Page 397 - Acquisition and Processing of Marine Seismic Data

388 7. SUPPRESSION OF MULTIPLE REFLECTIONS

FIG. 7.22 (A) An NMO corrected CDP gather, (B) its τ-p domain representation after Radon transform, (C) muting out of
the primary reflection amplitudes in the τ-p domain to keep the multiple amplitudes, and (D) output CDP gather consisting of
only multiple reflections after inverse Radon transform.

preventing us from picking the velocities of pri- 7.5 WAVE EQUATION MULTIPLE
mary reflections for late arrivals, where the REJECTION
amplitudes of the primaries are all hidden
behind the multiples. In Fig. 7.25B, however, Generation of multiple reflections in the
there are no contours and enclosures connected water column can be approached by wave field
to the multiple energy on the semblance: cor- modeling. Using seafloor topography and
rectly picking the primary velocities is possible acoustic wave equation modeling, it is actually
after a Radon velocity filter. possible to estimate all multiples via surface
In practice, a first-pass velocity analysis is seismic data. The method is known as wave
performed before the Radon velocity filter to equation multiple rejection (WEMR) and was
use during the Radon transform. After applica- first proposed by Wiggins (1988), who sug-
tion of a Radon filter in the τ-p domain, a gested that once the long period multiples are
second-pass velocity analysis is done to accu- modeled, then they can be subtracted from the
rately pick the velocities of the deeper pri- input seismic data by an adaptive subtraction.
maries buried under the dominant multiple Short period multiples cannot be eliminated by
amplitudes. WEMR, but their amplitudes are reduced.