The P-wave reflection coefficient at an interface separating two media is known to vary with angle of incidence. The manner in which it varies is strongly affected by the relative values of Poisson's ratio in the two media. For moderate angles of incidence, the relative change in reflection coefficient is particularly significant when Poisson's ratio differs greatly between the two media.Theory and laboratory measurements indicate that high-porosity gas sands tend to exhibit abnormally low Poisson's ratios. Embedding these low-velocity gas sands into sediments having 'normal' Poisson's ratios should result in an increase in reflected P-wave energy with angle of incidence. This phenomenon has been observed on conventional seismic data recorded over known gas sands.

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