Tuning and the Wedge
Learning objectives
- Explain tuning as constructive interference of a thin bed's two reflections
- Read the tuning curve: amplitude peaks near a quarter wavelength
- See that below tuning the apparent thickness over-reads the true thickness
- Know to read amplitude, not time, for beds thinner than tuning
The Thin Bed Problem
The last section showed that closely spaced reflections interfere. This one turns that into the single most useful chart in interpretation. Take a bed that thins from thick to a knife edge. Its top and base reflections are opposite in polarity, and as the bed thins those two wavelet copies slide together.
Three regimes appear, and you can watch all three by dragging the probe.
Reading the Tuning Curve
Well above a quarter wavelength, the top and base are two clean events and the apparent thickness, the time between the peak and the trough, tracks the true thickness. As the bed thins toward the tuning thickness, near a quarter wavelength, the top peak and the inverted base align and add: the amplitude climbs to a maximum. This is the bright, tuned reflection that makes thin beds conspicuous. Thin further and the two opposite reflections increasingly cancel, so the amplitude falls back toward zero at the knife edge.
The second lesson is subtler and more dangerous. Below the tuning thickness the peak-to-trough time stops shrinking. The apparent thickness pins at the tuning value and over-reads the true thickness, so time no longer measures how thin the bed is. What still varies is amplitude. That is why net-pay mapping in thin reservoirs reads amplitude, not isochron: below tuning, amplitude carries the thickness information and the time separation does not. This tuning behaviour is a property of the band-limited wavelet, so it is present in every convolutional synthetic, and understanding it is what separates a thickness map you can trust from one you cannot.