AVO: Definition, Classes, and Direct Hydrocarbon Indicators

What Is Amplitude Variation with Offset?

Amplitude variation with offset (AVO) describes the systematic change in seismic reflection amplitude as the distance between a seismic source and receiver increases, revealing subsurface lithology and pore-fluid content at reflector boundaries. Geophysicists worldwide apply AVO analysis to identify direct hydrocarbon indicators (DHIs) before drilling, directly reducing exploration risk.

How Amplitude Variation with Offset Works

When a compressional (P-wave) seismic wavelet strikes an interface between two rock layers, the wavelet partly reflects and partly transmits. The relative energy split depends on the contrast in acoustic impedance (P-wave velocity times density) across the boundary. At normal incidence (the source directly above the reflector), only the impedance contrast matters. As the angle of incidence increases, however, the shear-wave velocity and density of both layers begin to influence how much energy reflects. This angular dependence is described precisely by the four Zoeppritz equations (Karl Zoeppritz, 1919), which express the four wave modes (reflected P, reflected S, transmitted P, transmitted S) in terms of the elastic properties on each side of the interface. Because pore fluids strongly affect P-wave velocity while leaving S-wave velocity nearly unchanged, the angular dependence of reflectivity provides a lever to separate fluid-related effects from lithology-related effects.

In practice, the Zoeppritz equations are nonlinear and difficult to invert directly from field data. Roger Shuey (1985) introduced a two-term approximation valid for angles up to roughly 30 degrees: R(θ) = P₀ + G sin²(θ), where P₀ is the zero-offset intercept (proportional to acoustic impedance contrast) and G is the AVO gradient (sensitive to shear-wave contrast and Poisson's ratio). The Shuey approximation enables geophysicists to generate intercept (P) and gradient (G) attribute volumes from conventional 3D seismic data by fitting a line through amplitude values at near, mid, and far offsets on each common-midpoint (CMP) gather. A third term, the curvature (C), extends validity toward wider angles and is sometimes included in AVO inversion workflows targeting anisotropic or deep targets. The intercept-gradient crossplot is the workhorse visualization: background shales define a "fluid factor line," and anomalous clusters displaced off this trend indicate potential hydrocarbon-bearing sands or carbonate porosity changes. Additional derived attributes include the pseudo-Poisson's ratio contrast (Δσ), the fluid factor (F = ΔVp - (Vp/Vs·ΔVs)), and the product P × G, which amplifies gas-sand AVO anomalies.

Preserved-amplitude seismic processing is a non-negotiable prerequisite for reliable AVO. Standard exploration processing applies amplitude corrections (spherical divergence, absorption, surface-consistent deconvolution) that can inadvertently destroy the offset-dependent amplitude signal. AVO-specific processing retains true-relative amplitudes by applying only physically justified corrections, carefully balancing near and far offset stacks, and preserving the wavelet character across offsets. Multiples are particularly damaging because their AVO behavior differs from primaries and can generate false anomalies in shallow water where long-period water-bottom multiples overlap with target reflections. Once a geophysicist has conditioned intercept and gradient volumes, AVO results are validated against synthetic AVO models derived from well log data at nearby penetrations, using rock physics transforms such as the Biot-Gassmann relations to predict how the amplitude response changes when one fluid is substituted for another (gas-for-brine or oil-for-brine fluid substitution).

AVO Classes

Rutherford and Williams (1989) formalized a classification scheme based on the zero-offset acoustic impedance contrast between a sand and its encasing shale, and this scheme remains the standard reference framework worldwide.

Class I (high-impedance sand): The sand velocity and density together exceed those of the surrounding shale, producing a positive zero-offset intercept (hard kick, same polarity as the seabed). Amplitude decreases with offset and may approach zero or reverse polarity at wide angles. Gas saturation shifts the Class I sand toward lower amplitudes because gas reduces P-wave velocity, softening the hard kick. Class I sands are common in compacted Paleogene and older sequences in the North Sea and deep U.S. Gulf of Mexico.

Class II (near-zero intercept sand): Acoustic impedance of the sand closely matches the surrounding shale, so the zero-offset reflection is small or absent. The amplitude response is dominated by the gradient; a polarity reversal is possible somewhere in the offset range. Class IIp (polarity-reversal) sands are particularly diagnostic because the bright spot disappears at near offsets and grows at far offsets with opposite polarity. Class II responses are prone to misidentification because near-offset stacks show little or no anomaly.

Class III (low-impedance sand, classic bright spot): The gas-filled sand is slower and less dense than the encasing shale, producing a negative zero-offset intercept (trough on standard polarity). Amplitude increases in magnitude with offset, generating the "bright spot" amplified at all offsets. Class III is the most recognizable and most commonly exploited DHI pattern, prevalent in shallow Miocene sands in the U.S. Gulf of Mexico, offshore West Africa, and the Nile Delta.

Class IV (anomalous low-impedance sand): Like Class III, the sand has lower impedance than shale, but the gradient is positive rather than negative, so amplitude decreases with offset despite starting from a bright zero-offset response. This counter-intuitive behavior can arise when the overlying shale is unusually fast or when the sand itself has a specific Vp/Vs ratio. Class IV sands are less common but important to recognize to avoid misclassifying them as dim or brine-bearing.