Tensor CSAMT Three Dimensional Forward Numerical Simulation

The most commonly used observation method of controlled source audio magnetotelluric method(CSAMT)is scalar observation.It is easy to determine the linear fault structure by using scalar measurement data,especially when the measurement dipole direction is just perpendicular to the fault(TM mode).This method can meet the needs of exploration.With the increasing complexity of exploration targets,this measurement method has been unable to meet the needs of high-precision processing and interpretation.The scalar measurement method can achieve good inversion interpretation results for simple one-dimensional and two-dimensional geoelectric structures,but for more complex three-dimensional geoelectric structures,especially when the formation has strong electrical anisotropy characteristics,full impedance tensor information is needed to obtain the complex structure or anisotropy information of the formation.Therefore,it is necessary to carry out three-dimensional forward modeling of tensor CSAMT with higher interpretation accuracy,which will lay a foundation for three-dimensional inversion of tensor CSAMT and accurate interpretation of electromagnetic data in the future.Firstly,the 2.5-dimensional CSAMT electromagnetic field coupling equation is derived in detail by using the Galerkin finite element method,and the 2.5-dimensional CSAMT finite element numerical simulation based on the total field is realized.The accuracy of the algorithm is verified by comparing with the analytical solution of uniform half space and layered model.The numerical simulation of the two-dimensional anomaly model has achieved good results.The Cole-Cole model is introduced to discuss the influence of each polarization parameter on CSAMT results.Secondly,the finite element method is used to derive the three-dimensional CSAMT electromagnetic field solution equation based on arbitrary anisotropic media in detail.The working principle of tensor CSAMT method and the specific process of three-dimensional CSAMT numerical simulation by COMSOL Multiphysics software are introduced.The reliability of numerical simulation is verified by comparison with analytical solution and previous calculation results.Finally,a series of three-dimensional models are numerically simulated by COMSOL Multiphysics software.The effects of different polarization directions of the emission source on the tensor and scalar CSAMT results,the response characteristics of the impedance tensor and the tipper vector of different models,the effects of electrical anisotropy on the tensor and scalar CSAMT results,and the effects of induced polarization on the tensor CSAMT results are discussed.The results show that when the underground is a one-dimensional medium,the tensor is close to the scalar CSAMT calculation result;when the underground medium is three-dimensional,the tensor CSAMT is almost not affected by the polarization direction of the emission source,and can accurately describe the horizontal boundary of the abnormal body.This observation method is more suitable for practical work.Impedance tensor and tipper vector have unique advantages in characterizing the horizontal boundary of anomalous bodies.The existence of terrain has a certain influence on the electromagnetic field response of tensor CSAMT.It is suggested to consider the influence of terrain in the process of actual data processing and interpretation.The response of tensor CSAMT is sensitive to the resistivity of the principal axis in the x and y directions,and is not sensitive to the change of the resistivity of the principal axis in the z direction.When the value of the principal axis resistivity is close to the background resistivity,the tensor CSAMT can still clearly describe the horizontal boundary of the anomalous body.Tensor CSAMT is sensitive to the changes of three Euler angles.Compared with the scalar observation method,the tensor CSAMT method has more advantages in the identification of anisotropic media.The induced polarization effect will have a serious impact on the results of tensor CSAMT.Therefore,the influence of induced polarization effect should be considered in the processing and interpretation of CSAMT actual data.