Adaptive decomposition of multicomponent ocean-bottom seismic data into downgoing and upgoing P- and S-waves

摘要

With wavelield decomposition, the recorded wavefield at a certain depth level can be separated into upgoing and downgoing wavefields as well as into P- and S-waves. The medium parameters at the considered depth level (e.g., just below the ocean-bottom) need to be known in order to be able to do a decomposition. In general, these parameters are unknown and, in addition, measurement-related issues, such as geophone coupling and crosstalk between the different components, need to be dealt with. In order to apply decomposition to field data, an adaptive five-stage decomposition scheme was developed in which these issues are addressed. In this study, the adaptive decomposition scheme is tested on a data example with a relatively shallow water depth (～120m), consisting recordings from of a full line of ocean-bottom receivers. Although some of the individual stages in the decomposition scheme are more difficult to apply because of stronger interference between events compared to data acquired over deeper water, the end result is satisfying. Also, a good decomposition result is obtained for the S-waves. The extension of the decomposition scheme to a complete line of ocean-bottom cable data consists of a repeated application of the procedure for each receiver. The resulting decomposed upgoing P- and S-wavefields are processed, yielding poststack time migrated images of the subsurface. Comparison with the images obtained from the original (i.e., not decomposed) measurements shows that wavefield decomposition just below the ocean bottom leads to a strong attenuation of multiply reflected events at the sea surface and better event definition in both P- and S-wave sections. Other decomposition effects like improved angle-dependent amplitudes cannot be evaluated in this way.