oa A New Method for Stress Orientation Measurements by Using Borehole Deformation and its Applications to the Active Faults

A new sensor tool for borehole deformation using a laser displacement sensor has been developed for stress orientation measurement at shallow depths. The new method for estimation of stress field using this tool is based on following principles: 1) The borehole drilled in a stress field is quasistatically deformed due to viscous property of rock around a borehole. 2) The orientation of maximum horizontal compressive stress (SHmax) is determined from a minor axis of an ellipse fitted to a deformed shape of borehole cross section. One of the advantages of this method to estimate stress orientation is a remarkably lower cost than existing methods, such as stress release and hydraulic fracturing methods. Radial displacements of the borehole wall under creep deformation are measured with the laser displacement sensor rotating continuously around a borehole axis. A total resolution of 0.7μm is achieved by stacking of several thousands data. A field performance test confirms that the new tool can detect the creep deformation, about ten μm increasing of the borehole radius in a measurement of half a day just after drilling. After the performance test, we applied this new method to active faults in order to evaluate stress state of the faults. In the measurement for the Kego fault in Fukuoka Prefecture, western Japan, the average orientation of SHmax is about N60°E for the two measurement point around the southern edge of the fault. The estimated orientation is found to be unfavorable for a strike slip on the Kego fault. In the application to the Atotsugawa fault, central Japan, the orientation of SHmax is estimated to be ENE-WSW by the measurements in a mine cavity near the fault. This orientation is consistent with the trend of the directions of P-axes of the focal mechanism solutions of ultra-micro earthquakes along the fault.