An analysis of the seasonal variation of disturbance in georesistivity

Four types of disturbances in the seasonal variation in georesistivity are presented in the light of specific circumstances in China. As the results of theoretic calculation for the models and in comparison of the calculated results with the seasonal variations observed at 40 georesistivity stations, the following points are found: (1) The seasonal variation in georesistivity is caused by the change in resistivity of the surface layer, especially by the annual variation of phreatic level when the sounding depth is too shallow. This is one of the disturbances and thus has no relation to the origin of earthquake. The feature and amplitude of the seasonal variation depend on the types of disturbance models, geoelectric cross sections and sounding depth. (2) The seasonal variation in 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 types of geoelectric cross sections are concerned, the abnormal seasonal variation in georesistivity starts to appear with a certain electrode spacing, when the interference layer is located in the first layer. (3) Both abnormal and normal seasonal variations in georesistivity will be smaller than 2% when the sounding depth is greater than 300 m. Therefore, the seasonal variations can be removed or restrained when the location of observation has been properly chosen or the electrode spacing is enlarged enough to obtain a sufficient sounding depth.