We propose a depth imaging approach for OBS (Ocean Bottom Seismometer) reflection data in active-source structural surveys using wavefield separation and PSDM (Prestack Depth Migration). Wide-angle and long-offset OBS data includes a lot of valuable signals, not only reflection but refraction. The data will contain upgoing and downgoing waves because it is recorded on sea bottom. Therefore, even considering just reflections, the wavefields is contaminated with various modes of waves that degrade the quality of the depth image. In our OBS survey, the spacing of OBS locations is 3-5 km and typical water depth is more than 2km. The sparse spacing and ultra deep sea water means that signal-to-noise (S/N) ratio enhancement by signal stacking is not expected. We investigate the characteristics of OBS wavefield and separate it into four categories which include primary and multiple reflections. Each of them has similar features in the signal, and this approach can enhance the S/N ratio. Applying adequate PSDM to the four separated wavefields, we obtain the four depth images without contamination of the different type of wavefields. We apply the proposed approach to field data and demonstrate its effectiveness to enhance the final depth image quality with high reliability.


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