Study And Application Of 1D Forward And Inversion For TEM Of Long Wire Source

Transient electromagnetic methods(TEM)is a time domain electromagnetic method,which utilizes ungrounded loop or grounded-wire to send pulsed magnetic field(primary field)toward the underground,in the period the first field's intermission,pure secondary field is received by coil or grounded electrode.The received secondary field carries geoelectrical information about the underground geological body,Therefore,the purpose of detecting subsurface geological bodies can be achieved by processing and analyzing these secondary field data.According to the difference of excitation field source,the transient electromagnetic method can be divided into magnetic source and electric source.Among them,the electrical source transient electromagnetic method is one of the faster developing electromagnetic methods in recent years,especially playing an increasingly important role in the exploration of mineral resources in the middle-deep region.In order to enhance the amplitude of the received signal and improve the signal-to-noise ratio in actual exploration,electrical source transient electromagnetic method usually utilize sufficiently long grounded wires to increase the electrode distance,this measurement method is generally referred to as long wire source TEM.This method has the characteristics of great working efficiency,large exploration depth,high resolution,and sensitivity to low-resistance objects,compared to magnetic source transient electromagnetics,it is more suitable for complex terrain such as mountains and swamps,in recent years,it has played an increasingly important role in many kinds of fields such as mineral exploration,energy exploration,hydrology,engineering and environment.In this paper,one-dimensional forward modeling and inversion of Grounded-wire transient electromagnetic method are studied,and this method has been successfully applied to the interpretation of long wire source transient electromagnetic data in graphite mine.Firstly,from the Maxwell equations,frequency domain response expressions of grounded-wire transient electromagnetics in horizontal layered media are derived.Through frequency domain solution,outer layer integral calculation and time domain conversion numerical calculation process,the response expression of long wire source transient electromagnetic method in the time domain is solved,and all-time apparent resistivity is defined by the translation algorithm.Secondly,On the basis of numerical calculations to achieve grounded-wire transient electromagnetic response and verify the accuracy of the calculation method,The influence of variations in formation parameters and device parameters on the transient electromagnetic response of long wire source transient electromagnetic is studied.Thirdly,the damping least squares algorithm is studied and implemented in grounded-wire transient electromagnetic inversion.The correctness and applicability of the inversion method are verified by geoelectric model calculation.Finally,the grounded-wire transient electromagnetic method was applied in the exploration of graphite ore body and achieved remarkable results.It proved that the accuracy and validity of the previous theoretical research,also proved that the scientific and feasibility of this method in the exploration of mineral resources in the middle-deep region.