In-situ stress state is a fundamental and important parameter in geoscience and geoengineering fields. To obtain a data set of current in-situ stress in a scientific drilling project carried out by Geological Survey of Japan (GSJ), National Institute of Advanced Industrial Science and Technology (AIST), Japan related to earthquake prediction, an attempt to determine three-dimensional stress tensor was carried out. We successfully applied the corebased method called anelastic strain recovery (ASR) method for two core samples retrieved from a depth of approximate 470 m in the Niihama-Kuroshima drilling sites in Shikoku Island, southwest Japan. As a preliminary result, the stress orientations determined by the ASR method were reported and compared with the hydraulic fracturing results. The three-dimensional principal stress orientation by the ASR method showed that stress regime in this site seems to be in-between strike slip and normal faulting stress regimes. The ASR measurements and hydraulic fracturing tests were consistent in terms of the maximum horizontal stress orientation.
Determination of in situ stress state at a large depth is a very important measurement issue in both the geoengineering and geoscience fields. Drilling is the most popular way to access into the deep part in underground and to conduct various in situ measurements, experiments, monitoring, and to get geological materials called drill core samples. Thus, developments of techniques to determine in situ three-dimensional in situ stresses by using core samples is necessary and important because there is no foolproof method by which both of the magnitudes and orientations of three-dimensional stresses can be reliably measured at large depths, although various in situ experiment and laboratory measurement techniques have been proposed. For scientific deep-drilling projects, the combined application of borehole methods and core-based methods is suitable to obtain a complete stress tensor and to enhance the reliability of the stress measurement results. As a corebased method, a simple and inexpensive method called anelastic strain recovery (ASR) method which has a relatively explicit theoretical basis was first proposed by Voight (1968) and further developed by Teufel (1983). These studies assumed that vertical stress was one of the three principal stresses and performed only two-dimensional measurements in horizontal plane.
