Page 294 - Acquisition and Processing of Marine Seismic Data

5.6 GAIN RECOVERY 285

P
P
FIG. 5.46 A comparison of t correction and AGC to represent the relative amplitude preservation by t correction.
P
(A) Stack section with t correction containing several gas chimneys as narrow transparent columns. (B) After AGC correction,
the transparency characteristics of the chimneys are completely destroyed.

higher than the late arrivals, this means that the continental shelves to deep abyssal plains via
gain is too low, or the AGC window is too long steep continental slopes along the continental
(Fig. 5.47C). On the contrary, if the amplitudes margins. Fig. 5.49 shows an example seismic line
over the whole line are absolutely too close to that requires variable P values according to the
P
each other, then the AGC window is too short variable water depth for t correction to keep
(Fig. 5.47D), which may result in loss of resolu- the relative amplitude information. In
tion in the final stack data. Fig. 5.49A, P ¼ 3 is used for whole line; although
In the case of t P correction, values for P this is suitable for deeper parts of the section, it
between 1.5 and 4 produce acceptable results excessively suppresses the amplitudes in the
for most seismic data, yet several tests may be shallow parts. Slightly increasing P values along
required to determine the most suitable P value the downslope solves the problem and produces
for each seismic line. If the amplitudes of the a more regular amplitude distribution along the
early arrivals are low with respect to the late line while preserving the relative amplitudes.
arrivals, then the P value is too high and should In high-resolution seismic surveys like single
be reduced (Fig. 5.48). Since a suitable P value is or multichannel sparker surveys, the recording
a function of recording time, seabed depth and length may be considerably smaller when com-
the total recording time affect the optimal P. Fur- pared to the conventional multichannel surveys,
thermore, a number of different P values may be e.g., between 100 and 300 ms. A relatively small
required in the case of a variable seabed depth AGC window length must be selected in the case
along the seismic line. This is particularly of small recording lengths to recover the deeper
required for those seismic lines from shallow amplitudes properly.