| In the Transient Electromagnetic Method(TEM),the polarization effect often appears in the field data,and the apparent resistivity value cannot be directly calculated,which makes the resistivity inversion result unable to accurately distinguish the ore body position.Therefore,in order to obtain more accurate inversion results and obtain more abundant information about underground anomaly,polarization effects must be valued seriously.The polarization effects are mainly characterized by four parameters,such as zero-frequency resistivity,charging rate,time constant and frequency correlation coefficient.This paper adopted particle swarm optimization to extract polarization parameters,under the support of SQUID's key technology research on magnetic polarization-induction dual-field joint detection in the science and technology support program of Jilin Province.The main research contents are as follows:1)Based on the theory of TEM in uniform half space,the central magnetic field expression in frequency domain under the step wave is derived.Then the cosine transform filtering algorithm and convolution algorithm are used to derive the TEM response with arbitrary emission waveform.To obtain the electromagnetic-polarization response expression in the uniformly polarized half-space,the real resistivity in the above expression is replaced by the complex resistivity in the Cole-Cole model.Finally,the influence of zero-frequency resistivity,charging rate,time constant and frequency correlation coefficient on the polarization response is obtained.The larger the zero-frequency resistivity is,the smaller the time constant is,and the stronger the polarization effect is.2)Based on the electromagnetic-polarization response expression in the uniform polarization half space,the polarization parameter extraction problem is transformed into the minimum optimization problem,and the fitness function is constructed.Then a set of polarization parameters is selected to calculate the uniform half space.The electromagnetic-polarization response under the trapezoidal wave,and the single particle extraction and multi-parameter extraction were performed by the basic particle swarm optimization algorithm.The results show that the relative error between the theoretical value of the polarization parameter and the optimal parameter value is less than 5%,within the allowable range of error;the corresponding fitness curve is also close to zero.Finally,the theoretical data of Gaussian white noise is added as the measured data,and the optimized particle swarm optimization and basic particle swarm optimization algorithm are used to extract the parameters respectively.The results show that although both algorithms can achieve parameter extraction,the optimized particle swarm optimization algorithm perform faster at the computational speed and convergence.3)Based on the equivalent circuit model of the anomaly loop,the expression of the magnetic field response of the induced current in the circular anomaly loop at the center of the transmitting coil is derived.A terrain correction method suitable for SQUID is proposed.The relationship between the real magnetic field and the inclination of the terrain directly corrects the magnetic field,eliminates the influence of the terrain,and verifies the effectiveness of the terrain correction method from the theoretical model and the field experiment,respectively.The accurate explanation laid the foundation,for the inversion of the transient electromagnetic measurement data.4)The processing flow of superconducting transient electromagnetic data is summarized,and the polarization parameters of field data onto Inner Mongolia are extracted by basic particle swarm optimization and optimized particle swarm optimization respectively.The results show that the optimized particle swarm optimization algorithm is more accurate,and verify the effectiveness and applicability of the particle swarm optimization algorithm to the parameter extraction problem. |
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