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The construction of subsurface illumination and amplitude maps via ray tracing

ExxonMobil Upstream Research Company, Houston, Texas, U.S.

Ian Watson

ExxonMobil Exploration Company, Houston, Texas, U.S.

Corresponding author: glenn.w.bear@exxon.sprint.com

Editor's note: This article was selected as the Best Poster Paper at SEG's 1999 Annual Meeting. Due to its quantitative nature, the 1999 Best Student Poster Paper will not be published in TLE but in a future issue of G, without undergoing peer-review.

The first 20% of the full text of this article appears below.

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 salt-prone 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).

View larger version (101K): [in this window] [in a new window]   Figure 1. Total hits in each surface bin resulting from a 3-D seismic survey collected along east-west lines. Distances are in kilofeet.

View larger version (95K): [in this window] [in a new window]   Figure 2. Amplitude of a subsurface (subsalt) horizon recorded by the 3-D survey in Figure 1. This amplitude map was constructed by collecting ray-trace results (amplitudes of each hit) at each subsurface bin on the 3-D . . . [Full Text of this Article]