Hydrocarbon related "AVO anomalies" may show increasing or decreasing amplitude variation with offset. Conversely, brine-saturated "background" rocks may show increasing or decreasing AVO. Amplitude-versus-offset interpretation is facilitated by crossplotting AVO intercept (A) and gradient (B). Under a variety of reasonable geological circumstances, A's and B's for brinesaturated sandstones and shales follow a well-defined "background" trend. "AVO anomalies" are properly viewed as deviations from this background and may be related to hydrocarbons or lithologic factors. The common three-category classification of gas-sands developed by Rutherford and Williams is incomplete. We propose that an additional category (Class IV) be considered. These are low impedance gas-sands for which reflection coefficients decrease with increasing offset; they may occur, for example, when the shear-wave velocity in the gas sand is lower than in the overlying shale. Thus, many "classical" bright spots exhibit decreasing AVO. If interpreted incorrectly, AVO analysis will often yield "false negatives" for Class IV sands. Clearly, the conventional association of the term "AVO anomaly" with an amplitude increase with offset is inappropriate in many instances and has led to much abuse of the AVO method in practice. Similarly, interpretation of partial stacks is not as simple as looking for relatively strong amplitudes at far offsets. We recommend that all AVO analysis be done in the context of looking for deviations from an expected background response.

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