Source analysis of acoustic emissions in Aue granite cores under symmetric and asymmetric compressive loads

Details Source analysis of acoustic emissions in Aue granite cores under symmetric and asymmetric compressive loads Source analysis of acoustic emissions in Aue granite cores under symmetric and asymmetric compressive loads

Dec 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998geoji.135.1113z&link_type=abstract

Geophysical Journal International, Volume 135, Issue 3, pp. 1113-1130.

Astronomy and Astrophysics

Astronomy

21

Acoustic Emission, B-Values, Cracks, Fault-Plane Solutions, Fracture, Rock, Tomography

Scientific paper

The effect of stress anisotropy on the brittle failure of granite is investigated under uniaxial compression. Non-standard asymmetric compression tests are performed on cores of Aue granite (diameter 52 mm, length 100 mm), in which 20 per cent of the core top surface remains unloaded. The edge of the asymmetric steel loading plate acts as a stress concentrator, from where a shear rupture is initiated. The propagation of the fracture-related process zone from top to bottom of the core is mapped by microcrack-induced acoustic emissions. Compared to standard uniaxial tests with symmetric loading, in the asymmetric tests both a greater quantity and more localized distributions of emission event hypocentres are observed. The maximum event density doubles for asymmetric (20 events per 10-6 m3) compared to symmetric tests. The cluster correlation coefficient, a measure of strain localization in the faulting process, reaches 0.15 for symmetric and 0.30 for asymmetric tests. The clustering of events, however, is found post-failure only. Three different amplitudes are used to determine b-values discussed as a possible failure precursor. Focal amplitudes determined at a 10 mm source distance and maximum amplitudes measured at eight piezoceramic sensors lead to b-values that drop before rock failure. First-pulse amplitudes automatically picked from emission wavelets show no anomaly. First-motion polarity statistics of amplitudes indicate that a shear-crack-type radiation pattern is responsible for 70 per cent of the failure of granite, irrespective of stress boundary conditions. For type-S events with an equal percentage of dilatational and compressional first motions, focal mechanisms are determined by fitting measured first-pulse amplitudes to an assumed double-couple radiation pattern. While hypocentres of large type-S events align parallel to the later fracture plane, their fault plane solutions show no coherent pattern. Spatial views of fracture planes reconstructed from X-ray computed tomograms reveal local small-scale changes in fracture plane orientation. Nodal planes from average fault plane solutions of the microscopic acoustic emission events coincide with the overall orientation of the macroscopic fracture plane azimuth (strike angle) determined from thin sections and tomograms.

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