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PS (P-wave downgoing, S-wave upgoing) reflections in DAS VSP: deepwater field, Gulf of Mexico
Cherayil, Joseph J.
Cherayil, Joseph J.
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2024
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Many geophysical applications, including downhole deployments for vertical seismic profiling (VSP), use distributed acoustic sensing (DAS), a cost-effective and non-intrusive technology. This study uses DAS VSP data from an injection well in a deepwater field in the Gulf of Mexico. The objective includes 3D pre-stack Kirchhoff depth migration for both PP (P-wave downgoing, P-wave upgoing) and PS (P-wave downgoing, S-wave upgoing) reflections followed by joint PP/PS inversion to separate pressure and saturation changes during injection. However, linear noise contaminated many shots. Only the shots without linear noise in the updip part of the shot carpet at far offsets displayed PS reflections. Differing dips allowed us to separate the PP reflections from the linear noise. However, the linear noise overlapped with the PS reflections in dip, frequency, and amplitude—making PS reflection migration infeasible. The scarcity of PS reflections in the shot carpet, coupled with their diminished amplitudes, further hindered PS migration. The shot carpet displayed PP and PS reflections updip, but only PP reflections downdip. To understand why we only saw PS reflections updip, we ran simulations using the finite-difference method to determine how dip affects PP and PS reflection amplitudes in DAS VSP. DAS VSP modeling found that PP reflections were stronger than PS reflections, and that PS reflections were weak or non-existent downdip. The key conclusion—that PS reflections are stronger updip and weaker downdip—matches the observations in the DAS VSP data. This implies that in geologic settings with dipping reflectors, geophysicists should place sources updip to record PS reflections in DAS VSP for a predominantly vertical well trajectory. The observed data and the DAS VSP modeling support the hypothesis that both the geologic dip and the well trajectory modulate PS reflection amplitudes in DAS VSP. Future studies can use these findings to understand PS reflection amplitudes in DAS VSP for better well-reservoir facility management decisions involving time-lapse waterflood monitoring and well-rate optimization.
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