3D Electromagnetic Forward Modeling Of Vertical Finite-length Wire Source

As we all know,the borehole-surface electromagnetic method is a high-precision electromagnetic survey method that utilizes existing mines or drillings and provides high-power alternating current for vertical finite-length wire in the wells.Its wide exploration area,large depth of exploration,and high resolution have attracted more and more attention.This article relies on the 863 subproject “Large power borehole-ground electromagnetic imaging system”.The source of the borehole-surface electromagnetic method is simplified as a vertically finite-length wire in the wells.the forward modeling study of vertical finite-length wire source electromagnetic fields is carried out to study the electromagnetic response of 3D electrical abnormality in the artificial source electromagnetic field,which will be used to lay the foundation for the electromagnetic three-dimensional inversion interpretation.In this paper,starting from Maxwell's equations,the analytical solution of the vertical electric dipole electromagnetic field is deduced by means of the vector potentiality.The electromagnetic field components of the vertical electric dipole are integrated along the wire direction to obtain the surface electromagnetic response of the vertical long wire source in the well.through a series of model calculations,compared with the one-dimensional forward CSEM program results of the electrical coupling of Kerry Key to verify the correctness of the analytical formula of the vertical electric dipole electromagnetic field.By comparing the electromagnetic field excited by the vertical electric dipole and the finite-length wire source,it is found that a certain number of electric dipoles can be used instead of the finite-length wire source.Secondly,we analyze several key factors affecting the response of vertical long-lead sources in a one-dimensional model,such as the resistivity of the underground reservoir,the relative position of the source and the reservoir,the arrangement of the measurement points,etc.It is found that the borehole-surface electromagnetic method is more sensitive to the high-resistance body,and when the emission source and the abnormal body are in the same horizontal line or the measuring point is located in the well,the borehole-surface electromagnetic method reflects the abnormal body more clearly.In the three-dimensional forward simulation,we use multiple vertical electric dipole to approximate finite-length wire source.Through simple model calculations,we compared the one-dimensional analytical solutions of electromagnetic fields to verify the correctness of three-dimensional forward modeling of vertical finite-length wire source electromagnetic fields.By comparison with horizontal finite-length wire source,we have found that the borehole-ground electromagnetic method has obvious advantages in terms of depth of detection and resolution,and proves the feasibility of the borehole-ground electromagnetic method in the deep and hidden areas.Finally,we studied the influence of some parameters on the electromagnetic response of the borehole-ground electromagnetic method in the absence of interference,such as the influence of parameters such as the length,position,emission frequency,emission current of the emission source,and abnormality resistivity.It is found that in the practical work of the borehole-ground electromagnetic method,we should try to make the source longer,the emission current larger,the emission source and the anomaly body at the same level as much as possible.At the same time,we must choose the appropriate transmission frequency to ensure that the ground can receive enough electromagnetic signals.The response,at the same time,laid the foundation for the next three-dimensional electromagnetic inversion interpretation of wells.