The construction of subsurface illumination and amplitude maps via ray tracing

Abstract

The goal of a seismic survey is to illuminate subsurface geologic formations that may hold hydrocarbon accumulations. Conventional seismic survey design relies on the assumption that uniform midpoint coverage will lead to uniform illumination in the subsurface as long as each midpoint is hit by a sufficient range of offsets. In areas of complex velocity structure, severe wavefield distortions lead to irregular subsurface illumination patterns, even if surface midpoint maps show a uniform distribution. A more appropriate approach is to design seismic surveys to ensure illumination of key subsurface horizons.The difference between midpoint coverages and subsurface illumination patterns is particularly large in saltprone areas (Muerdter et al., 1997). Due to severe wave distortion through complex, high-velocity salt bodies, conventional design methods that result in relatively uniform surface coverage ( Figure 1) generate uneven amplitudes and shadow zones on subsalt horizons, an effect that is shown clearly by ray-trace modeling of an entire seismic survey ( Figure 2). Methodology.We begin by building a model of the known geology. Typically, this model is based on the velocity model used for migration or on previously available seismic interpretations and includes any horizons of particular exploration interest. We can then predict the illumination and amplitude distributions that should be achieved on any horizon from a given survey by full-offset ray tracing of the survey coordinates (either planned coordinates or actual coordinates from navigation data obtained during the collection of a survey). If the modeled illumination and amplitude maps are to be compared with similar data extracted from processed seismic, it is important to ray trace only those shots and receivers that were used in the processing (i.e., the trace coordinates to be ray traced should be extracted during the processing flow).Ray-tracing results must next be reduced in a manner consistent with the seismic processing used for the survey. In particular, the subsurface bins into which modeled hits or amplitudes will be accumulated need to be equivalent to the subsurface bins used in processing. Similarly, the offset range of the ray-traced results should be limited to the range used in processing. Also, it is critical that multiple ray-tracing hits within each offset bin be handled as they will be in seismic processing. Finally, any gain treatments that will be applied during processing need also to be applied to the raytracing results.Fresnel-zone smoothing effects may be introduced to improve the match between ray-tracing results and seismic data. This may be accomplished by spreading each hit over adjacent subsurface bins that are within the first Fresnel zone of the reflection point (Schneider and Winbow, 1999). Fresnelzone smoothing often has the side effect of masking the acquisition overprint. Proper reduction of the ray tracing results leads to amplitude maps that approximate the result of prestack depth migration. Iterative ray tracing ...

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