geophysics · impedance
Acoustic Impedance
Z = ρ × Vp
click formula to derive ↑
Inputs
g/cc
ft/s
Description
Computes acoustic (P-wave) impedance as the product of bulk density and P-wave velocity. Contrasts in impedance between adjacent layers generate seismic reflections, making AI the central link between well logs and seismic amplitude.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Z | (g/cc)·(m/s) | Acoustic Impedance |
| ρ | g/cc | Bulk density of the formation, typically from the density log. |
| Vp | ft/s | Compressional (P-wave) velocity, typically derived from sonic slowness (Vp = 1/DT) or from a check shot/VSP. |
Assumptions
- Density and Vp are measured or estimated at the same depth and represent the same rock volume
- The medium is treated as isotropic for the purposes of this single-value impedance (no azimuthal velocity variation)
- Vp is the true formation velocity, free of major borehole-induced cycle skips or noise in the sonic log
Limitations
- A single AI value does not capture anisotropy or shear-wave information needed for full elastic characterization — see Vp/Vs and Gassmann calculators
- Sonic logs can be affected by gas, washouts, or poor borehole conditions, biasing Vp and therefore Z
- AI alone cannot uniquely determine lithology or fluid — it is best used in crossplots with Vp/Vs or other attributes
Use Cases
- → Seismic inversion: Provide the well-log-derived AI trace used to calibrate and constrain seismic acoustic impedance inversion volumes.
- → AVO class determination: Combine AI contrasts with Vp/Vs to classify expected AVO response (Class I-IV) at a target interface.
- → Rock physics crossplotting: Plot AI against Vp/Vs or porosity to build rock physics templates for lithology and fluid discrimination.
Related Calculations
Region Notes
Permian Basin
Wolfcamp shale AI typically falls in the 8,000-11,000 (g/cc·m/s) range; carbonate-rich benches can exceed 12,000, useful for distinguishing facies on inversion volumes.
Gulf of Mexico
Unconsolidated Miocene sands often show AI below 6,000, with gas sands as low as 3,000-4,000 — strong negative AI anomalies are classic DHI targets.
Global
Always QC sonic-derived Vp against a check shot or VSP before using AI for inversion calibration — uncorrected sonic drift is a common source of inversion mismatch.
References
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