Monitoring of the extent of high temperature zone by resistivity tomography during in-situ heating test in the sedimentary soft rock

Abstract

One of the major issues in disposal of nuclear waste is that the long term behaviors of sedimentary soft rock can be affected by various environmental factors such as temperature or hydraulic conditions. Therefore, it is necessary to develop a method for evaluating the long term stability of caverns in sedimentary soft rock as subjected to changes of environment. We have conducted an in-situ heating test to evaluate the influence of high temperature to the surrounding rock mass at a depth of 50 m. As resistivity monitoring was thought to be effective to map the extent of a high temperature zone, resistivity tomography was conducted during the heating test. The results demonstrated that the resistivity of the rock mass around the heating well decreased and this low resistivity zone was gradually expanded with the heating. The relationship between resistivity and temperature can be explained by an existing empirical formula between aqueous solution resistivity and temperature. Resistivity may change by many other factors, but it is expected that resistivity change by the other factors is very little in this test. Hereby, it can be concluded that spatial distribution of temperature can be mapped by resistivity tomography. Consequently, resistivity tomography is expected to be one of the promising methods to monitor the high temperature zone heated by nuclear waste.