Study On The 1D Forward And Calculation Method Of Resistivity Of Surface-to-Underground Transient Electromagnetic Method

The Surface-to-Underground transient electromagnetic method adopts the observation system in which signals are launched from ground and information are received from tunnel;While realizing the high-power launching,the receiving device is close to the target body and has a good coupling,and its exploration depth and abnormal resolution are high.However,at present,its data processing and interpretation level is not high.The lack of corresponding interpretation technology,greatly restrict the extension and application of the method.Therefore,in this paper according to the deficiencies of ground-Tunnel transient electromagnetic method in data processing and interpretation,the research on the definition and calculation method of one-dimensional forward and full-domain apparent resistivity of the Surface-to-Underground transient electromagnetic method is carried out;and the main research contents are as follows:(1)The one-dimensional forward algorithm of the Surface-to-Underground transient electromagnetic method is presented.Based on the electromagnetic field excited by horizontal electric dipole,the vertical magnetic field frequency domain expression excited by the rectangular loop of the layered model is given by using the idea of splitting and superposition.Then the time-frequency conversion is carried out by using the sine and cosine transform to realize the one-dimensional forward calculation of the Surface-to-Underground transient electromagnetic method.Finally,the accuracy of the forward algorithm is verified by homogeneous earth model.(2)The one-dimensional forward calculation of the Surface-to-Underground transient electromagnetic method was performed.The response characteristics of the vertical induction electromotive force and magnetic induction of a typical isotropic horizontal layered electric model with the spatial location changes of receiving point are analyzed: As the depth of the receiving point increases,the shape of the vertical induced electromotive force curve changes greatly,transiting from "direct descent" to "first rise and then descend",and maximum point appears.The maximum point appears delay at the time with the depth of the receiving point increases;the vertical magnetic induction curve shape is less affected by the spatial position changes of the receiving point,which is generally expressed as the change of the whole amplitude.The vertical induction electromotive force and the magnetic induction intensity are more influenced by the electrical parameters of the surrounding strata at the early stage.(3)The full-domain apparent resistivity of the Surface-to-Underground transient electromagnetic method is defined.Based on the analysis of the functional relationship between the vertical induction electromotive force and the magnetic induction intensity of the Surface-to-Underground transient electromagnetic method and the homogeneous earth model resistivity,based on the inverse function theorem,iterative algorithm is proposed to define the full-domain apparent resistivity of the Surface-to-Underground transient electromagnetic method by using vertical magnetic induction,realizes the fulldomain apparent resistivity calculation regardless of the distance or the time.In this paper,a typical horizontal layered model is used to validate the above-mentioned fulldomain apparent resistivity algorithm,and the response characteristics of the fulldomain apparent resistivity curve with the change of strata parameters are analyzed.The influence of the receiving point depth and the offset distance on the calculation results of the full-domain apparent resistivity of the Surface-to-Underground transient electromagnetic method is studied.(4)The detection capability of Ground transient electromagnetic method and Surface-to-Underground transient electromagnetic method was discussed.Taking the single low resistivity layer and the double low resistivity layer model as an example,the calculation results of the full-domain apparent resistivity show that: compared with the Ground transient electromagnetic method,the Surface-to-Underground transient electromagnetic method has weak detection ability to the shallow low resistivity layer,strong detection ability for the deep low resistivity layer,and time-depth conversion systems is different;Ground transient electromagnetic method and Surface-toUnderground transient electromagnetic can not effectively distinguish the double low resistivity layer,the Surface-to-Underground transient electromagnetic method mainly reflects the geological anomaly information near the measuring point.