It has been recognized for some time that seismic amplitude variations with offset contain lithologic and pore-fluid information. However, the exact formulas for plane-wave reflection coefficients are not easily interpreted in terms of the effects expected from classes of geologic models. For fluid contacts within a rock unit (which may show up as “flat spots” in seismic data), simplified approximate formulas can be derived to predict reflection coefficients for compressional (P) and shear (S) waves. The formulas derived in this note predict (under the assumption that changes in pore fluid do not affect the elastic properties of the rock matrix) that the P and S reflection coefficients from a gas-brine, gas-oil, or oil-brine contact increase monotonically with offset, and that for a gas- brine contact, the ratio of the two coefficients is sensitive to gas saturation.

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