The Joint Inversion Of Cross-Borehole Seismic CT Technology And Direct Current Resistivity

With the continuous development of society,people are increasingly paying attention to issues concerning the safety of engineering construction,the safety of ecological environment,and the protection of geological hazards,which are closely related to the safety of individual life and property.The solution to this problem is inseparable from shallow geophysical exploration methods.Seismic CT technology and DC resistivity method are two important shallow exploration methods and are widely used in engineering surveys and other fields.The two methods have a good exploration effect in the common shallow strata.However,when there are low-speed geologic bodies under the local conditions,they are affected by ray sparsity and the seismic imaging results are not good.When there is a high-resistance body or an intermediate thin layer underground,the electric field lines are less distributed,and the accuracy of DC resistivity exploration is insufficient.In order to make up for the disadvantages of the two methods,the predecessors developed cross-gradient joint inversion of the seismic data and the DC resistivity data on the basis of the rectangular mesh.This method achieves the complementary advantages between different methods of physical properties to a certain extent,and improves the accuracy of the inversion of the two methods.However,it is difficult to simulate the undulating terrain and complex models by the rectangular grid division method.For this reason,this paper developed a cross-gradient joint inversion algorithm of cross-hole seismic CT technology and DC resistivity method based on irregular triangle meshes.The similarities and differences between this algorithm and its predecessors mainly include the following three points:(1)The use of irregular triangle meshes;(2)The cross-hole seismic first-arrival travel time linear interpolation method based on irregular triangle mesh have developed in this paper as the seismic forward method;(3)For the non-differential form of cross-gradient function discretization,the method of linear hypothesis of physical parameters of adjacent triangle units is used.In addition,in order to play a comparative role,the separate inversion of the cross-hole seismic CT technology and DC resistivity method was performed in this paper.The forward modeling of the cross-hole seismic CT technology uses the new algorithm developed in this paper,and the inversion is based on the damping least squares inversion method developed by previous researchers.The DC resistivity method forward modeling uses the finite element method based on the point source electric field boundary value problem,and the inversion adopts a DC resistivity regularized focusing inversion algorithm based on the minimum gradient support function developed by previous researchers.Through the calculation and comparison of the synthetic example,the following three conclusions are drawn:(1)The method of the linear interpolation for first-arrival wave travel time based on irregular triangle meshes developed in this paper have the ability to simulate complex first-arrival traveltime models;(2)Cross-gradient function solving method based on linear hypothesis of adjacent triangle units is feasible;(3)The cross-gradient joint inversion of the cross-hole seismic CT technology and DC resistivity which is based on irregular triangle meshes,makes the different physical models constrain each other.However,the obtained inversion result still needs to be further improved.