Page 275 - Acquisition and Processing of Marine Seismic Data

266 5. PREPROCESSING

FIG. 5.24 An example stack section and amplitude spectra of its different parts from shallow to deep zones (from A to D),
illustrating the dramatic decrease in the frequency content in the late arrivals.

change along the time axis of the seismic sec- in deeper parts of the seismic section. The aver-
tions. TVF is generally applied to stack sections age amplitude spectra for each window are also
which are first divided into different time win- provided. The frequency bandwidths of the
dows, and then band-pass filters with different input section in Fig. 5.25A are approximately
cut-off frequency values are applied to each time 6–200 Hz, 4–80 Hz, and 4–40 Hz for windows
window. Normally, wider pass-bands are used A, B and C, respectively. Applying a TVF using
for the shallow zones while the high-frequency these frequency bands for three time windows
cut-off value is gradually decreased for deeper removes most of the high-frequency random
windows. noise amplitudes for the deeper parts, and hence
Fig. 5.25 shows an application of TVF to a the low amplitude deeper reflections become
stack section using three time windows, indi- much more visible (Fig. 5.25B). Time windows
cated by A, B and C in Fig. 5.25A. Since the must be determined by considering the domi-
frequency band decreases at late arrivals, high- nant frequency characteristics and the structural
frequency random noise becomes more distinct style of the geological formations, as in