Page 286 - Acquisition and Processing of Marine Seismic Data
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5.6 GAIN RECOVERY 277
FIG. 5.36 (A) Filtered input shot gathers with no gain, and spherical divergence correction results for (B) homogenous,
(C) layered media.
approximate velocities first, and then after the 5.6.2 Time Raised to a Power
P
velocity analysis, the correction is removed and Correction (t )
reapplied with the correct velocity distribution
obtained from a velocity analysis. Fig. 5.36 shows Gain correction based on a power of arrival
example shot gathers with and without a spheri- time of seismic amplitudes is one of the most
cal divergence correction, using correct velocities common gain recovery methods in processing.
for both homogenous and layered subsurfaces. It consists of trace-by-trace multiplication of seis-
Amplitudes of late arrivals become visible mic amplitudes by a gain function, defined as
after removal of the spherical divergence effect P
gtðÞ ¼ t (5.10)
from the seismic data. This type of gain recov-
ery, however, does not contain any correction where t is the recording time of the seismic sam-
for the absorption effect and the amplitudes of ple and P is a constant typically between 0.5 and
high-frequency components consumed by the 4.0. The method does not require velocity infor-
absorption cannot be recovered at all. The most mation on the subsurface, is easy to apply, and
important advantage of spherical divergence also preserves the relative amplitude changes
correction is that it preserves relative ampli- in the seismic data.
P
tudes of the seismic data, termed relative ampli- t is a gain function that decreases the ampli-
tude processing. Therefore, it is preferred for tudes of early arrivals for t < 1.0 s and increases
data collected for oil and gas exploration, where the amplitudes of late arrivals for t > 1.0 s.
P
relative amplitude changes are critically impor- Fig. 5.37 shows t gain functions for different P
tant since they may indicate the subsurface values.Theefficiencyoftimeraisedtoapowercor-
hydrocarbon accumulations. rection is directly proportional to the value of P:
