Improved geoelectrical resistivity monitoring for earthquake precursors near the Xinfengjiang reservoir of southern China, with a fixed Wenner array of downhole electrodes in four separated boreholes

Abstract

Over the past five decades, a geoelectrical resistivity monitoring experiment using fixed electrode arrays has been conducted at Heyuan in the Guangdong Province of China for detecting possible earthquake precursors near the Xinfengjiang reservoir. A pair of surface quasi-Wenner arrays designed to monitor temporal resistivity variations have been used in two oblique directions of N27°E and N80°W with electrode spacings AB/MN of 997 m/315 m and 1032 m/328 m, respectively. To reduce the effects of near-surface environmental changes, an improved experiment has been introduced in August 1992, using a subsurface Wenner array in the direction of N80°W with electrode spacings AB/MN of 54 m/18 m, consisting of downhole electrodes in four horizontally separated boreholes at the same depth of 65 m. During the experimental period of August 1992 to December 1996, a continuously downward trend in apparent resistivity with the improved downhole array and a peculiar annual variation with the conventional surface arrays have been observed. For the same period, no notable earthquakes occur in and around the Xinfengjiang reservoir. The field tests indicate that the downward trend of apparent resistivity was due to the aging of one of the current electrodes. By using simple layered models, we develop a sensitivity analysis to design a monitoring array with minimum environmental effects and to explain the observed resistivity variations, such as a paradoxical resistivity behavior related with negative sensitivity. No significant resistivity variations, which could be attributed to earthquake activity of magnitude 3.4 or less in the study area, are detected. We prove that the two monitoring methods with the modified subsurface Wenner and the conventional surface quasi-Wenner arrangements can complement each other to efficiently estimate a change in the electrical properties of rocks and reduce the ambiguity in the interpretation of apparent resistivity data.