Page 454 - Acquisition and Processing of Marine Seismic Data

9.3 VELOCITY ANALYSIS IN PRACTICE 445

FIG. 9.18 Effect of fold number on the semblance plots on a real marine supergather. Semblance plots calculated from
(A) 126-, (B) 60-, (C) 40-, and (D) 27-fold CDPs.

smoother and smoother, and the details are lost. absolute amplitudes of a full cycle of the seismic
For the example semblances in Fig. 9.21, a time wavelet. During selection, it must be taken into
gate of 30 ms seems to be suitable. In practice, consideration that the dominant period is time
the time length of the gate must be determined dependent, that is, it is small for early arrivals
as close as the dominant period of the input but large for deep reflections, because of the
dataset (generally 10–40 ms) to include all the absorption effects.