Page 307 - Acquisition and Processing of Marine Seismic Data

298 5. PREPROCESSING

around the mound (Fig. 5.61C) because of the defined instead of using a unit step function in
sparse picking of the top mute polygons on multiplication during the muting process
the shot gathers. (Fig. 5.62B). This ramp zone provides a slight
The only parameter that must be selected transition from zero to the maximum amplitude
before muting is the mute ramp value, in milli- of the trace, to prevent the Gibbs effect in the
seconds. During the mute process, undesired forthcoming frequency domain operations.
portions of the trace(s) are simply multiplied The amplitudes in the ramp zone are not
by zero, whereas the areas of genuine reflections directly transferred into the mute output; they
are multiplied by 1.0, which means that the seis- are multiplied by a scaling factor gradually
mic data is actually multiplied by a unit step changing from 0 to 1. This makes the edge of
function on a trace-by-trace basis, as indicated the mute zone not as sharp, but constitutes a
in Fig. 5.62A. This multiplication zeroes out smooth transition zone. The time length of this
the undesired part of the data and does not affect ramp zone is important for an effective muting,
the signal zone. However, the sharp transitions especially for the top mute process, because the
in the time domain signal, such as the edges of amplitudes within the ramp zone are slightly
a box-car function, create the Gibbs effect in suppressed. In the case of top mute applications,
the frequency domain, as explained in these amplitudes correspond to seabed reflec-
Section 5.5.1 in detail. In the case where a fre- tions. Fig. 5.63 shows several example stack sec-
quency domain operation is performed on these tions before and after a top mute process using
trace(s) later on after muting, a ramp zone is different ramp zones to demonstrate the effect

FIG. 5.62 Schematic explanation of top muting on a seismic trace. (A) Each seismic trace is basically multiplied by a unit
step function (indicated by the red line) in a trace-by-trace basis for top muting. (B) In practice, a transition or ramp zone is
defined to multiply with the seismic data instead of using a unit step function to avoid the Gibbs effect in subsequent fre-
quency domain applications.