Source Mechanisms of Acoustic Emissions due to Failure Around a Borehole in a Triaxial Lab Experiment

At the Norwegian Geotechnical Institute, a triaxial laboratory experiment was performed on a sample of Vosges sandstone. To study the development of borehole breakouts, a horizontal, borehole was drilled at mid-height through the sample. During the experiment, 12 piezoelectric receivers recorded waveforms of acoustic emissions. To locate events, the InSite software package (ASC) was used. Waveforms are complex and characterised by long coda waves produced by interaction of emitted waves with the surface of the borehole and the specimen’s walls. Often, no S-wave can be distinguished. In order to suppress effects of wave propagation, we perform a moment tensor inversion of P-wave first-onset amplitudes. For inversion, a subset of 162 high-quality events was chosen. Green’s functions were computed assuming a homogeneous velocity model. The source mechanisms basically coincide with the orientation of fractures developing in the sample. Since the waveforms are affected strongly by the presence of the horizontal borehole and probably also the specimen’s walls, a full waveform inversion and a 3-D velocity model taking into account the geometry of the sample would have to be applied. To this end, we will employ a spectral element method (SEM) to compute Green’s functions.