Precise Modeling of Seismic Responses from Fractured Geological Media

Abstract

The goal of this article is the investigation of seismic responses from fractured medium through the precise simulation of dynamic processes in heterogeneous elastic media. The base of the method proposed is the direct solution of a system of elastic equations in full 3D case. For describing the geometry of rock massif parallelepided meshes are used. For taking into account the internal mathematical features of the govern system of eqations the grid-characteristic method is used. It allows us to set correct boundary conditions and to describe full set of elastic seismic waves: Rayleigh and Love surface waves and all body waves. In this work the direct modeling of the internal geometry of fractured media is used. All cracks consist of two parallel borders and contain two different saturators: nothing and fluid. In the first case the corrector for setting of day-surface (normal zero-force) condition is used and in the second one -the approach of free gliding is used. With the usage of proposed method the seismic response from a set of 31 vertical geological cracks was obtained. This cluster was placed at 2 km below the day-surface and has fractured density equals to 1 crack \ 100 m. Dynamic wave fields for successive times were obtained and synthetic multi-component seismograms were drawn. The detailed analysis of these results provided interesting facts. The main part of the seismic response from a system of geological cracks consists of complex combination of scattered P-waves and S-waves. The intensity of reflected waves is less than the intensity of propagated waves approximately by the factor of 1,8 for empty cracks and 0,2 for fluid-filled cracks. In general, seismic response from empty cracks is significantly higher than from fluid-filled cracks. The dominant component of seismic response for empty cracks is the S-wave registered on X-component, for fluid-filled cracks -P-wave registered on Z-component. After the arrival of first responces of P ans S-waves following them tail may be clearly identified connected with the crack-crack interaction.