An interactive wedge model for thin‑bed tuning. Change velocity, frequency, wavelet and polarity; watch the top and base reflections merge at tuning thickness, and read the tuning curve and apparent‑thickness curve that interpreters use to calibrate thin beds.
A seismic wedge model is the simplest experiment in seismic resolution. One layer thins from well above the wavelength down to zero, inside a uniform host rock, so the only thing that changes along the model is bed thickness. Convolving the two reflection coefficients (top and base) with a wavelet produces the synthetic section above, and the way the two reflections interfere as the bed thins is the whole story of thin‑bed tuning.
Three regimes appear from left to right:
The lower‑left plot is the tuning curve: the maximum absolute amplitude of the composite waveform at each thickness. Interpreters use it in two ways:
The lower‑right plot, apparent versus true thickness, shows why time picks fail below tuning: apparent thickness (peak‑to‑trough separation) overshoots slightly near tuning, then flattens at the wavelet's own breadth no matter how thin the bed really is.
Before an acquisition survey is designed we run a wedge model with the expected velocities and the bandwidth the survey can realistically deliver at target depth. If the thinnest bed the geologist needs to map sits below the tuning thickness the design can achieve, the choice is clear: either extend the bandwidth (broadband source, point receivers, finer sampling, Q‑compensation) or set the objective to detection and amplitude calibration rather than direct thickness measurement. Deciding this on a model costs hours; discovering it after acquisition costs the survey.
A synthetic model of a single layer whose thickness decreases linearly to zero inside a uniform host rock. Convolving its top and base reflections with a wavelet shows how the two events interfere as the bed thins, which is the standard way to study tuning thickness and thin‑bed resolution.
The bed thickness at which top and base reflections interfere constructively and composite amplitude peaks. For a zero‑phase wavelet with opposite‑polarity reflections it is about λ/4 = V/(4f). Below it the two reflections cannot be separated in time.
Composite reflection amplitude plotted against true bed thickness. It rises toward λ/4, peaks at tuning, then falls roughly linearly to zero. It is used to convert amplitude to thickness for beds below tuning.
Below about λ/8 the composite waveform stops changing shape as the bed thins; only amplitude decreases, proportionally to thickness. λ/8 is therefore the practical detection limit, distinct from the λ/4 resolution limit.
Above tuning, time picks track true thickness. Near tuning, side lobes push the picks apart so apparent thickness slightly exceeds true thickness; below tuning the picks lock to the wavelet breadth and stop changing while the bed keeps thinning. Time picks below tuning overestimate thickness.
Yes. Opposite‑polarity top and base reflections reinforce near λ/4. Same‑polarity reflections cancel near λ/4 and reach their amplitude peak closer to λ/2, so the "tuning thickness" for a given wavelet depends on the sign of the reflection coefficients as well as on wavelength.
We start from the thinnest bed that must be mapped, model it, and design the source, receiver and sampling parameters that can actually deliver the required bandwidth at depth.
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