Detection Of Bedrock Fracture Zones With Cross-Borehole Electrical Resistivity Tomography

Deep geological disposal is internationally recognized as a safe and feasible way of high-level radioactive waste disposal.Beishan,Gansu Province is a key pre-selected site for high-level radioactive waste disposal in China,where there are a large number of water flowing fractured zones.Water flowing fractured zone is an important channel for radionuclide diffusion to the biosphere.The identification of potential water flowing fractured zone provides an important reference for the site selection of high-level radioactive waste disposal.At the same time,it is of great significance to ensure ecology and life safety,accelerate the development of the nuclear power industry,upgrade the transformation of my country’s energy industry structure,promote green and sustainable development,and boost the construction of ecological civilization.Based on the electrical resistivity tomography(ERT)as the main mean,this thesis focused on the identification quality of cross-borehole electrical resistivity tomography on potential water flowing fractured zone in the rock mass.The model resolution of the conventional electrode arrays is poor,which can not meet the requirements of large-scale fine imaging in the field.In general,we improve the detection accuracy of conventional arrays by expanding the type and number of arrays,which is also called the arrays splicing method.The arrays splicing method is easy to implement and widely used.In recent years,the strategy of automatically selecting arrays has become a popular method for arrays optimization,and the most representative ones are the GF strategy and the CR strategy.Based on the analysis of the arrays splicing method,GF strategy,and CR strategy,this thesis implemented these three electrode optimization methods.Based on the geometrical factors,the Cross-borehole ERT arrays were selected by the arrays splicing method.The symmetry of electrode arrays sensitivity was analyzed,GF strategy and CR strategy are optimized,and the speed of the strategy was improved.The influence of different parameters such as iteration step,sorting function and orthogonal limit value on CR strategy were tested,and the CR strategy with the best sorting function and orthogonal limit value was obtained.The electrode arrays optimized by the arrays splicing method,GF strategy,and CR strategy were used to simulate the continuous and discontinuous fractured zones.Numerical experiments show that the arrays splicing method can effectively improve the resolution and has strong applicability,but the improvement effect lags behind GF strategy and CR strategy;GF strategy has less calculation time,and the improvement of model resolution in the early stage of the optimization process is comparable to CR strategy;the arrays selected by the CR strategy have the best imaging quality on the identification of the fracture zone,but the calculation time is longer.The influence of Beishan-scale factors on the identification of fracture zones was explored.When the resistivity ratio between the rock mass and the fracture zone is large,the imaging quality will be severely distorted.This phenomenon will become more serious as the distance between boreholes decreases.Increasing the borehole spacing will reduce the imaging resolution in the middle of the panel,and it will be more difficult to restore the resistivity value,but this phenomenon will be alleviated as the resistivity ratio between the rock mass and the fracture zone increases.Finally,the fracture zone was successfully identified by Cross-borehole ERT technology in Beishan site,Gansu Province,which is consistent with the rock coring results,indicating that the cross-borehole electrical resistivity tomography can achieve accurate detection of potential water flowing fractured zone in large-scale conditions.