Research On Three-dimensional High-precision Frequency-domain Borehole-to-surface Electromagnetic Method

In order to overcome the defects of conventional surface electromagnetic exploration methods,frequency domain well-to-surface electromagnetic method proposes an electromagnetic exploration technology that combines the advantages of logging and conventional surface electromagnetic method.Compared with the conventional ground electromagnetic method,the emission source of well-ground electromagnetic method is closer to the target body,so it has more advantages in deep resource exploration and geological structure investigation in complex environments.Because the well excitation is closer to the target body,the interference of surface electromagnetic noise is greatly reduced,the imaging is clearer and the accuracy of underground detection is improved.However,due to technical and other reasons,the well-ground electromagnetic method has not been widely used in geophysical exploration.Therefore,it is necessary to carry out the forward simulation research of the well-ground electromagnetic method,aiming to lay the foundation for the future high-precision inversion imaging.Therefore,this paper intends to study the response of three-dimensional well-ground electromagnetic method and its influencing factors in frequency domain.Firstly,starting from Maxwell equations,the basic theoretical formula of frequency domain well-ground electromagnetic method is deduced in detail,the basic principle of well-ground electromagnetic method is expounded,and the simulation process of frequency domain well-ground electromagnetic method using COMSOL Multiphysics software is introduced.Secondly,by comparing with the analytical solution,the feasibility of using COMSOL software to simulate the frequency domain well-ground electromagnetic method is verified.A series of three-dimensional geoelectric models are designed to carry out forward simulation.In the three-dimensional space,COMSOL software is used to study the electromagnetic response of three-dimensional anomalous body under the influence of different emission frequencies,line source length and buried depth,anomalous body size and resistivity,anomalous body spacing and the combination of high and low resistivity anomalous bodies,layered strata and different terrains(peaks and valleys).Finally,taking the exploration project of a coal mine goaf in Shanxi as an example,the actual engineering data are interpreted and processed by using the wellground electromagnetic method,and the reasonable inference and conclusion are given on the scale and location of water accumulation in the coal mine goaf.The research shows that when the frequency is high,the exploration depth is shallow but the accuracy is high.When the frequency is low,the exploration depth is deepened but the accuracy is reduced.The response amplitude of underground abnormal body decreases with the increase of frequency,which conforms to the basic propagation theory of electromagnetic field.The increase of the excitation line source length can improve the detection effect of the abnormal body in a certain range,but when the line source length is too long,it is not conducive to the detection of the target body;By increasing the buried depth of the line source,the strength of the received signal can also be enhanced to a certain extent,which is conducive to the detection of the target anomaly.The influence of resistivity of different layered strata on the detection effect of underground anomalies is studied.It is found that the response amplitude of low resistivity anomalies under the influence of high resistivity layered strata is larger than that of low resistivity layered strata.For the case containing terrain,the abnormal appearance effect of the target body under the influence of flat or small-scale terrain(small peaks,small valleys)is better than that under large-scale terrain(large peaks,large valleys).The results also show that the frequency domain well-ground electromagnetic method can also effectively distinguish the simulated small anomalous body and the complex anomalous body combination.Compared with the high-resistivity anomaly body,the well-ground electromagnetic method is also more sensitive to the detection of low-resistivity anomaly,which is conducive to the detection of water accumulation in coal mined-out mines looking for low-resistivity anomaly.On this basis,the actual working effect of frequency domain well-ground electromagnetic method in Shanxi coalfield is good,and the position of goaf and water abundance in coalfield are accurately inferred.The numerical simulation and practical engineering results show that the frequency domain well-ground electromagnetic method is a high-precision exploration method for detecting mineral resources and coalfield goaf,which is of great significance to the actual production and prevention of mine water disasters.