet” zone just before the seabed reflec-
shallow and deeper parts of the data become tion on the shot gathers, the length of which
much more balanced. AGC application becomes exactly equals half of the AGC operator length
progressively ineffective as the operator length (Δt/2) in the case of the application to the central
increases. In practice, the suitable operator length samples of the AGC window. If a smaller AGC
is selected by considering the recording length of operator is used, this quiet zone will be narrow.
the input data. Operator lengths between 250 and Above this zone is the shadow zone in the water
1000 ms generally produce acceptable results; column on the shot gathers. This quiet zone is
however, depending on the record length, the not only observed on shot gathers, but also evi-
most suitable operator length can be determined dent on the final stack sections if an AGC is
by trying several different values and visually applied after stacking (Fig. 5.44A). When there
checking the results after each trial. is a strong reflection on the stack data, there
AGC blows up the noise in the water column may be a low amplitude band of Δt/2 millisec-
and produces a relatively high-amplitude noise onds width just below the high amplitude reflec-
zone just above the seabed (Fig. 5.43), which is tion after AGC (Fig. 5.44B).