Interpreting Avo Anomalies

We investigated the effects of changes in rock properties on<br>AVO responses. In the slope-intercept domain, reflections<br>from wet sands and shales fall on a trend, the Fluid Line.<br>Reflections from the tops of sands containing gas or light<br>hydrocarbons fall on a trend below the Fluid Line;<br>reflections from the base of gas sands fall on a trend above<br>the Fluid Line. The distance of these trends from the Fluid<br>Line depends upon pore fluid compressibility; i.e., distance<br>increases with increasing compressibility. But, if all other<br>factors are equal, base of sand reflections are displaced<br>further from the Fluid Line than top of sand reflections.<br>Consequently, base of sand reflections, which identify<br>down-dip limits and fluid contacts, will be more prominent<br>than top of sand reflections. Porosity changes affect<br>acoustic impedance, but do not significantly impact the<br>Vp/Vs contrast. As a result, porosity changes move the<br>AVO responsea long trends parallel to the Fluid Line.