Hydrogeologic factors of geoelectric field and diurnal computation of preferred orientation of crack water seepage

Xichang and Tianzhu geoelectric field observational arrays were set up in 2007. In the following two years, TGF-A waveform appeared obviously in Xichang array, while TGF-B waveform was observed in Tianzhu array. Geoelectric fields at different stations in either of the arrays were highly correlated, which was supported by tidal mechanism of geoelectric fields. Tidal peak-to-trough values at different stations or in different orientations of the same station are obviously different, and this is closely related to such factors as rock characteristics, degree of cracking, preferred orientation of cracks, water ratio, seepage rate, degree of water mineralization and pressure difference of crack water, according to tidal mechanism and hydrogeologic data. Tidal electrical signals come from periodic movement of crack water or electric charge. When charge is adsorbed to rocks or breaks away from rocks, noise is produced. The signal-to-noise ratio appears almost identical within the same array, and it is closely related to the signal generating process and electromagnetic background on the site. Tidal harmonic amplitudes were used to compute preferred orientation of crack water seepage, which yielded the same result as with peak-to-trough value method, avoiding accidental error caused by taking peak and trough values. Before the Wenchuan M_S8.0 earthquake in 2008, there were short-term and impending variations of preferred orientation of crack water seepage in both of the arrays, but more obvious in Xichang array.