Question

When using AVO analysis, what specific change in the reflection coefficient versus the angle of incidence characterizes a Class III &#x27;bright spot&#x27; anomaly?

Accepted Answer

A Class III AVO anomaly is characterized by a strong negative reflection coefficient at near-zero angles of incidence that becomes increasingly more negative as the angle of incidence increases. In AVO analysis, which stands for Amplitude Versus Offset, experts study how seismic reflection amplitudes change as the distance between a source and a receiver increases, as this distance relates to the angle at which seismic waves hit rock layers. The reflection coefficient represents the ratio of the reflected wave amplitude to the incident wave amplitude at a boundary between two rock layers with different physical properties. A negative coefficient at zero degrees means the acoustic impedance, which is the product of rock density and seismic velocity, decreases across the boundary. A Class III anomaly typically indicates a high-porosity, gas-filled sand encased in lower-velocity shale. Because the gas significantly lowers the bulk modulus and density of the sand, the initial reflection is strong and negative. As the angle of incidence increases, the influence of the contrast in Poisson&#x27;s ratio—a measure of how much a material expands when compressed—further amplifies this negative trend. This results in the characteristic bright spot on seismic data, where the reflection amplitude grows stronger in the negative direction as the offset increases.

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When using AVO analysis, what specific change in the reflection coefficient versus the angle of incidence characterizes a Class III 'bright spot' anomaly?