ANALYSES OF THE SEASONAL VARIATION OF DISTURBANCE IN GEORESISTIVITY

Four models for the seasonal variation of disturbance in georesistivity are presented in the light of specific conditions in China. As the result of theoretic calculation of the models and in comparison of the calculated results with the seaonal variations observed at 40 georesistivity stations, the following points are found: (1) the seasonal variation of georesistivity is caused by the change in resistivity of the surface layer, especially by the annual variation of groundwater level when the sounding depth is too shallow. This is one of the interference changes and is irrelevant to the occurence of earthquakes. The feature and amplitude of the seasonal variation depend on the types of disturbance models, geoelectric sections and the sounding depth. (2) The seasonal variation of georesistivity usually has the same sign as that of the change in resistivity of shallow layer and thus the normal seasonal change appears. However, as far as the K or Q type of geoelectric section are concerned, the abnormal seasonal variation in georesistivity starts from a certain electrode separation when the interference layer is located in the surface layer. (3) Both abnormal and normal seasonal variations in georesistivity will be smaller than 2% when the sounding depth is deeper than 300 m. Therefore, the seasonal variations can be removed or limited when the location of observation is properly chosen and larger electrode .spacing is used, so that the sounding depth is large enough.