Firstarrival Tomography of Seismic OBS Data and Prestack Depth Migration of MCS Data from the Sumatra Continental Margin

Prestack depth migration is applied to multi-channel seismic streamer data acquired by RV Sonne cruise 186 near Simeulue island offshore Sumatra. The sea floor depth varies between 50 m near Simeulue and 5 km in the oceanic trench. The sediment is up to 4 km thick in the trench. The oceanic crust is imaged over a distance of 90 km landwards from the trench. It is subducting at an angle of 5-6 degrees on average. The P wave velocities of the sedimentary layers are determined by focusing analysis. These velocities are used to construct an initial model for seismic first arrival tomography, which is applied to wide-angle Ocean Bottom Hydrophone and Seismometer data recorded along the same profile as the MCS data. The inversion of more than 9000 traveltimes from 24 stations with source-receiver offsets smaller than 70 km was performed on a 1000 x 100 grid, which covers 250 km along the profile and 30 km in depth. The inversion converged after 14 iterations with decreasing regularization parameter to a root-mean-square traveltime residuum of 0.060 s, which is of the size of the experimental error. The velocity model shows high P-wave velocities of 6 km/s in the northeast, where the crust is of continental origin. The obtained 2D velocity model is used as input for the prestack depth migration. Seismic tomography was also applied to two addtional OBH/OBS experiments in the same area. The P-wave velocity profiles show a velocity increase of 0.7/s for the first 8 km depth below sea floor with a sea floor depth of 2 km. Detailed bathymetric maps of the survey areas are presented.