Seismic Wave Characteristics Research In Fracture Media

With the developments of detecting technologies of oil-gas mineral resources exploration, the research of fractured oil-gas reservoir has become an important exploration field. Fractured oil-gas reservoirs are of small scales with highly heterogeneities and anisotropies. To carry out the exploration of fractured reservoirs, one need study the propagation of seismic wave in crack media first, which can help us to analyze and interpret the actual data more reasonably. Therefore, this paper focuses on the studies of the method of analyzing the propagations of seismic waves in crack media.This paper discussed the crack wave staggered grid high-order finite difference numerical simulation method, analyzed source loadings and wave field properties in numerical simulations, and studied the behaviors of seismic wave propagation in fractured medium through velocity anisotropies, shear wave splitting and range AVO. The main results are summarized as follows:1. It researches the Simulation method of high order staggered-grid finite difference in crack media, we respectively derived the PML and MPML absorbing boundary condition in 3D and 2D3C simulation. The seismic wave field high-order staggered-grid finite difference simulation method is improved, the relevant software is amended and supplemented, the 2D 3C and 3D numerical simulation are also developed to be suitable for any observable system and flexible loading sources. A new interchange way of symmetry axis and observation axis in media is proposed, which leads to the observation results can take the place of 3D observations, avoids a large number of 3D data operation, and can effectively study the S-wave splitting phenomenon in crack media.2. It not proposes relations of the loading modes of seismic sources and wave field prperties but also analyses physics mechanism. The conclusion is that:to get pure P wave sources, the pressure sources should be loaded on the normal stress terms; to get pure S wave sources, shear force source should be loaded on shear stress terms; and (horizontal, vertical, horizontal) concentrated force sources should be loaded on (along x, y, z direction) volume force terms to can get both P wave and S wave source directions. The result has guiding significance in actual application.3. It researches the mechanism of S-wave splitting and wave field characteristics in crack media in detail, media, and analyzed the correspondences between SV, SH waves and S1, S2 waves. Based on the concrete models, the paper provides the influencing factors of S-wave splitting ranging over source type, azimuth, medium, interface and observation system, et al.In particular, we studied the time differences, and the variations of amplitude and energy in terms of azimuth angle between fast and slow waves after the shear wave splitting. And, the conclusion turns out to be that, the time difference between the fast and slow waves increases with the azimuth angle, and the amplitudes of fast and slow wave components will change according to certain rules, and the energy in fast and slow waves will be redistributed. The result provides the important application value.4. It analyses the velocity anisotropy and azimuth AVO in crack media, gives the influences of anisotropic parameters on phase velocity, group velocity and PP wave reflection coefficients in crack media are summarized.