In deep water Gulf of Mexico (GOM), defining accurate salt geometry is essential for imaging geological structures beneath complex salt bodies. Reverse time migration (RTM) handles complex wave propagations in any direction without dip limitation, and is now the standard imaging algorithm in salt model building. With increasing computing capacity and integrated imaging and interpretation, our ability to use RTM imaging as an interactive tool for delineating salt geometry has been significantly improved. The conventional RTM salt model building is a top-down approach which consists of various steps of migration and interpretation: sediment flood RTM and picking the Top of Salt (TOS), salt flood RTM and picking the Base of Salt (BOS), and overhang RTM and interpretation as needed. However, in the areas with complex salt geometry, such as narrow mini-basin with surrounding salt bodies, the conventional top-down salt building approach may not be conducive to correct salt body interpretation and can lead to poor images beneath the salt. In this paper, we demonstrate the application of RTMbased prismatic wave imaging in complex salt model building, in particular the introduction of dual flood RTM to evaluate the prism waves and correctly interpret the salt horizon. A 2D synthetic case study is utilized to demonstrate the prismatic wave work flow. The technique is then applied to a marine wide azimuth dataset in Walker Ridge, Gulf of Mexico, in order to improve subsalt imaging in the complex structural setting.
Conventional Kirchhoff migration can be made much faster by a priori calculation of wave arrival directions, and tracing rays back only along the measured directions; this is known as parsimonious migration. For surface seismic data, where acquisition with 2D arrays is common, the wave arrival directions can be measured by using local slant stacking to calculate the two horizontal slowness components. The vertical slowness is then deduced by using the dispersion relation. For vertical seismic profile ͑VSP͒ data, where multiline acquisition is not possible, only the vertical component of the slowness vector can be measured, which is insufficient to constrain the wave arrival direction. We overcome this limitation for three-component VSP data by polarization analysis of the incident waves. Polar and azimuth angles are calculated at the receivers by estimating the data covariance matrix and the vertical slowness, assuming isotropic media and linear polarization of both P-and S-waves. For P-waves, the data covariance matrix defines the polarization ellipsoid, which is then used to calculate the wave arrival direction. For S-waves, an additional step is required because the polarization is orthogonal to the propagation direction. Ray tracing along only the measured propagation directions eliminates the traditional traveltime table calculation, and enhances the efficiency of Kirchhoff migration of three-component VSP data. Distortions of the polarization caused by factors such as anisotropy are assumed to be corrected independently of the migration process described here. Synthetic examples for a flat reflector and for a salt flank model demonstrate the procedure and the image quality.
The top-down method for salt interpretation starts with the search for salt-sediment interfaces, which begins at the shallowest depths and progressively moves deeper. This search is typically conducted on intermediate seismic products such as sediment flood, salt flood volumes, overhang sediment flood, and overhang salt flood depth migrations. However, with this approach, poorly imaged subsalt areas become known only after spending considerable time interpreting intermediate salt features. We evaluated a new method for streamlining this traditional approach to salt-model building wherein a reference salt geometry was obtained earlier in the salt interpretation process. Having a reference seismic volume helped to identify poorly imaged subsalt targets much sooner. A hybrid of model-based and data-based interpretations was then performed for the poorly imaged subsalt areas, whereby interpretation was completed by proposing geologically viable models that were continuously verified by their impact on geophysical (seismic) data. In poorly imaged areas, we looked bottom-up, rather than top-down, to establish a salt geometry that best fit the geologic model as well as the geophysical data. Our hybrid target-oriented approach is useful for not only reducing the interpretation cycle time but also for improving images beneath the salt.
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