We propose a newly-developed depth imaging approach for OBS (Ocean Bottom Seismometer) reflection data in active-source structural surveys using wavefield separation and PSDM (Prestack Depth Migration). OBS data includes a lot of valuable signals, not only reflection but refraction. However its wavefield is contaminated with various kinds of waves that degrade the quality of the depth imaging of OBS reflection. Multiple reflected waves, for example, have been considered as a strong source of noise in the imaging. However, we have found out that the multiples have a wide spread of reflection points and we take advantage of this feature to improve the depth imaging after careful processing of acquired OBS data. In our OBS survey, the spacing of OBS locations is typically 3-5 km. The sparse spacing means that signal-to-noise (S/N) enhancement by signal stacking is not expected. Therefore, we require higher quality data input to PSDM to obtain good depth image. OBS data contains upgoing and downgoing waves because it is located on sea bottom. We investigate the characteristics of OBS wave fields and classify them into four categories which include primary and multiple reflections. Each of them contains similar features in the signal, and this wave field separation 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 wave fields.


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