Abstract
The underground rock is composed of rock skeleton and pore fluids. When seismic waves propagate in underground medium, it will show complicated change influenced by pore and pore fluids in rocks. It is very important to study the characteristics of reflection and trans- mission coefficients of seismic waves at the interface and to analyze the properties of the lithology and pore fluids of porous medium, which can reveal the oil and gas bearing in underground medium. Based on the relationship among wave functions, displacement and stress in porous medium, the equation of reflection and transmission coefficients at the interface of porous medium saturated with an ideal fluid is derived. A geological model with sandstone porous medium in the top layer and mudstone porous medium in the bottom layer, the rock skeleton parameters of which vary with porosities, is established. Based on the equation and the model, the variation of reflection and transmission coefficients with the incident angle at the interface of porous medium is studied under the conditions of dif- ferent pore fluids filling and different porosities. The study shows that the existence of the pore and pore fluids will impede the reflected and transmitted abilities of seismic waves at the interface of porous medium. Combining with the theory of rock physics and well data, the porosities and pore fluids of porous medium can be identified qualitatively by studying the variation of the reflected fast P- and SV- waves with incident angle. The values of reflection and transmission coefficients of slow P-wave are very small, but the variation of that is relatively large due to the influence of the pore and pore fluids.
Highlights
The underground rock is a porous medium composed of rock skeleton and pore fluids
The pore shape, pore size and the properties of pore fluids all affect the propagation of seismic waves in porous medium
The variation characteristics of reflection and transmission coefficients with incident angle at the interface of porous medium saturated with an ideal fluid are studied
Summary
The underground rock is a porous medium composed of rock skeleton and pore fluids. The properties of rock are related to various mineral properties that make up the rock skeleton, and influenced by many factors, such as pore shape, pore size, pore fluids filling, and so on. When the seismic wave impinges perpendicularly at the interface of porous medium, some scholars [Geertsma and Smit, 1961; Silin et al, 2004; Silin and Goloshubin, 2010] analyzed the reflection and transmission problems. They considered that the reflection and transmission coefficients can be expressed as a low frequency asymptotic formula. Combining coefficient equation with geological model, the variation characteristics of the reflection and transmission coefficients with incident angles are analyzed under different conditions of porosities and pore fluids filling, respectively. Some meaningful conclusions are obtained in NUMERICAL ANALYSIS, which provides theoretical support for the identification of oil, gas and water in reservoirs
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