Three-dimensional FEM Numerical Simulation Of Borehole-ground Potential With Gradient Current Source

Borehole-ground potential method is an effective technology means to analyze the distribution of the residual oil in high water cut oil field and explore the concealed deposit in depths and margins of crisis mines. This method is also widely used in the survey of hydrologic geology, engineering geology and environmental geology. Borehole-ground poten-tial method with gradient current source that supply the wells high-power current with gradient layout of point current source, measure potential distribution in the surface or the wells, in order to study the changes of underground geological body how to affect distribution law of ground electric field or underground electric field. There is theoretical and prac-tical significance in the study of efficient and high accuracy simulations suitable for complex electric media. In this paper, the3D FEM forward of borehole-ground potential with gradient current source is mainly studied, and becomes the basis of the inversion imaging.The total potential is usually divided into normal potential and abnormal potential because of the singularity of potential in source. The influence of singularity in vicinity of the source was eliminated through calculating the abnormal potential, and the energy functional expression on the variation problem of the point current sources abnormal potential can be obtained which is the foundation of the finite element numerical calculation. We use the3D FEM forward with gradient current source whose the continuous conductivity among the nodes to simulate the complex underground geological conditions, and the abnormal response of the abnormal body on the surface is solved. In view of the features of the linear equations coefficient matrix formed in the solution process of the finite element method, SSOR-PCG (the symmetrical super relaxation preconditioned conjugate gradient method) combined with CSR (the compressed sparse row) technique is an effective method to solve the equations which improves the computational efficiency and the stability of the calculation results.Several different geoelectric models are calculated by the3D FEM of borehole-ground potential with gradient current source, the abnormal response characteristics and laws of the target geological body on the surface are obtained when the depth of power supply is different. With the advantages of high vertical resolution, we take advantage of distribution law of gradient potential difference which can effectively delineate the spatial location of the target geological bodies such as layered media, the concealed deposit in margins, and so on. All these research results have laid a good foundation for further inversion imaging of borehole-ground potential with gradient current source, also provide certain basis for field production practices.