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Seismic surveys are often constrained by the time needed to activate all the required sources so the source signals do not interfere with each other. Simultaneous source seismic acquisition, also referred to as blended acquisition, is an effective method for reducing the cost and improving the quality of seismic surveys by eliminating the requirement that the sources do not interfere with each other. Independent simultaneous shooting is a unique form of blended acquisition in which sources operate independently of each other and the receiver recording is continuous. This acquisition method is particularly efficient and robust in obtaining high-density source grids for land and marine surveys. Processing the simultaneous source data depends on the randomness of the shot times to create a situation where the signal is coherent, and the interference is random in common-receiver gathers. Although the older and simpler method of separating interfering shots with random noise attenuation works well when the residual interference noise left by the random noise attenuation is acceptable, higher separation quality is possible using a shot separation process based on sparse inversion and compressive sensing methodology. We found that the resulting simultaneous source surveys produced images that were equivalent to or better than conventional seismic surveys, while requiring less acquisition effort, thus reducing costs.

Trinidad and Azerbaijan offshore areas are strongly affected by shallow gas anomalies which greatly attenuate seismic signals. Building velocity models in such areas with shallow water depths and gas can be a difficult task. Here we present two alternative ways to build reliable velocity models in the presence of shallow gas; one that is suitable to very shallow (<100m) and poor data quality areas and the other for deeper water depths. In the first instance, we make use of Diving-Wave refraction tomography method to build shallow velocity models offshore Trinidad and Azerbaijan. Previous use of this method has been limited to processing seismic data to produce a shallow velocity model to determine static corrections in time processing. Our success is in using the velocity model derived from Diving-Wave tomography as a starting model for reflection tomography in depth processing. We show that Diving-Wave method is a robust technique that produces reliable near surface models in the presence of gas and in areas with low signal to noise ratio. In the second case, we show that where data has reasonable offset to work with, reflection tomography can produce fairly accurate and high fidelity velocity models that can be further improved with iterative migration velocity analysis. As a result, depending on available data quality, either Diving-Wave derived shallow velocity model or reflection tomography derived model can be used to improve the ultimate product from iterative pre-stack depth migration and reflection tomography.

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