Time Domain Ground–airborne Electromagnetic Induction Polarization Simulation And Parameter Extraction

With the continuous improvement of China's scientific and technological level,there is a higher demand for mineral resources.The shortage of resources restricts the improvement of the economy and people's living standards.Therefore,there is an urgent need for an efficient means of mineral resources exploration to solve the resource shortage in China.In the ground-to-air electromagnetic detection,an electrical source is mainly used as a signal transmitting device to excite the earth to produce a secondary induction field by loading step current to obtain abundant underground electrical information.The surface to air electromagnetic method covers the advantages of the ground electromagnetic method and airborne electromagnetic method.It has high efficiency and wide coverage and is widely used in geophysical exploration in China.Three-dimensional numerical simulation based on the FDTD method is an important means to analyze electromagnetic response characteristics.At present,the time-domain 3D electromagnetic numerical simulation technology in China is mainly based on flat terrain,and only the existence of the induction field is considered.However,in the actual exploration process,more than half of the mineral resources are mainly distributed in the mountain jungle areas.Most mineral resources belong to polarizing bodies,such as graphite,copper,silver and other common mineral resources have obvious polarization effect.Therefore,in the process of numerical simulation,only considering the induction field in flat terrain and ignoring the existence of polarization field has great limitations.Also,because of polarizers,the traditional skin depth formula based on DC conductivity calculation is no longer applicable,and a new method for calculating generalized skin depth of polarized media is needed.Supported by the provincial science and Technology Department Project,"Research on Key Technologies of magnetic polarization induction dual field joint detection based on SQUID" and "Research on anomalous diffusion mechanism of time-domain electromagnetic detection based on fractional finite difference method" sponsored by provincial science and Technology Department,this paper performed time-domain electromagnetic induction polarization simulation.A time-domain 3D electromagnetic numerical simulation method based on wave-field transformation is proposed.For calculating the skin depth of the polarized medium,the generalized skin depth formula of the polarized medium is derived,and the skin depth of the polarized medium is calculated.When the underground medium has the polarization effect,only using DC conductivity for data interpretation will lead to serious deviation of interpretation results.A multi-polarization parameter extraction method based on particle swarm optimization is proposed to solve this problem,which realizes the parameter extraction and imaging interpretation of underground polarization information and improves the interpretation accuracy of measured data.The main research contents are as follows.(1)Based on the wave-field transformation theory,the Maxwell equations are transferred to the virtual wave-field using the corresponding relationship between the virtual wave field and the real diffusion field.In this paper,the integral transformation method is used to transfer the calculation results back to the real diffusion field.A 3D electromagnetic numerical simulation method based on wave-field transformation is proposed.The method is verified by homogeneous half-space,layered,and undulating terrain models.The characteristics of the electromagnetic response of the plate model,mountain terrain model,and sea-land interaction model are analyzed.(2)In the virtual wave field,the typical Cole-Cole model's governing equations are constructed.The control equations of the typical Cole-Cole model are solved to realize electromagnetic induction polarization simulation based on wave-field transformation.The theoretical model is used to verify the method's correctness,and the electromagnetic response characteristics of the three-dimensional polarized body model are analyzed.The influence of geometric parameters of time-domain ground to air electromagnetic measurement on polarization response results is pointed out.It provides an excellent theoretical basis for practical polarization measurement.(3)Based on the plane wave propagation theory,the generalized skin depth formula of polarized media based on the Cole-Cole model is defined.The influence of polarization parameters such as conductivity,polarizability,the time constant,and dispersion coefficient on skin depth under infinite frequency is analyzed.The frequency range of polarization parameters affecting skin depth is summarized.(4)To interpret the polarized media,a multi-polarization parameter extraction method based on particle swarm optimization(PSO)is proposed.The theoretical model is established,and the imaging interpretation is carried out.The theoretical model verifies the effectiveness of this method.The multi-polarization parameter extraction method based on particle swarm optimization is applied to Luanchuan and Lianyungang field experiments.The underground polarization information is extracted,and resistivity depth and polarizability depth imaging are realized.These ticks can significantly improve polarization data interpretation accuracy.