Page 203 - Acquisition and Processing of Marine Seismic Data

194 3. NOISE IN MARINE SEISMICS

typically have higher velocities, and therefore suppress the guided waves by a simple band-
they arrive at the receivers much earlier than pass filtering.
high frequency components, hence, they exhibit Guided waves can be significantly attenuated
lower dips on the shot records. during stacking, and because they are linear on
Guided waves generally arise at shallow the shot gathers, a τ-p transform (slant stacking,
waters where a hard seafloor exists, and are Section 4.9) can be applied to discriminate and
sometimes very strong at early arrivals of far off- suppress their amplitudes. Fig. 3.25 shows an
set traces lying parallel to the head waves. analysis of guided waves in τ-p domain. On
Fig. 3.23 shows an example shot record with τ-p gathers, guided waves are aligned along
extremely strong guided wave interference a narrow vertical band around the p value
and its ray path. In this case, the noise makes determined by their dip on the time section
the far offsets of early arrivals completely use- (Fig. 3.25B). If this narrow zone is muted out
less. Fig. 3.24 shows an f-k analysis of a shot in the τ-p domain (Fig. 3.25C), guided wave
record contaminated by strong guided wave amplitudes are significantly removed in the time
amplitudes. The f-k panel in Fig. 3.24A suggests domain after an inverse τ-p transform
that the guided waves have a frequency band (Fig. 3.25D). Although τ-p transform, trace mix-
between 20 and 150 Hz, which is clearly in the ing and f-k filters may be effective in suppressing
frequency band of the primary reflections. This the guided waves, sometimes it is not possible to
situation implies that it is not possible to completely remove them from the seismic data

FIG. 3.23 (A) An example shot record with strong guided wave interference (red arrows) in early arrivals of far offsets, and
(B) the ray path of a guided wave.