Research On Transient Electromagnetic Finite Element Reverse Time Migration Imaging Method

China has moved towards high-quality development,and the consumption of mineral energy will continue to increase.Therefore,the requirements for various mineral resource surveys will certainly continue to increase.Transient electromagnetic method(TEM)is a geophysical method widely used in petroleum mineral exploration,coalfield hydrogeological investigation and engineering survey,etc.It is considered as one of the preferred methods for conducting various mineral resource investigations in China.However,since the TEM data interpretation method can only obtain the physical information of the spatially continuous distribution of subsurface materials,and considering that the fluctuation field has a high resolution for the subsurface discontinuous interface and the similarity between the electromagnetic wave diffusion field and the wave field propagation,a new TEM data interpretation method called transient electromagnetic pseudo-seismic imaging is developed.The transient electromagnetic pseudo-seismic imaging method is based on the wave field inverse transformation theory,which transforms the electromagnetic diffusion field data into virtual wave field data.Then,with the help of the mature migration imaging method in seismic exploration,the identification and positioning of the underground discontinuous electrical interface is realized,thereby improving the resolution of the TEM.Due to the ill-posed problem of wave field inverse transform,this paper intends to use the sweeping time wave field transform technology to obtain a more stable virtual wave field value.However,the virtual wave field data obtained after transformation contains the first arrival signal with strong energy,which will affect the accuracy of the imaging results.Therefore,the data cannot be directly used for migration imaging,but should be removed first arrival.In addition,the transient electromagnetic pseudo-seismic migration imaging is based on the zero offset observation system.In the same survey line,if the offsets of each receiving point are not equal,the virtual wave field data needs to be processed by normal moveout correction to be used for migration imaging calculation.For the part of transient electromagnetic pseudo-seismic imaging algorithm,this paper introduces the finite element migration imaging processing method in seismic data interpretation into transient electromagnetic pseudo-seismic imaging technology,so as to quickly and accurately distinguish the underground discontinuous electrical interface.Transient electromagnetic finite element reverse time migration imaging method firstly starts from the fluctuation equation,discretizes the spatial domain and uses the weighted residual method to establish the finite element control equation of the three-dimensional fluctuation equation in the time-space domain,and then uses the conditionally stable central difference scheme to obtain the finite element reverse time migration recursive formula.When solving the recursive formula of finite element reverse time migration,the element analysis of the calculation area is first carried out and then integrated into the overall matrix.For the large sparse matrix,the CSR storage format is used for storage and the pardiso solver in the MKL library is used to iteratively solve the equation.Finally,by constructing different layered models and typical threedimensional geological bodies for model testing,the migration imaging results are in good agreement with their corresponding theoretical models,which verifies that the transient electromagnetic finite element reverse-time migration imaging algorithm proposed in this paper can effectively reconstruct the spatial distribution information of subsurface materials.