Deep-hole resistivity anomaly was observed in Heping geoelectric station before the earthquakes occurred in Heyuan. Different from the resistivity anomaly of most resistivity stations in China, the daily mean value curve of resistivity in Heping station shows long trend of decline in quiet periods while turns flat or rising before the earthquakes. In this paper, based on the geological and hydrological information near the station, a series of three-dimensional finite element models were constructed to analyze and verify the disturbance caused by the electrode polarization, the changes of moisture content and water table in the rock masses. The results of simulation indicate that 1) under constant stress in the quiet period, with the electrode polarization, the apparent resistivity has a downward trend; 2) before the earthquake, with the compressive stress increased, the comprehensive effect of rising high-conductivity particles and increasing water table will jointly lead to the flat or rising trend.
Earth resistivity observation is known as a resultful earthquake precursor method. In China, ground surface electrodes are widely applied in the apparent resistivity observation, and the electric current concentrates upon the surface of the earth and it is easily interfered by the environment changes such as temperature, rainfall, human activity etc. The resistivity observation with deep-hole electrodes is similar to the whole space measurement so it is efficacious in reducing the interference effect and increasing the observation accuracy (Liu et al. 1994).
Heping geoelectric station is located in Heping Village Heyuan city, about 6 km east of Hsinfengkiang Dam where the 1962 Ms6.1 earthquake occurred. This survey area is with smooth terrain and small relative height difference. This low hill red rock terrace with altitude 70 m is mainly composed of Cretaceous sandstone and conglomerate. The electrodes are more than 500 m away from west and southwest river.
Schlumberger array is used in this observation, the array orientation is N80°W, the source electrodes spacing is 54 m (AB) and the measuring electrodes spacing is 18 m (MN), electrode buried depth is 65 m (see Figure 1). The electrodes are made by lead with solid cylinder shape. Observation instrument is CAT-S digital earth-resistivity equipment developed by earthquake administration of Guangdong province, China.
