Page 42 - Acquisition and Processing of Marine Seismic Data

1.3 FUNDAMENTALS OF MARINE SEISMICS 33

FIG. 1.22 Theoretical reflection hyperbolas obtained from (A) a single horizontal reflector model and (B) a single inclined
reflector model. If the interface is horizontal, the recorded hyperbolas are symmetrical about the shot location, whereas it is
asymmetrical in the case of an inclined interface and zero-offset time moves in an updip direction.

require NMO times as high as possible to work reflection events in the prestack data. In some
properly. cases in marine seismics, however, reflections
The NMO time of a specific reflection hyper- do not display hyperbolic moveout, especially
bola increases with offset distance. Since long in areas of rapid morphological changes.
offsets provide higher NMO times for the reflec- Fig. 1.24 shows a migrated section through a
tion hyperbolas, longer streamer cables are pre- steeply dipping continental slope crosscutting
ferred during the acquisition of marine seismic several morphological structures of submarine
data. Maximum NMO time is also larger for canyons in different scales with steeply dipping
shallow reflections, and it decreases with walls, which constitute morphological varia-
increasing reflector depth (Fig. 1.23A). In addi- tions both on the seafloor and within the subbot-
tion, NMO time decreases as the propagation tom sediments. Example shot gathers from
velocity increases (Fig. 1.23B). In the real world, different locations along the seismic line are also
the velocity of the subbottom sediments com- provided in Fig. 1.24 to analyze the effect of mor-
monly increases with increasing reflector depth, phological variations on the shape of the reflec-
and therefore, as a combined effect of reflector tion hyperbolas. Especially in areas where the
depth and velocity, we can expect an overall rough seafloor exists, reflection hyperbolas on
decrease in the maximum NMO time for deeper the shot gathers are distorted due to the multiva-
reflection hyperbolas (Fig. 1.23C). These conse- lued reflections from steep walls of the subma-
quences restrict the quality of the velocity field rine channels. In these areas, velocity analysis
obtained from seismic data by velocity analysis, does not provide accurate results and the quality
which is discussed in Chapter 9. of the stacking output is degraded since it is
In seismic data processing, some crucial impossible to flatten such reflection hyperbolas
processing steps, such as NMO correction or with an imperfect moveout during NMO
velocity analysis, assume perfectly hyperbolic correction.