Experimental Studies On The Changes Of Rock Resistivity Image And Anisotropy Under Fixed-Rate Uniaxial Loading And Unloading

The observational and experimental results show that earth resistivity and its anisotropy would change obviously and abnormally during the preparation and occurrence of a strong earthquake and large geological disaster, so earth resistivity has always been one of the important parameters used in earthquake motoring and prediction in our homeland. It has played an important role in earthquake motoring and prediction in nearly 50 years. Case Studies have reported that, prior to some strong earthquakes, not all earth resistivity recorded along every direction at a station has striking abnormal changes even if this station is very close to their epicenters via the fixed Schlumberger sounding array mainly used in current earthquake-related resistivity observations. This may lead to the increase in misjudgment on earthquake-related resistivity anomaly. Thus, exploring a new observation technique may be beneficial to supplement our current earthquake-related resistivity observation system and has the practical significance in earthquake prevention and disaster reduction in our homeland. In this paper, we apply the electrical resistivity tomography, a mature technique in geophysical exploration, to the experimental studies on the changes of rock resistivity image and its anisotropy. Apparent resistivity values were acquired during the uniaxial compression and unloading with a fixed rate on five sets of man-made samples. Then we constructed the resistivity and its anisotropy images at different stress, then studied their changes with stress, and discuss the possibility of resistivity image and its anisotropy abnormal changes as earthquake-related anomalies.In the process of uniaxial compression and unloading on five sets of man-made samples consisting of relatively homogeneous(sample Ⅰ)、built-in a high-resistivity block (sample Ⅱ、Ⅲ) and built-in a low-resistivity block (sample Ⅳ、Ⅴ) samples, we constructed the relative resistivity change (RRC) images corresponding to four radial measuring lines intersecting with the center of surface of a sample using electrical resistivity tomography, and plotted the curves of apparent resistivity anisotropy factor λ* and azimuthal angle of anisotropy axis a versus stress and depth. λ* and a represent the rate and direction of crack generation and/ or development respectively. Our results indicate that, (1) all RRC images show the same change trend with stress:with the increase of stress, the resistivity-decreased region (RDR) in the RRC images would shrink gradually, while the resistivity-increased region (RIR) would expand gradually. In the process of unloading, with the decrease of stress, the RIR continues to expand, and RDR continues to shrink. (2)The high- or low-resistivity block embedded in a sample has a great influence on the resistivity-increased amplitude just in its location and surroundings, but little effects on the trending change of resistivity image. (3)For the samples with originally electrical anisotropy, λ* decreases with the increase of stress; Cracks produce and develop mainly in the shallower part of a rock sample, while in the deeper part, the rate of crack production and/ or development is much lower. In this paper, our experiment results reveal that the rock resistivity image and its anisotropy have presented regular change stress, which shows that the abnormal changes of resistivity image and its anisotropy may be used as earthquake anomalies.