Moment tensor and stress inversion solutions of acoustic emissions during compression and tensile fracturing in crystalline rocks

Abstract

We investigate the accuracy and robustness of moment tensor (MT) and stress inversion solutions derived from acoustic emissions (AEs) during the laboratory fracturing of prismatic Barre granite specimens. Pre-cut flaws in the specimens introduce a complex stress field, resulting in a spatial and temporal variation of focal mechanisms. Specifically, we consider two experimental setups: (1) where the rock is loaded in compression to generate primarily shear-type fractures and (2) where the material is loaded in indirect tension to generate predominantly tensile-type fractures. In each test, we first decompose AE moment tensors into double-couple (DC) and non-DC terms and then derive unambiguous normal and slip vectors using k-means clustering and an unstructured damped stress inversion algorithm. We explore temporal and spatial distributions of DC and non-DC events at different loading levels. The majority of the DC and the tensile non-DC events cluster around the pre-cut flaws, where macro-cracks later develop. Results of stress inversion are verified against the stress field from finite element (FE) modeling. A good agreement is found between the experimentally derived and numerically simulated stress orientations. To the best of the authors’ knowledge, this work presents the first case where stress inversion methodologies are validated by numerical simulations at laboratory scale and under highly heterogeneous stress distributions.

Keywords

• Induced seismicity
• Acoustic emission (AE)
• Moment tensor (MT) inversion
• Stress inversion
• Finite element (FE) modeling