Seismic Properties of Thin Beds

Hai-Man Chung

ABSTRACT

In this dissertation, the seismic properties of thin layers are studied. The normal-incidence properties of one layer and two layers are studied in terms of amplitude, frequency, and complex attributes of the composite wavelet. The offset-dependent properties of one layer are also studied.

The amplitude results for one-layer models indicate that, as the thickness increases from zero to the (l/8)λ d , the amplitude changes quadratically. However, if the two reflection coefficients have equal magnitudes and opposite polarities, the amplitude increases linearly. At (l/4)λ d thickness, all four models show tuning effect.

The amplitude results for two-layered models show that the amplitude changes quadratically as the thickness of one of the two layers increases from zero to the (1/8)λ d value, with tuning effect occurring close to the (I/4)λ d thickness. However, the model with alternating polarities for the three reflection coefficients exhibits a minimum at approximately the (1/16)λ d thickness, and a maximum at close to the (l/4)λ d thickness. These properties do not change appreciably as the thickness of one of the two layers increases within a range of five fold.

In the frequency study, the results indicate that, as the thickness increases, the peak frequencies of the composite reflections decrease slowly. However, for the one-layered model whose reflection coefficients have unequal magnitude and opposite polarities and the two-layered model whose reflection coefficients have alternating polarities,the peak frequencies increase as the thickness increases from zero to the (1/16)λ d , and then decrease as the thickness increases further.

The complex attributes study indicates that the instantaneous frequency is useful for studying wavelet interference. Amplitude tuning effect combined with frequency tuning effect appears to be a good indicator of the existence of thin layers. However, the use of complex attributes remains largely empirical and a pattern recognition tool.

The results of the offset-dependent study show that tuning effect can change drastically the effect of lateral changes in Poisson's ratio in terms of amplitude, peak frequency, and complex attributes. To interpret AVO effect properly in thin-bed interpretation, the effect of offset-dependent tuning must be accounted for.

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