Continuous spatial stress variations within a granitic rock mass: Revealed by cross-sectional ellipticity of an array of boreholes

Résumé fourni par la source

Understanding the spatial variations of in-situ stress is crucial for optimizing subsurface resource development and mitigating geological hazards. However, due to the spatial sparsity of conventional stress measurement methods (e.g., hydraulic fracturing and overcoring), critical stress variations might be overlooked. To enhance the continuity of stress profiling, we utilize borehole cross-sectional ellipticity resulting from stress-induced borehole deformation to obtain continuous and abundant stress information. We developed a workflow to extract borehole cross-sectional ellipticity from the acoustic televiewer (ATV) logs, which provide high-resolution measurements of borehole cross-sectional geometry. Based on a borehole array in the Bedretto Underground Laboratory in Switzerland, we characterized the stress variations within a hectometer-scale granitic rock mass using borehole cross-sectional ellipticity. We found that each borehole exhibited significant variations in cross-sectional ellipticity along its depth, indicating strong heterogeneity of the stress field. Subsequently, we employed a grid search algorithm to invert the continuous stress variations along each borehole based on its cross-sectional ellipticity. The results indicate that the rock mass is generally in a normal faulting stress regime, but the stress orientations and relative stress magnitudes vary significantly. The causes of such stress variations could be related to local stress concentrations caused by fractures or stress perturbations resulting from fault slip. Our work provides a quantitative characterization of continuous stress variations within rock masses without direct stress measurements, which is beneficial to various geoscientific and subsurface engineering applications.

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