geophysics · avo
Reflection Coefficient
RC = (Z₂ − Z₁) / (Z₂ + Z₁)
click formula to derive ↑
Inputs
(g/cc)·(m/s)
(g/cc)·(m/s)
Description
Computes the normal-incidence (zero-offset) reflection coefficient between two layers from their acoustic impedances. The magnitude and sign of RC control seismic reflection amplitude and polarity, forming the basis for synthetic seismograms and bright/dim spot interpretation.
Variables
| Symbol | Unit | Description |
|---|---|---|
| RC | — | Reflection Coefficient |
| Z₁ | (g/cc)·(m/s) | Acoustic impedance of the layer above the interface (ρ × Vp), computed from log data above the target horizon. |
| Z₂ | (g/cc)·(m/s) | Acoustic impedance of the layer below the interface (ρ × Vp), computed from log data below the target horizon. |
Assumptions
- Wave travels at normal incidence (zero offset) — angle-dependent amplitude effects (AVO) are not represented by this single value
- The interface is a sharp, single boundary between two homogeneous layers rather than a gradational or thin-bed contact
- Impedance values are representative of the rock volumes immediately above and below the interface at seismic scale
Limitations
- Real seismic amplitudes are affected by tuning (thin-bed interference), attenuation, and noise that can mask or amplify the true RC
- Does not capture offset-dependent amplitude variation (AVO) — pair with the Vp/Vs and Gassmann calculators for a full elastic analysis
- A single RC value cannot distinguish between different fluid/lithology combinations that happen to produce the same impedance contrast
Use Cases
- → Synthetic seismogram generation: Convert a log-derived impedance series into a reflectivity series for forward modeling and well-to-seismic calibration.
- → Bright/dim spot analysis: Identify intervals with anomalously strong RC values that may indicate hydrocarbon-related impedance contrasts.
- → AVO baseline: Establish the zero-offset reflectivity baseline against which offset-dependent AVO behavior is compared.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring shale-on-carbonate contacts can produce RC magnitudes of 0.10-0.20, often visible as strong, continuous reflectors used for zone correlation.
Gulf of Mexico
Class III AVO gas sands in Pliocene-Pleistocene sediments commonly show RC around -0.15 to -0.25 at the top of sand — classic bright-spot DHIs.
Global
Always tie computed RC values to an extracted seismic wavelet amplitude at the well location before interpreting amplitude anomalies regionally.
References
Primary source
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