Study On Propagation Characteristics Of Seismic Wave In Dual Phase Coal Rock Medium

Seismic exploration technology has a wide application prospect in coal geological structure detection and coal dynamic disaster early warning.In seismic exploration,coal and rock media are usually regarded as isotropic,anisotropic or two-phase media,especially anisotropic media theory and two-phase media theory are the two basic theories of seismic exploration.Based on anisotropic medium theory and two-phase medium theory,combined with pseudo spectral method,this paper systematically researches the wave field characteristics and propagation law of seismic wave in coal and rock medium.Firstly,the coal and rock medium are regarded as isotropic or anisotropic medium,the wave equation of coal and rock medium is derived based on the theory of elastic wave dynamics.The grid generation,stability conditions and boundary conditions of the pseudo spectral method are analyzed simultaneously.Secondly,a Matlab program of pseudo spectral method for isotropic and anisotropic media is compiled to simulate the wave field propagation of single-layer and multi-layer coal and rock media.In accordance with results,the propagation law of seismic wave in coal and rock is analyzed.Then,the isotropic and anisotropic elastic wave equations and the recurrence formula of pseudo spectral method are derived by using the two-phase medium theory.After that,a Matlab program is written to simulate the wave field.Finally,using the theory of elastic wave dynamics,the AVO equation is deduced,and a Matlab program is compiled for forward modeling to study the characteristics of coal and rock AVO curve.The effect of coal thickness,incident angle and wavelet frequency on AVO curve is researched.Through theoretical derivation and numerical simulation analysis,the following conclusions are obtained:(1)The pseudo spectral method can be regarded as the case when the order of the spatial derivative of the finite difference method tends to be infinite.The pseudo spectral method has the characteristics of high calculation accuracy,small numerical dispersion,less space sampling points and short calculation time.These features make the pseudo spectral method can simulate the wave field of isotropic,anisotropic and two-phase media well.(2)The comparison shows that the seismic wave excited in single-phase homogeneous isotropic medium propagates around in the form of P-wave,and its front is circular,while the seismic wave in single-phase anisotropic medium propagates forward in the form of quasi-p wave and quasi shear wave,and the wave front is diamond;when the longitudinal wave in the isotropic medium propagates to the interface of the medium,it produces reflection and transmission,and produces the reflection longitudinal wave.The quasi-p wave and the quasi shear wave in anisotropic media have similar reflection and transmission laws.(3)In two-phase anisotropic media,except the pseudo fast P-wave and pseudo shear wave,the pseudo slow P-wave also exist,which has lesser amplitude in solid phase and major amplitude in flow phase.Among the three types of waves,the slow Pseudo-P wave has the slowest velocity,and the pseudo-s wave has the most serious attenuation.(4)For thin coal seam,the reflection coefficient of PP wave is related to incident angle,incident frequency,interlayer thickness and physical parameters of roof and floor strata.For 1-10 m thick coal seam,with the enlarges of incident angle,the thin layer reflection coefficient decreases;with the expands of incident frequency and coal seam thickness,the reflection coefficient increases.