The AVO Classes

Part 6, Part 6: Elastic and AVO

Learning objectives

  • Read the intercept-gradient (R0 versus G) crossplot
  • Place a reflection into Class I, II, III, or IV
  • Recognise the background trend of water-wet rocks
  • See a hydrocarbon sand as a deviation off that trend

Two Numbers, One Map

Shuey handed the interpreter two numbers for every interface: the intercept R_0R_0 and the gradient GG. Plot one against the other and a remarkable thing happens. Rutherford and Williams, and later Castagna and Swan, found that top-of-sand reflections do not scatter randomly across the R0R_0-GG plane; they fall into a small family of AVO classes, each with its own amplitude-versus-angle behaviour.

  • Class I (R_0>0,G<0R_0>0,\ G<0): a hard sand with a bright peak at zero offset that dims as the angle opens. Think tight or well-cemented gas sand under a softer shale.
  • Class II (R_0approx0,G<0R_0\approx 0,\ G<0): almost invisible on the stack, then a trough that grows at far offsets. The subtle one, easy to miss.
  • Class III (R_0<0,G<0R_0<0,\ G<0): the classic gas bright spot, a strong trough that gets even more negative with offset.
  • Class IV (R_0<0,G>0R_0<0,\ G>0): a soft trough at zero offset that dims with angle, common in unconsolidated or shaley sands.

The AVO classesIIIIIIIVintercept R0 →gradient Gbackground trend (brine sands, shales)Each interface is two numbers, R0 and G. Water-wet rocks lie on the dashed trend; a gas sand pulls off it toward Class III.

The Background Trend and the Anomaly

The most important line on the crossplot is not any single point but the background trend: water-wet sands and shales cluster along a line through the origin (the dashed line here). A brine sand and the shale above it differ mostly in lithology, and their (R_0,G)(R_0,G) points land on that trend. Fill the same sand with gas and its point pulls off the trend, usually down and to the left toward Class III. That deviation, not the raw amplitude, is the fluid signal.

This reframes what AVO detects. You are not hunting for the biggest reflection; you are hunting for the reflection that leaves the crowd. Drag the intercept and gradient across the plane and watch the class label and the amplitude-versus-angle curve change together. The next section stops classifying and starts building: a real angle gather that rebuilds as the fluid changes through Gassmann.

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