Page 187 - Acquisition and Processing of Marine Seismic Data

178 3. NOISE IN MARINE SEISMICS

as a strong seabed reflection. This reflection from the seabed, but also the reflected energy
energy then passes through the streamer, reach- from subbottom sedimentsrepeats itself asa com-
ing to the sea surface, and reflected back plete reflection packet. Sometimes the amplitudes
towards the seabed since air-sea surface bound- of multiple reflections are so strong that primary
ary is also a very good reflector. This signal will reflection amplitudes are completely masked.
act as a secondary seismic source, producing its One of the most common multiple reflection
own reflections from seabed and shallow sub- types is peg-leg multiple, originating from a
surface sediments. This reflection process may high amplitude reflection from a strongly reflec-
repeat itself on the data several times depending tive deeper boundary, such as an acoustic base-
on the water depth, source strength and record- ment, and the reflection repeats itself in the
ing length, and all of these reflections are water column (Fig. 3.9). Hence, a phantom inter-
received by the hydrophones and recorded in face that mimics the real reflector occurs, and the
the seismic data. time interval between the two reflections always
The arrival time of the first multiple reflection equals to the water depth, since the peg-leg
from seabed is exactly twice the seabed reflection reflection has an additional travel time from its
arrival time (Fig. 3.7). If the sea bed is inclined, propagation in the water column.
the inclination of multiple reflections increases Multiple reflections propagate mostly in the
on the stack sections (Fig. 3.8). In most cases, the water column and in shallow subsurface
repeating amplitudes are not only the energy sediments; hence their propagation velocity is

FIG. 3.7 Multiple reflection hyperbolas (red arrows) on shot gathers.