Reverse Time Migration of offshore wide-angleseismic data: an efficient method to image deep crustal structures

Abstract

We image the Moho discontinuity and deeper crustal layers by applying the Reverse Time Migration (RTM) method to wide-angle seismic (WAS) data that were acquired along two different profiles in the NW and SE offshore Brazil. The application of this method is quite uncommon to ocean bottom seismometers (OBS) data due to the OBS wide spacing deployment and low folds. Also, we applied the method to each OBS without making any assumptions on the type of propagation generated inside the model. The long offset of the refractions obtained, allows us to image deeper crustal layers.Being able to obtain an image of these deeper layer is useful when MCS streamer data are not available or, even when they are, do not image the same depths.We analyze the effectiveness of the RTM method when applied to the reflectivity of the WAS data. We can image the structures by cross-correlating the forward and backward wavefields given by the acoustic seismic equation. This allows us to use each interface crossed by a ray as a source but also as a receiver.The velocity models used to perform RTM were previously obtained by applying a procedure of two-dimensional forward ray-tracing followed by a damped least-squares travel time inversion.The results obtained have an unexpected large contribution from the wavefield traveling as refractions within the earth. We obtain strong and continuous refractors for depths that correspond to the basement and the Moho discontinuity. As we move inland, the refractors that correspond to the Moho discontinuity disappear due to the move of this discontinuity to deeper depths. The obtained results are promising for a wide range of applications at a crustal scale seismic exploration, with wide-angle seismic data.Funding: This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020- IDL