Abstract

Seismic data processing is one of the three key stages of 3D seismic surveys for oil & gas exploration i.e. data acquisition, data processing and data interpretation. Without good data processing, it is impossible to get good subsurface image for data interpretation. Pre-stack depth migration (PSDM) is one of the tools to get subsurface image in depth for geologic interpretation. For PSDM processing, interval velocity model building scheme is key to arrive at optimum velocity model and to arrive at global optima, the starting interval velocity model is critical. The paper illustrates an innovative way of obtaining optimum interval velocity model used for PSDM. The velocity model building scheme comprises two steps. Firstly, RMS velocity refinement is attempted by running PSTM twice. Secondly, after a few tomographic iterations, average interval velocity is fixed for all the horizons and new velocity model is created. One more tomographic iteration after target line PSDM and residual moveout analysis provided the best interval velocity model. This velocity model building scheme for PSDM processing has resulted in very good subsurface imaging. The velocity model obtained showed higher confidence. The velocity variation observed within the layer (between Hz-5 and Hz -6) seems to be genuine and caused due fluid influx from below or some localized lithologic changes or it might be a mud volcano. Velocity maps depicting structural features including fault systems are unique outcome of this study.