Seafloor Multiple Reflections in Sub-bottom Profiles

Chesapeake Times, Vol 2 | July 2020

The interpretation of marine seismic profiles is complicated by the fact that operators must separate useful information about subsurface geometry from a wealth of background interference and noise typical of marine surveying. One of these problems is the presence of multiple reflections or reverberations. These slowly decaying wave trains usually arise in the water layer, which tends to trap the sound wave bouncing back and forth between the seabed and the sea surface. The most prominent of these multiples and usually the first to arrive are harmonics associated with the seafloor reflection.

Take for example the ideal situation shown in Figure 1 where we can see the marine trace collected from a seismic source and receiver near the surface. The resulting primary reflections off of the seabed and a single reflecting interface beneath the seabed are drawn. In this case, there are no complicating reverberations to confuse the interpretation.
Figure 1: (left) Cartoon of the primary reflections off of the seabed and a reflecting interface, (right) Example seismic trace resulting from the situation on the left
However, if the seismic source bounces off of the sea surface and back down to the seabed a second time, we would get the situation shown in Figure 2. In this new case, the seismic trace shows both the primary water bottom reflection (in black) and the 2nd order harmonic of the bottom reflection (in red). The harmonic occurs because the sound bounces down to the seafloor twice (hence, 2nd-order) before being recorded at the receiver. Depending on the strength of the source and the seafloor reflection coefficient, many harmonics are possible. In any case, once harmonic reflections appear in the record it becomes difficult to sort out which reflections are primary and which are multiples of reflectors we have already observed.
Figure 2: (left) Cartoon of the primary reflections off of the seabed and a reflecting interface along with a second bottom reflection (in red), (right) Example seismic trace resulting from the situation on the left showing the second-order harmonic of the water bottom reflection.
Figure 3 shows a seismic profile with strong 2nd-order multiples. At the beginning of the trace (far left side), the trace starts showing primary reflections at around 380 ms (blue line). The second order bottom multiple occurs at 2x the primary so we look for the 2nd order multiple of the seafloor at around 760 ms (red line). Below this point, the whole profile begins to repeat itself.
Figure 3: (left) uninterpreted seismic trace. (right) interpreted seismic trace. The primary bottom reflection is shown in blue and the 2nd-order harmonic (multiple) of the bottom trace is shown in red. Note that the 2nd order harmonic is always twice the 2-way travel time of the primary it will change with the water depth.
The exact position of the seafloor multiple depends on the depth of the source and the altitude of the receiver above the seabed. In order to assist interpreters in identifying these important multiples, plans are for SonarWiz 7.07 to include an interactive multiple identification tool. When the tool is active, SonarWiz will identify the location of the bottom reflection and predict the position of all of its harmonic multiples, marking the multiples on the sub bottom display at the appropriate depth. This tool should help to prevent interpretation mistakes.

There are many types of reverberation present in marine seismic traces other than seafloor reflector harmonics. But, because it is usually the first to arrive, the first harmonic of the seabed reflector is an important marker in the seismic trace. Once this first multiple has arrived, interpretation below this point in the profile should proceed with caution because other reverberation artifacts are likely to be present in addition to primary features.
-David Finlayson, Chief Scientist

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