High-resolution stress mapping using mine borehole data

The majority of in-situ stress information in the World Stress Map database comes from earthquake focal mechanisms, and petroleum regions where oil and gas industry technologies enable the collection of contemporary crustal stress information using borehole logs and tests. As a result, there is a limited stress data in many other areas, particularly in regions with low seismicity due to their tectonic settings or limited hydrocarbon exploration and production. In recent years, borehole image logs have become a standard tool in the mining industry as well, used for geotechnical and structural analysis. These logs provide a pseudo-image of borehole walls, allowing the characterization of stress-related deformations, such as borehole breakouts and drilling-induced tensile fractures, to better understand the present-day stress state.

We investigated the orientation of present-day horizontal stresses (S_Hmax and S_hmin) in various mine sites in Australia and Mozambique, inferred from the analysis of acoustic televiewer logs (ATVs) from over 1500 boreholes. This resulted in great understanding of in-situ stress orientation in regions with limited prior stress data. Unlike petroleum boreholes, where image log data is available for specific intervals (e.g., reservoirs), most open-pit mine boreholes are logged from near the surface, providing stress information from shallow depths and sometimes extending to 1.5 km. In addition, boreholes in mine industry have close spacing (sometimes less than 30 m apart) that provide a great opportunity to investigate the local variability of the stress state. It is e.g. possible to track rotations of the orientation of maximum horizontal stress S_Hmax near geological structures.

The S_Hmax orientations analysed at at the mine-site and basin scales in this study align closely with regional stress patterns, highlighting the role of large-scale tectonic forces as the primary drivers of crustal stress patterns. However, the high-resolution data used in this study — such as closely spaced boreholes (sometimes less than 30 meters) and S_Hmax orientation data spanning from near the surface to depths of 1.5 km — reveal small-scale S_Hmax rotations (ranging from 10° to 90° on a spatial distance of 1 to 100 meters) induced by stiffness contrasts, rock fabric, and geological structures. These small-scale S_Hmax rotations have significant implications for geotechnical and geomechanical applications across various disciplines.