Study On 1D Forward And Inversion Algorithm Of Grounded Source Transient Electromagnetic

Transient electromagnetic method(TEM)has been widely applied in shallow resource exploration and engineering geophysical prospecting.Unlike the electromagnetic method in frequency domain,TEM detects underground targets by collecting the secondary field response in time domain.The computation of electromagnetic response in time domain is more complicated than the frequency domain,and the computation is huge.Forward and inversion of 2D and 3D TEM are still in the theoretical research stage,and so in actual production TEM is still mainly based on 1D forward and inversion.On the one hand,anomalies response of secondary field in the time domain are relative weaker than in the frequency domain,which puts higher requirements for calculation precision and calculation speed of 1D forward and inversion.On the other hand,some factors such as the complexity of the time domain emission waveform,high requirements for the performance of the instrument,the relative complexity of theory,high requirements for theoretical requirements of field workers,have restricted the popularization and application of the transient electromagnetic.Therefore,it is of great significance to further study the 1D forward inversion method of TEM,to improve its calculation precision and speed,and to find out the propagation characteristics of the electromagnetic field in the time domain.In this paper,starting from the introduction of the Maxwell electromagnetic field equation set,analytical expression of transient electromagnetic forward response of electric dipole in the case of uniform half space and layer is derived in the second chapter,and at the same time,Implementation method of getting frequency domain analytic formula through Hankel transform and conversion algorithm from frequency domain to time domain are discussed.A hybrid optimization algorithm combining Guptarsma algorithm and polygonal line approximation is proposed in this paper.Both of them complement each other and improve the computation efficiency and forward precision and compare the two.In the third chapter,based on the forward algorithm of electric dipole,the realization methods of electrical source of conducting wire are studied and compared.On the basis of the forward of electrical source,the propagation law of electromagnetic field of underground and surface is discussed respectively,which provides a reliable reference for how to collect the signal of the electromagnetic field component to improve noise-signal ratio.Then,the law of forward response of different parameters is discussed,and the characteristics of transient electromagnetic response of different geoelectric sections are calculated.Finally,the early and late apparent resistivity of transient electromagnetic is studied,and the translation algorithm and dichotomous method are used to obtain the apparent resistivity of the whole region,which is compared with the early and late apparent resistivity later.In the fourth chapter,the 1D inversion algorithm of transient electromagnetic is mainly discussed.The least squares constrained inversion of transient electromagnetic damping is realized,and the influence of various factors such as the distance between the transceiver and the initial model on the inversion results is discussed.Besides,the weighted constraint method is introduced,and the 1D inversion of several theoretical models is used to verify the correctness of the inversion method.