A Study On Faults Detection Using Electrical Resistivity Tomography Method

Fault zone is geological structures which are widely distributed in the earth’s crust. It breaks the continuity and integrity of rock mass, and can separate the dif-ferent aquifers; it therefore has a special significance in hydrology. In the bedrock area, faults are a dominant factor that controls groundwater occurrence and its migra-tion. In case of high-level radioactive waste disposal, fault zone is an important channel for the movement of radio nuclides to the biosphere, and therefore it is an important to study the fault location and its water transmissibility of repository in the evaluation of repository performance. Investigation of the location, occurrence, and the width of the fault may provide basic information for fault hydro-geological characteristics.This paper mainly presents:(1) simulation of faults zone at laboratory and in-jection of water on one side of the fault, the resistivity changes were monitored us-ing high-density electrical resistivity tomography method during water injection and draining process of cross-fault profiles,and discussed the transportation and distribu-tion of water in the fault zone;(2) establish the physical model of heat conduction in the fault zone at laboratory, and use of high-density electrical resistivity tomography method to monitor the apparent resistivity section changes caused by heat conduction and evaluate heat conduction process in the fault zone;(3) established different width and depth physical model of fault zone at laboratory to study its anomalous characteris-tics via laboratory experiment, and established different angle mathematical models of the faults to study its anomalous characteristics through forward simulation, and devel-oped the same depth, width, angle mathematical models of the faults but using different electrode space to study its anomalous characteristics with forward simulation;(4) the exploration work in the radioactive waste disposal on F4fracture at Gansu Beishan preselected area, obtained three across fracture of high density resistivity profile, and the image of resistivity distribution of three profiles were obtained by removal of ab-normal data to carry out the inversion, combined with the actual geological conditions analyses the abnormal characteristics of F4fracture, and width, angle of the fault were also calculated.From the above studies following results are obtained:(1).Analysis of the resistivity change on cross fault profile during water injec-tion, the process and the distribution of water in the fault migration was obtained, water movement in fault reflected by apparent resistivity section consistent with that observed phenomena, indicating that it is feasible technique, using high density resis-tivity method to monitor fault zone water injection process. Water will give priority to saturated fine grained medium, when water level in the fine medium is higher than that of the capillary pressure it can support, water will flow toward coarse medium and form a fixed water level difference between the two sides. When the groundwater flow direction is perpendicular to the water blocking faults, it will form a difference water level on both sides of the fault zone, and the size of the level difference can be used as evaluation of water resistance ability of the fault zone.(2) The results from the heat conduction process monitoring indicate that the heat current will change because of the existence of fault. So it is possible to provide the reference for fault zone hydro-geological study by monitoring the cross fault resistivity changes influenced by seasonal temperature variations. But due to the influence of sea-sonal temperature change is in the range of a few meters to tens of meters underground, it is therefore limited to the shallow depth scale.(3) The results of different width and depth of the fault zone assessment at lab-oratory scale shows that the larger the width, the more shallow the depth of the fault zone and the greater the abnormal apparent resistivity, otherwise abnormality would be smaller. The result of different techniques for the detection of different angle fault zone through forward simulation shows that the dipole-dipole, differential and Wenner method all can reflect fault zone abnormalities. Dipole device probing depth is largest, followed by differential methods, and Wenner method’s depth is not that much deep, but Wenner method has best detecting effect.(4) Analysis the inversion result of f4fault resistivity profile, combining with the actual geological conditions, determined the relationship between fracture zone and resistivity profile of f4, abnormal characteristics of f4fault in the resistivity profile is the southeast tilt and low resistance banded anomalies, occurrence and broadband information obtained by calculation, f4fault width is30-40m, dip angle is of about60degrees.