Inversion of drilling-induced tensile fracture data obtained from a single inclined borehole

We investigate the feasibility of estimating an in situ three-dimensional str ess field by using data of drilling-induced tensile fractures (DTFs) observed in a single inclined borehole. The principal assumptions in this investigation are that the rock is isotropic, homogeneous and elastic. A DTF is a longitudinal crack consisting of many small parallel cracks which are oblique to the borehole axis. A DTF is characterized by its circumferential position (theta(mD)) along the borehole surface and the inclination (gamma(m)) of the small cracks with respect to the borehole axis. We show how it is possible to estimate the three-dimensional stress field by using the variation of theta(mD) and gamma(m) with respect to borehole orientation (i.e. azimuth and inclination of a borehole). Based on the variation of theta(mD) and gamma(m) as functions of borehole orientation which changes with depth, an inverse problem is formulated to estimate the three-dimensional stress field. Tests with synthetic data sets (theta(mD) and gamma(m)) show that it is feasible to estimate the three-dimensional stress field and that the statistical approach is appropriate for the inversion practically. Finally, we discuss a DTF data set (theta(mD) and gamma(m)) measured in a real borehole in the northern area of Japan Main Island and apply the inversion technique to estimate the stress field. (C) 1998 Elsevier Science Ltd.