Application of Electrical Resistivity Tomography for Investigating the Internal Structure and Estimating the Hydraulic Conductivity of In Situ Single Fractures.

Fractures greatly impact the hydraulic and mechanical characteristics of fault zones; thus, the hydraulic characteristics of single fractures are critical for better understanding the hydrology of regional fault zones. A number of geophysical methods are applicable on the in situ field measurement scale. Electrical resistivity tomography (ERT) is a sensitive and nondestructive approach capable of imaging the spatiotemporal resistivity variations within a fractured medium. In this study, we conducted two water infiltration monitoring experiments along in situ single fractures within tight rock using the ERT method. First, we constructed a three-dimensional (3-D) variable-density model subdivided into tetrahedra identical to the in situ fracture and verified the feasibility and accuracy of the fracture-tracking ERT (FT-ERT) through numerical simulations. Second, based on the verification of FT-ERT, the fracture resistivity variations were obtained, and the spatial and temporal variations in electrical resistivity inside the fracture were used to delineate the internal water infiltration fronts. Finally, we depicted the infiltration fronts, calculated the water infiltration velocity, and estimated the fracture permeability. The electrical response characteristics of the fractured medium revealed obvious preferential flow during infiltration. Moreover, the results indicate that the internal structure of field fractures is discontinuous and heterogeneous and that FT-ERT monitoring can not only effectively capture the spatial structures of subsurface fractures but also provide quantitative data for assessing the water conductivity of discontinuous media with strong spatial anisotropy and heterogeneity. Studying the internal hydraulic characteristics of fractures is particularly valuable for understanding the water-conducting and water-resistive characteristics of fault zones. [ABSTRACT FROM AUTHOR]