RESISTOR NETWORK AS MODELING TOOL FOR FRACTURE DETECTION IN CRYSTALLINE ROCKS

摘要

The rock massif with sparse fracture network can be considered as highly inhomogeneous. The fractures enable electric current to flow in otherwise relatively non-conductive country rock. The measured apparent resistivity in such a medium strongly depends on the mutual position of the individual electrodes in relation to the conductive fractures. Commonly used 2D resistivity models are built up by rectangular blocks with finite dimensions and characterized by their resistivity values. The fractures are however very thin compared to the model block size. Electrical resistivity of fractures can be in this case modelled only by reduction of resistivity value in the individual blocks containing fractures. We have developed a resistor-network model where the medium is replaced by discrete 2D orthogonal resistors lattice. Instead of resistivity blocks this model is built up by individual resistors which are characterized by their resistance value. The proposed resistor network model can be used for evaluating the electric resistivity measurements in underground storage monitoring projects for the interpretation of the ERT measurements carried out directly on the surface of crystalline rocks with individual fractures. The model can be used for modelling the artificial conductors as well. The effect of fractures and fracture zones is presented together with an example from real measurement in an underground tunnel.