A Review of the Anelastic Strain Recovery (ASR) Technique for In-Situ Stress Measurements: A Suggested Test Protocol and Further Challenges

ABSTRACT: Determination of in situ stress states is one of the most important issues for rock mechanics, rock engineering, and geomechanics, but difficult challenges in its measurement technique remain. In the past two decades, we have conducted more than ten case studies related in-situ stress measurements by a core-based method called anelastic strain recovery (ASR) technique in both oceanic and continental deep drilling projects. The case studies covered a wide range in terms of rock types, retrieved depths of core samples, and research backgrounds. Through learning by trial and error of the applications of the ASR technique, we have established a practical test protocol and data processing procedure for determining three-dimensional in-situ stresses from the ASR measurement data. In this paper, we review previous case studies of stress measurements using the ASR technique and summarize the detailed test procedures, and suggest a standard test protocol for both oceanic and continental deep drilling project. In addition, we will also address challenges for the future. 1. INTRODUCTION Stress state is a key parameter to understand plate tectonics, structural geological setting of subduction zones and to ensure safe and efficient developments of underground energy resources in great depths. Therefore, determination of in-situ stress states is one of the most important issues for rock mechanics, rock engineering, and geomechanics, but difficult challenges in its measurement technique remain. Stress measurements are carried out both in tunnels with large overburdens in civil and mining engineering and in deep boreholes for natural underground resources developments and for scientific investigations. Here, we focus on the in-situ stress measurements in both deep ocean drilling and continental (onshore) drilling in this paper. In deep boreholes, methods of stress determination can be divided as downhole measurements such as hydraulic fracturing / extended leak off test, stress analysis using borehole breakouts and drilling induced tensile fractures, and core-based techniques. As the core-based techniques, Anelastic Strain Recovery (ASR), Diametrical Core Deformation Analysis (DCDA), Acoustic Emission (AE), Differential Strain Curve Analysis (DSCA), Deformation Rate Analysis (DRA) have been proposed (Amadei and Stephansson, 1997; Zoback et al., 2003; Zoback, 2007; Zang and Stephansson, 2010; Ito et al., 2013). In the past two decades, we have successfully conducted more than ten case studies of in-situ stress measurements by the core-based method called anelastic strain recovery (ASR) technique in both oceanic and continental deep drilling projects in geoscience and/or geoengineering areas. In this paper, we review previous case studies of stress measurements using the ASR technique, summarize the detailed test procedure, and suggest a standard test protocol mainly for deep boreholes.