Offset-dependent tuning effects of thin layers and a model study of two layers
Hai-Man Chung, Donald C. Lawton
The offset-dependent amplitude of a single low-velocity thin layer embedded in a homogeneous thick layer is studied. It is found that for the P-wave maximum peak amplitude, the change in amplitude as a result of a change in Poisson's Ratio is adversely affected by the effects of offset-dependent tuning. In the absence of the tuning effects, the P-wave amplitude increases by more than 90% for some offsets as Poisson's Ratio for the thin layer changes from 0.25 to 0.1. But with tuning, the percentage drops to less than 20%. For the PS-wave maximum peak amplitude, the effects of tuning are less severe.
The study of two-layer model indicates that the tuning thickness for the upper layer remains at 1/4 of the predominant wavelength, regardless of the thickness of the lower layer. A plot of the trough to peak amplitude appears to be more diagnostic of the thickness of the lower layer than the plot of absolute maximum amplitude of the reflected composite wavelet. The instantaneous frequency also appears to be useful to differentiate the subtle waveform changes introduced by varying the thickness of the lower layer.