Three-dimensional Anisotropic Forward Modeling And Inversion Research Of CSEM In Frequency-domain

Controlled source electromagnetic method is a kind of artificial source low frequency electromagnetic exploration method used in geophysical exploration.With transmitting electromagnetic wave through the dipole source and receiving total electromagnetic field response,we can get the electrical information of subsurface structure.In the past,in order to alleviate the non-uniqueness of inversion and save the computational cost,most of the domestic and foreign three-dimensional controlled source electromagnetic forward and inverse methods are based on the assumption of electric isotropic underground medium.If we still use the traditional isotropic modeling and inversion method under this situation,the inversion results will be vulnerable to be affected by the observe response which containing anisotropic information and easily produce inaccurate or even false isotropic anomaly around the original abnormal bodies.To solve this problem,this paper mainly focuses on the development of an efficient and stable 3D anisotropic CSEM algorithm which could be applied on land and marine exploration.In order to calculate the forward modeling response of complex three-dimensional medium,we firstly separate the total electric field into primary field which is transmitted by the source directly(background field)and secondary field produced by the abnormal bodies.The primary field could be calculated by one dimensional Green's function in layered medium while the secondary field is based on the Maxwell equations and boundary conditions.After deducing the finite difference discrete forward equations,we can introduce the anisotropic conductivity through current density into the equations,and finally solve the staggered partial finite differential equations.On the basis of the anisotropic forward modeling method,this paper proposed athree dimensional CSEM triaxial anisotropic inversion algorithm and apply it on theoretical models.The inversion results are acceptable.The main reason for choosing triaxial anisotropic conductivity is that there are few independent conductivity components in it which could alleviate the non-uniqueness of inversion and get reliable inversion results.On the other hand,the triaxial anisotropic model is also suitable for the theoretical analysis for typical geological conditions.Moreover,this paper tested the inversion effect and compared two different transmitter-receiver configurations in order to verify the validity and stability of the algorithm.