Abstract

Prestack depth migration (PSDM) should be the ultimate goal of seismic processing, producing angle-dependant depth images of the subsurface reflectivity. But the expected quality of PSDM images is constrained by many factors. Understanding all of these factors is necessary to improve depth imaging of geologic structures. In all PSDM approaches, e.g., Kirchhoff or wave-equation, migration always includes compensating for wave propagation in the overburden (back propagation, downward continuation, etc.), before focusing back the reflected/diffracted energy at each considered location in depth (imaging). Ideally, we would like to retrieve the reflectivity of the ground as detailed as possible to invert for the elastic parameters. But the waves perceive the reflectivity through “thick glasses,” seeing blurred structures, and not necessarily all of them, depending on the illumination. Only a filtered version of the true reflectivity is therefore retrieved. Being able to estimate these filters, the so-called resolution is the key to a better understanding of the imaging results and improving them, either at an early stage by careful survey planning, or, possibly at a later stage, by properly tuning migration parameters. In addition, interpreters should be aware of such effects. The seismic “wave filters” are easily explained and calculated with ray-based approaches. Once obtained, they can also be used to simulate PSDM images. Ultimately, they could be used to compensate (deblurring) for some of the resolution effects.

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