Using the OBS wide-angle reflection/refraction velocities to perform a pre-stack depth migration image of the “single bubble” multichannel seismic: example of the Moroccan margin

Abstract

The SISMAR marine seismic survey acquired over 3600 km of deep seismic reflection profiles on the Atlantic margin of Morocco, one of the oldest on earth. We present here a crustal scale cross section of the Moroccan margin off El Jadida. To penetrate below the salt layer, a nonconventional, low frequency seismic source was used in the “single bubble” mode (4805 in. 3), and recorded by Ocean Bottom Instruments (hydrophones and seismometers) and a 4.5-km-long streamer that recorded multichannel seismic (MCS). This profile was also reshot with a conventional reflection seismic source (2369 in. 3), in order to evaluate the differences with the single bubble source image. In this study, the single bubble mode source was the best source to image deep structures due to the strong energy produced in low frequency band. In the second part of this study, two velocity models were constructed in order to perform a pre-stack depth migration of the MCS section. The first velocity model was obtained by a depth-focussing error analysis of the MCS data and the second from modelling of the Ocean Botton Seismometer data located along the profile. A comparison of these two velocity models shows 10% higher mean P-wave velocities for the OBS velocity model, probably due to anisotropy. After reducing the OBS velocities by 10%, we show that there are no significant differences between the pre-stack depth migration images obtained by using MCS velocity analysis and the OBS velocity model. Using OBSs data is another way to provide a velocity model to perform pre-stack depth migration of MCS data.