geophysics · avo
AVO Intercept/Gradient Classification
Class from (R₀,G) sign/magnitude; screening attributes A×B = R₀·G, A+B = R₀+G
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Inputs
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Description
Classifies an AVO intercept/gradient (R0, G) pair into the Rutherford-Williams (1989) Class I-III / Castagna-Swan (1997) Class IV gas-sand scheme, and computes the A×B and A+B screening attributes commonly used as standalone hydrocarbon-indicator volumes.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Class | — | Rutherford-Williams/Castagna Class |
| A×B | — | Intercept × Gradient (Screening Attribute) |
| A+B | — | Intercept + Gradient (Far-Angle Proxy) |
| R₀ | — | Zero-offset (normal-incidence) reflectivity, typically taken from the Shuey/Aki-Richards intercept or a seismic intercept-stack attribute volume. |
| G | — | AVO gradient describing amplitude change with offset, typically taken from the Shuey gradient or a seismic gradient-stack attribute volume. |
Assumptions
- R0 and G were derived consistently (same convention/sign), ideally from shuey_avo or an equivalent attribute volume using the same polarity convention
- The target is a single, isolated reflector (top-of-sand-style contrast), not a tuned/complex multi-reflector package
- Class boundaries are gradational, not sharp physical thresholds — see limitations
Limitations
- The R0≈±0.02 class boundaries used here are a commonly-taught rule of thumb, not a universal precisely-defined physical cutoff — literature crossplot templates use a background trend line, not a fixed number
- Class assignment alone cannot confirm hydrocarbons — lithology, tuning, and processing/polarity errors can all mimic an anomalous class
- Requires R0/G in a consistent SEG-normal-polarity convention; a reversed-polarity dataset will misclassify
Use Cases
- → Attribute-volume screening: Classify an (R0,G) attribute volume pixel-by-pixel for hydrocarbon-indicator classes ahead of prospect ranking.
- → Well-tie class confirmation: Confirm which Rutherford-Williams/Castagna class a specific well-tied horizon belongs to before choosing a stacking/inversion strategy.
- → DHI screening attribute: Compute the standalone A×B product attribute commonly used as a bright-spot screening volume.
Related Calculations
Region Notes
Gulf of Mexico
The original Rutherford-Williams (1989) dataset was Gulf of Mexico gas sands — Class III (R0<0, G<0) remains the dominant, best-calibrated class in this basin.
Permian Basin
Wolfcamp/Bone Spring targets frequently plot as Class IIp or Class I (small-to-large positive R0) rather than classic Class III — do not force a Gulf-of-Mexico-style Class III expectation onto Permian carbonate-adjacent targets.
Global
Always calibrate class boundaries against a local background (wet) trend crossplot where possible, rather than relying solely on the fixed |R0|≈0.02 rule used here.
References
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