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The anatomy of AVO crossplots

Rock Physics Associates, Harpenden, U.K.

Roy White

London University, U.K.

Richard Uden

Continuum Resources, Houston, Texas, U.K.

Corresponding author: rob.simm@rockphysassoc.demon.co.uk

The first 20% of the full text of this article appears below.

AVO crossplots are a simple and elegant way of representing AVO data. Offset variations in amplitude for reflecting interfaces are represented as single points on a crossplot of intercept and gradient. The advantage of this type of plot is that a great deal of information can be presented and trends can be observed in the data that would be impossible to see with a standard offset (or angle) versus amplitude plot. The crossplot is an ideal way of examining differences in AVO responses that may be related to lithologic or fluid-type variations. Commonly used techniques for revealing these differences include color-coding samples from the crossplot and using this as an overlay to a seismic display or creating weighted (or "equivalent angle") stacks (i.e., linear combinations of intercept (R0) and gradient (G).

The early literature approached AVO crossplots from the point of view of rock properties. Acentral concept that emerged from this work was the "fluid line," a hypothetical trend based on a consideration of brine-filled rock properties together with simplifications of the reflectivity equations (Figure 1). If the intercept is plotted on the x axis and the gradient on the y axis, then for consolidated sand/shale rocks the top and base reflections form a trend from the upper left to the lower right quadrant of the crossplot that passes through the origin. When it was realized that data points for equivalent hydrocarbon-filled rocks plot to the left of this line, it became clear that normalizing the data against the fluid line might provide an optimum AVO indicator.

View larger version (20K): [in this window] [in a new window]   Figure 1. AVO classes and the AVO crossplot.

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