Three-dimensional Numerical Simulation Of Borehole-to-surface Electrical Method

Electrical exploration is a very important method of geophysical exploration,which plays an important role in the fields of deep structure,metal resource exploration,hydrology,and geological engineering.As for the traditional ground-excitation and ground-receiving mode,since the current density will decrease exponentially with increasing depth,the resolution will decrease rapidly as the buried depth of the target anomaly increases.Compared with the traditional ground-excitation and ground-receiving mode,borehole-surface electrical method supplies a large-intensity electric current into the well,and the current flows through the well into the deep underground,then the potential is measured at the surface,and the power supply electrode can be as close as possible to the target anomaly,and As the electrode depth changes,multiple sets of observation data from a study area can be obtained,which can greatly improve the imaging ability of the underground target body.This paper simulates the detection of coal mine water goaf,which has positive significance for the prevention and control of mine water damage accidents.Because the three-dimensional finite element forward modeling has problems such as large memory usage and long calculation time,it is still in the research stage.As a 3D simulation software based on finite element method,COMSOL Multiphysics plays an increasingly important role in the fields of acoustics,electromagnetics,fluid mechanics and optics with its reliable simulation effects and precision.In this paper,COMSOL software is applied to the numerical simulation of borehole-surface direct electrical method.By simulating the electric field response of three-dimensional underground medium of various geological models,the related scientific problems affecting the borehole-surface electric method that the length of excitation source and the position,scale,and resistivity distribution of anomalous body affecting are studied,which lays a solid foundation for field construction and data processing of borehole-surface electrical method in the future.This article is divided into five chapters.The first chapter is the introduction,which mainly introduces the research background and research significance of the thesis,the research status of the numerical simulation of the borehole-surface electrical method and the main research contents of the full text.The second chapter is the forward theory of the borehole-surface electrical method,introduces the basic principles of the steady current field,current continuity conditions,boundary conditions and so on,and the expression of resistivity at any point in the ground under the excitation of DC line source is derived.The third chapter mainly introduces the working method of borehole-surface electrical method and the process of establishing underground model using COMSOL Multiphysics software.The fourth chapter "forward numerical simulation results and analysis" is the focus of the full text.Firstly,the potential response and the analytical solution results under uniform half-space conditions are compared.The feasibility of numerical simulation of borehole-surface method using COMSOL software is proved.Then through different geological models for comparison,the parameters such as the length of the source,the depth,thickness and resistivity of the anomaly and the influence of the complex model on the apparent resistivity response of the borehole-surface electrical method are discussed respectively.In the last chapter,the main work results are summarized,and the future work is prospected.The paper uses the AC/DC module in COMSOL Multiphysics software to carry out three-dimensional numerical simulation of the underground medium,and verifies the effectiveness of using the software to realize the numerical simulation of the borehole-surface electrical method.At the same time,the low-resistance model,the high-resistance model and the complex geoelectric model are simulated in three-dimensional space,and Apparent resistivity diagrams of plane and profile are plotted respectively.The results show that the shallower the depth of the anomalous body,the thicker the thickness,the greater the difference between the resistivity of the anomalous body and the surrounding rock,the more obvious the apparent resistivity response;the longer the length of the silo,the more sensitive the low-resistance anomaly is;the numerical simulation results also show that the well geoelectric method can achieve effective resolution for complex anomalies and their combinations.