New methods for estimating shallow crustal stress orientation from borehole creep deformation after drilling

摘要

We have been developing two new methods for estimating crustal stress orientation at shallow depth,rnbased on a common principle to monitor elliptical creep deformation of a drilled borehole due to thernviscous property of the rock. One method is to measure directly deformation of the borehole wall justrnafter drilling for a few hours with a newly developed downhole tool of a laser displacement meter.rnCompilation of the past results of the measurements applying to 10 borehole sites, where more than twornmeasurements were performed at different depths, provides us statistical information about thernfluctuations of the stress orientations estimated by the present method. The result shows that 60 % of thernmeasurements are within 20 degrees from the mean principal stress orientation at each borehole. Wernintroduced a flattening parameter of measured ellipsoidal borehole deformation as an indicator of therndata reliability when the fluctuations are relatively large. This parameter seems to be effective forrndetermining a dominant stress orientation from the scattered results. The other method is to analyze thernlong-term deformations measured by a multi-component borehole strainmeter. We have analyzedrnlong-term 4-component strain data observed at 12 sites, southwestern Japan nearby the Nankai Trough,rnfor several years from a viewpoint of the stress relaxation process after borehole drilling. Time-varyingrnprincipal stress orientations were estimated throughout the entire observed period. The obtainedrnorientations are distributed from NE-SW to SE-NW at 12 sites. We compared the orientations from thernpresent method with the results estimated by the hydraulic fracturing and borehole breakout at the samernborehole, suggesting that this method has a potential to provide a crustal stress orientation with therndifferent principle than before.