Amplitude variation with offset (AVO), or changes in P-wave reflectivity with incidence angle. (a) Typical cases with no change in Poisson's ratio at interface. Note negative gradient for positive intercept, and vice-versa. (b) Decreasing Poisson's ratio leads to more negative gradients. This is the case for reflections from the top of a gas reservoir. (c) Increasing Poisson's ratio causes more positive gradients. This is the case for reflections from the bottom of a gas reservoir