Nature of the scattered seismic response from zones of random clusters of cavities and fractures in a massive rock

Abstract

A B S T R A C TThe scattering of elastic energy by random clusters of fractures and/or cavities in a massive rock is studied. The interpretation of the scattered seismic response reveals crucial information about the clusters of inhomogeneities (fractures/cavities), which may correspond to reservoirs. The study is based on a new two-dimensional numerical-modelling method that relaxes the constraints on the location and orientation of the inhomogeneities, accounts for inhomogeneities that have almost no volume but a finite surface area (fractures) and improves the accuracy of the calculation when the size of the inhomogeneities is comparable to the mesh size.It is shown that the nature of the seismic response of zones of diffuse fracturing and/or cavities is associated with the non-uniformity of micro-inhomogeneities in such zones; accumulations of these micro-inhomogeneities are known as clusters.The relationship between the non-uniformity of micro-inhomogeneities and the strength of the seismic response has been established and measured.Considerable differences in the structure of the seismic response of zones of diffuse fracturing and diffuse cavities have been identified. Converted PS-waves dominate in the scattered wavefield associated with fractures. This is explained, as the modelling results show, by a greater transparency of fluid-filled fractures, which reduces the reflected energy of compressional waves. The wavefield associated with cavities is characterized by the predominance (in terms of strength) of compressional PP-waves. The strength of converted PS-waves in the scattered wavefields for both media is approximately the same.On the whole, according to the results of the modelling, the energy of the scattered response of fractured reservoirs is considerably less (about two times) than that of cavernous reservoirs.

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“…One of the peculiarities of the stated problem is that information can be obtained only from experimental measurements with the usage of acoustic sounding (see, e.g., [49]). A number of seismic detectors are located at points d i on the surface of the Earth, which record the vertical components V y (d i , t j ) of particles velocity in the reflected wave at time instances t j .…”

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“…One of the peculiarities of the stated problem is that information can be obtained only from experimental measurements with the usage of acoustic sounding (see, e.g., [14]). A number of seismic detectors are located at points d i on the surface of the Earth, which record the vertical componentsṼ y (d i , t j ) of particles velocity in the reflected wave at time instances t j .…”