geophysics · seismic resolution
Vertical Resolution (Tuning Thickness, Widess)
h_tuning = λ/4 = V / (4f)
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Inputs
ft/s
Hz
Description
Computes the Widess (1973) tuning thickness — the bed thickness (λ/4) at which reflections from the top and base of a thin bed interfere constructively to produce a peak-amplitude 'tuned' response. This is the conventionally accepted vertical resolution limit: beds thinner than this cannot be resolved as two distinct events and instead produce a single composite reflection whose amplitude, not shape, changes with thickness.
Variables
| Symbol | Unit | Description |
|---|---|---|
| h_t | ft | Tuning Thickness |
| V | ft/s | Representative interval velocity of the bed/target being assessed for resolvability. |
| f | Hz | Dominant (peak spectral) frequency of the seismic wavelet at the target level. |
Assumptions
- Bed is embedded in a uniform-velocity medium with equal-magnitude, opposite-sign reflection coefficients at its top and base (the classic Widess wedge model)
- Wavelet is effectively zero-phase and reasonably narrowband around the stated dominant frequency
- A single representative velocity and dominant frequency adequately describe the target interval
Limitations
- This is the classic peak-amplitude (tuning) thickness, not necessarily the smallest bed thickness that can be detected at all — Kallweit & Wood (1982) showed that a broadband zero-phase wavelet's practical resolution, defined using the wavelet's full frequency content (specifically its highest usable frequency) rather than a single dominant frequency, can resolve somewhat below this classic λ/4 limit. The exact refined relationship (the ratio between highest and dominant frequency, and the correct time-to-thickness conversion) could not be reconstructed with confidence from available secondary sources for this session — consult Kallweit & Wood (1982) directly for the rigorous broadband treatment rather than treating λ/4 as a hard floor.
- Actual detectability also depends on the reflection coefficient contrast and background noise level, neither of which is captured by a thickness value alone — a strong-contrast bed thinner than h_tuning may still be visible as an amplitude anomaly even though it isn't 'resolved' in the classic two-event sense
- Assumes a uniform velocity within the bed and surrounding medium — a real velocity gradient across the interval shifts the effective tuning point
Use Cases
- → Reservoir resolvability assessment: Compare an interpreted or type-log reservoir thickness against h_tuning to judge whether it can be resolved as a distinct seismic event on the available data.
- → Explaining amplitude anomalies: Recognize when a 'bright spot' or unusually strong amplitude is a tuning effect from a near-tuning-thickness bed rather than a true reservoir-quality or fluid indicator.
- → Acquisition/processing frequency targeting: Determine what dominant frequency would be needed to resolve a known target thickness, informing survey design or reprocessing decisions.
Related Calculations
Region Notes
Gulf of Mexico
Classic bright-spot plays sit close to tuning thickness by design (best DHI response) — always check whether an amplitude anomaly's mapped thickness is near h_tuning before treating amplitude strength as a direct reservoir-quality indicator.
Permian Basin
Thin, low-relief Wolfcamp benches (often under 10-15 m) frequently sit at or below tuning thickness for typical 25-35 Hz target-depth data — expect composite, non-resolved responses rather than distinct top/base picks.
Global
Always quote h_tuning alongside the velocity/frequency pair it was computed from — the same reservoir thickness can be comfortably resolved in a high-frequency shallow survey and completely tuned-out in a low-frequency deep one.
References
Primary source
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