Research On2.5D Forward Modeling And Inversion Technology Of Borehole-ground Array Electrical Resistivity Method And Application

Relative to the ground resistivity method, Borehole resistivity method can gain more abnormitythan the ground electrical method due to the detection electrode (including the current electrode andthe potential electrode) is close to the research target body, thus enhancing the detection depth;Meanwhile, Borehole electrical method can detect the research targets in different directions to obtainthe probe data of multiple directions, thus improving the detection precision and the longitudinal andlateral resolution. This method has been widely used in environmental geology, engineering geology,hydrogeology and mineral exploration, to get finer electrical structure characteristics of theunderground medium is an effective technology means for the city engineering geophysical、explorethe concealed ore in depths and margins of crisis mines and analyze the distribution of the residual oilin high water cut oil field.The traditional borehole resistivity method including borehole-to-ground、ground-to-boreholeand cross-borehole work style. Most of them adopt ground single channel (or two channels)instrument to measure, plotting the apparent resistivity curve along the borehole axis or the groundline in order to study the spatial distribution around the borehole medium, to expand the effectiveradius of drill hole, such as the discovery of borehole periphery and bottom hole deep blind ore,determination of the position、occurrence、shape、size of the ore body relative to the drill hole,recourse and delineation of ore body range of the ore body relative to the drill hole, determining thecontinuity of the ore bodies between the boreholes. The array of borehole-to-ground resistivitymethod in the paper mainly research point current which is in borehole or ground equal interval layoutarray electrode, including a variety of work ways such as borehole-ground(ground-borehole)、cross-borehole and borehole-ground-borehole, adopting the E-SCAN observation modemeasurements, you can collect intensive electrical measurement data from the borehole and ground. Afine subsurface electrical structure can be obtained through the2.5D electrical resistivity forwardmodeling and inversion of borehole-ground electrical resistivity method. The paper starts from the point source two-dimensional electrical medium, and details the basicprinciples of a point source of two-dimensional finite element method. Focus on improving the waysof borehole grid cell subdivision and the selection of wavenumber in the inverse Fourier Transform.By comparing the theoretical value and the results of numerical calculation of two ideal geoelectricmodel with homogeneous half space and vertical electrical contact surfaces to verify the accuracy andvalidity of the forward modeling. In terms of the inversion, establishing the inversion objectivefunction through the introduction of A.N.Tikhonow regularization principle, through adoptingreciprocity principle to calculate the Jacobi matrix, which makes the algorithm more simple. In theselection of regularization factor, several selection methods are presented, by comparison, the paperuses the method selected as the maximum row sum of the matrix product. The inversion results ofdifferent geoelectric model show that, this algorithm can better reflect the spatial location of thegeological body on geoelectric section,all plasmid and surrounding rock boundary are clearer. Finally,the borehole array resistivity method applied to detecting karst cave, crushed zone and water rich zonein subway construction of the distribution, through the selection of reasonable working parameters,fine process and interpretation, and after drilling verification, achieved a good application effects.