Abstract
The effects of changes in rock and fluid properties on the amplitude of reflection seismic waves are described. This analysis is commonly known as amplitude versus offset (AVO) analysis. Attributes characterizing the seismic response are useful for determining subsurface rock and fluid properties. In the slope-intercept domain, reflections from wet sands and shales fall on or near a trend, which we call the Fluid Line. Reflections from the top of sands containing gas or light hydrocarbons fall on a trend approximately parallel to the Fluid Line; reflections from the base of gas sands fall on a parallel trend on the opposing side of the Fluid Line. Typically, rock properties of sands and shales differ, and therefore, reflections from sand/shale interfaces are also displaced from the Fluid Line. The distance of these trends from the Fluid Line depends upon the contrast of the ratio of P-wave velocity, Vp, and S-wave velocity, Vs. This ratio is a function of the pore fluid compressibility and implies that distance from the Fluid Line increases with increasing compressibility. Reflections from wet sands are closer to the Fluid Line than hydrocarbon related reflections. Porosity changes affect acoustic impedance, but do not significantly impact the Vp/Vs contrast. As a result, porosity changes move the AVO response along trends approximately parallel to the Fluid Line. These observations are useful for interpreting AVO anomalies in terms of fluids, lithology and porosity.