To determine three-dimensional stress orientation, we carried anelastic strain recovery (ASR) measurements out using drill core samples taken from a scientific ocean deep drilling project. The lithology of the core samples is mudstone or siltstone with larger porosities ranged from 35% to 45%. We glued strain gauges on their cylindrical surface, and successfully obtained high quality anelastic strain data in at least six directions. And then, we determined the three-dimensional stress orientations by the strain-time curves. The stress orientations obtained from the ASR core measurements were consistent with those from drilling induced borehole breakouts and tensile fractures observed in electrical image of borehole logging.
Following Ocean Drilling Program (ODP), the Integrated Ocean Drilling Program (IODP) begun from 2003. Deep drillings related with geodynamics such as seismogenic zone drillings are one of its important scientific targets. Therefore, determination of in situ stress state is an important and necessary research item in such ocean drilling projects. As an IODP scientific deep drilling project, NankaiTrough Seismogenic Zone Experiments (NanTroSEIZE) is undergoing in the southwest Japan subduction zone to understand the physics of an active fault (Kinoshita et al., 2006). Determination of current in-situ stress is one of the main scientific objectives of NanTroSEIZE. Unfortunately, there is no foolproof method by which magnitudes and orientations of threedimensional in-situ stress can be reliably measured at large/great depth, although various field and laboratory measurement techniques have been proposed. In the cases of ocean scientific deep drilling projects, we suggest that a combined application of borehole method (s) and core-based method (s) be employed. As one of them, a simple and inexpensive method to determine in-situ stress from anelastic strain recovery (ASR) measurement of oriented cores can be considered as having a relatively explicit theoretical basis in comparison to other core-based methods.
