| The small-loop transient electromagnetic(TEM)method has great application prospects in national major projects such as urban underground space survey,shallow surface karst survey,Sichuan-Tibet railway geological survey and dam leakage in the South-to-North Water Diversion Project,which has the characteristics of high efficiency,small impact of the device,not limited by the site range and high resolution.As a new technology,it is a current research hotspot.However,the research on the forward theory and refined data processing method of this method is slightly lagging behind,which seriously restricts the application effect in shallow geological survey,making it a key problem that needs to be solved urgently.In this paper,a combination of theoretical analysis,numerical simulation and field experiments is used to carry out small-loop TEM method forward and inversion theory and application research,and the following conclusions are obtained:(1)For the small-loop TEM method forward response calculation problem,the electric dipole integral superposition method is proposed to solve it.The field source is divided into an infinite number of electric dipoles,the calculation expressions of the electromagnetic field components satisfied by the electric dipoles are deduced,and the forward calculation problem of the layered model of the small-loop TEM method is solved.Using Euler transform to perform frequency-time conversion calculation,the time consumption is only one tenth of that of the sine and cosine transform algorithm,and the calculation efficiency and accuracy are high.A layered model was established to analyze the attenuation law of the transient electromagnetic field of the small loop,and compared with the response of the large loop device.The results show that the small-loop TEM method can not only accurately reflect the abnormal information in the deep formation,but also be more sensitive to the detection of shallow targets.The research content provides the theoretical basis for forward modeling for the small loop transient electromagnetic method inversion research.(2)The three-component propagation law of transient electromagnetic field in the small-loop of typical geological anomalies is simulated and analyzed by using the finite difference time domain algorithm.The results show that: in a homogeneous medium,the propagation law of the horizontal components Hx and Hy of the magnetic field is like the wings of a "butterfly",spreading around;The same;when the abnormal body is located directly under the field source,the horizontal component of the magnetic field induces positive and negative responses,and the maximum value of the positive and negative responses is located at the end of the abnormal body,while the vertical component of the magnetic field induces a positive response;when the abnormal body is located to the left of the field source When the magnetic field is on the side of the magnetic field,the horizontal component of the magnetic field induces a negative response;when the abnormal body is located on the right side of the field source,the induction produces a positive response;the existence of the covering layer and the low-resistance layer delays the diffusion time of the transient electromagnetic field to the deep formation,and there is a low-resistance shielding effect.(3)In view of the problems of low approximate calculation and inversion accuracy in the "smoke ring" fast imaging method or the linear iterative method,it is proposed to apply the nonlinear optimization algorithm to the inversion calculation of the small-loop TEM method,using particle swarm and Firefly two optimization algorithms.The optimal parameter combination and convergence characteristics of the algorithm are analyzed,and an improved PSO-IFA hybrid optimization algorithm(PSO-IFAH)is proposed to solve the problems of low precision of PSO algorithm and low computational efficiency of FA algorithm.The results show that: compared with the PSO and FA algorithms,the improved PSO-IFAH optimization algorithm improves the calculation accuracy by 87.62% and the efficiency by 65.22%,and has the characteristics of high precision and high efficiency.(4)The "smoke ring" fast imaging method,the Occam linear iterative method,the PSO algorithm,the FA algorithm and the PSO-IFAH algorithm were applied to the small-loop TEM inversion.The influence of the anti-noise ability and the initial model search range of the three optimization algorithms on the accuracy and speed of the algorithm is analyzed.The results show that the inversion effect of the nonlinear optimization algorithm is better than that of the linear inversion method and the fast-imaging method;the improved PSO-IFAH optimization algorithm has high inversion accuracy and good stability for the overburden and low-resistance thin layer models;Model inversion processing.The research shows that the low-resistance anomaly has a volume effect,which makes the range of inversion results larger than the real model;affected by the low-resistance anomaly,the inversion results of the high-low-resistance anomaly model show a low-resistance anomaly;the improved PSO-IFAH algorithm The anti-noise ability is better than the FA algorithm and the PSO algorithm;when the model search range is small and close to the true value,the nonlinear optimization algorithm has high computational efficiency and accuracy;otherwise,the algorithm has low efficiency and large error;(5)Aiming at the search range of the initial model of the PSO-IFAH algorithm,the fast-imaging method and the linear iterative inversion method are used to constrain the calculation of the PSO-IFAH algorithm,and a refined FLCPSO-IFAH joint inversion method is proposed.The results show that the computational efficiency of the FLCPSO-IFAH joint inversion method is increased by more than 61%,the calculation accuracy is increased by more than 55%,and the algorithm accuracy and speed are further improved.Taking the measured data as an example for inversion calculation,in terms of inversion effect,the FLCPSO-IFAH refined joint inversion method has the best effect and accuracy,and the abnormal details are prominent.The research content solves the technical problem of fine inversion of small loop transient electromagnetic method,and provides an effective method for realizing fine detection and processing of shallow layers.(6)The small-loop TEM method is applied to the urban ground collapse emergency investigation project,and the FLCPSO-IFAH joint inversion method is used for refined data processing,combined with geological and drilling data for interpretation,and accurately and effectively reflects the superficial electrical properties.According to the distribution of the structure,the distribution of the bedrock surface and the dissolution development zone is inferred,and the main dissolution development zone is delineated.The research content is enriched and the theory of transient electromagnetic method is developed,which promotes the development of transient electromagnetic method technology,effectively solves practical engineering application problems,and provides professional technical guarantee for people’s life and property safety and social security and stability. |
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