Abstract
As most of the lithostratigraphic reservoirs in China are thin interbeds, the study of seismic responses in thin interbeds is an integral part of lithologic reservoir exploration. However, at present, the research on seismic reflection coefficients of thin interbeds in exploration seismology is still weak, which leads to the lack of theoretical basis for the subsequent interpretation of amplitude variation with offset (AVO) related to thin interbed. To solve this problem, in this paper, we proposed second-order approximate equations of the seismic reflection coefficients in thin-bed and thin-interbed layers. Under the assumption of a small impedance contrast in layered media, we made a second-order approximation with a more evident physical meaning to the reflection coefficient calculation method proposed by Kennett. Then, based on the test of the single thin-layer theoretical model, it was confirmed that the second-order approximation equation of the PP-wave (reflected compressional wave) is accurate at incident angles less than 30°, and that of the PS-wave (converted shear wave) is accurate at wider incident angles. Finally, based on the single-thin-bed equations, the approximate equations of seismic reflection coefficients in thin interbeds were established, the validity of which was verified by the theoretical model. Our equations will be applicable to the calculation of PP- and PS-wave reflection coefficients in thin interbeds where internal multiples are difficult to suppress and transmission loss is hard to accurately compensate. This lays a theoretical foundation for improving the seismic prediction accuracy of lithologic reservoirs.
Highlights
The technology of amplitude versus offset (AVO) or amplitude versus incident angle (AVA)has been used to predict rock properties and fluid in the subsurface reservoirs for decades [1,2,3,4].Conventional AVO analysis is based on Zoeppritz equations or their approximations; these equations are not suitable for thin-bed problems due to the mixing of the reflected waves, converted waves, and internal multiples, which are hard to separate [5,6,7].The problem of thin-bed layers has been studied for decades
Our single thin-bed reflection coefficient equation is deduced based on the exact reflection coefficient equation in stratified media proposed by Kennett [13,14]
We support for the subsequent AVO interpretation iωqβ h, performed a numerical simulation analysis on the esingle thin-bed model based on the Kennett equation and thatfrequency; the high-order multiples contributeofless to the overall reflection coefficient where ω is thefound angular h indicates the thickness region
Summary
The technology of amplitude versus offset (AVO) or amplitude versus incident angle (AVA). The existing approximate equations of thin-bed reflection coefficients are based on the assumption of small incident angle and weak impedance contrast. These equations are complicated and their physical meaning is not clear, so they do not provide theoretical support for the subsequent AVO interpretation of single thin beds or thin interbeds. We performed a numerical simulation analysis on the single thin-bed model based on the Kennett equation and found that the high-order multiples contribute less to the overall reflection coefficient of the single thin bed. We used a recursive algorithm [12] to extend the second-order approximation equation of the thin-bed reflection coefficient to the thin-interbed model and verified its accuracy by numerical simulation analysis
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