| Transient electromagnetic method(TEM)is the most widely used geophysical method at present.And the central loop device is the most commonly used transient electromagnetic sounding device for its advantage of optimal coupling with the target and the characteristics of simple and light.Shallow detection has become a research hot spot currently.The transient early signal is affected by the turn-off time,displacement current,and the inherent process of the receiving coil,which makes the early signal severely distorted.Since multi-dimensional forward problems of engineering geology involves the spatial region decomposition and time-frequency domain transformation,the numerical calculation is difficult.Therefore,the multi-dimensional forward problems also need to be solved.For above issues,firstly,this paper derives the response of vertical magnetic dipole and obtains the 1-dimensional transient response of the central loop according to the integral approach of the large loop source.The key steps involved in the calculation,Hankel transform and G-S transform are discussed.The MATLAB simulations show that 140-node Hankel filter coefficient and 16-node G-S transform filter coefficient can lead to a high computational precision.Hence this method can be used in the calculation of time domain response for TEM.Based on the analysis of 1-dimensional forward,this paper analyzes the effect of turn-off time and loop length on the response.The results show that the shorter the turnoff time and the smaller the loop length,the more early signal in the response,which is more capable for the shallow detection.While achieving shallow detection,additional effects(displacement current effects,receiving coil transitions)cannot be ignored.The paper discussed this issue and reached the following conclusions: 1)The greater the frequency is,the more significant of the influence of the displacement current.The influence of the displacement current on the time domain response is approximately within a few tens to hundreds of nanoseconds.The shorter the turn-off time the greater the influence.2)Based on the equivalent circuit of the receiving coil,discussed the transition process under different responses,the study shows: In the case of step response,when ?=1,the response decays faster and there is no overshoot.And in the oblique step response,the larger the damping coefficient,the faster the induced voltage rises and falls.Besides,polarity-reversal occurs in the induced voltage of the secondaryfield before and after turning off,and due to the inherent transition process of the receiving coil,the actual observed voltage is a superposition of the primary field and the secondary field,which leads to a longer effective sampling time and the early signal cannot be detected.For the difficulties of the forward of high-dimensional transient field in complex geological structure,this paper uses Maxwell software to establish the simulation model.The result demonstrates that it has high simulation accuracy(within 2.5%)with appropriate parameter settings,which means it is feasible to carry out the forward of TEM with Maxwell.Then the receiving coil model is built and the transient process with different distributed capacitors in the oblique step response is analyzed.The checked response is correspond to the theoretical curve.Finally,the model of a low resistivity is analyzed to get dome-like induced voltage curve.In order to improve the calculation accuracy of inversion,DE and PSO hybrid algorithm is proposed.The algorithm has a dual-population evolution strategy and information exchange mechanism,which can reduce the “premature” of traditional PSO and DE algorithms and improves the accuracy and efficiency of the inversion calculation.This method is capable when applied in transient electromagnetic inversion of the central loop. |
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