Characterization of acoustic emission sources in a rock salt specimen under triaxial compression

This study presents the application of an automated moment tensor analysis procedure on microcracks, which were generated during a triaxial compression test of a cylindrical rock salt specimen (diameter 150 mm, length 300 mm). The acoustic emission signals were detected in a frequency range between 20 kHz and I MHz using 12 acoustic emission sensors mounted on the surface of the specimen cylinder. The moment tensor analysis was applied to about 30,000 events, which were precisely located using at least 16 P- and S-wave arrival times. For more than 40% of these events, approximately 12,500 events, stable moment tensor solutions could be evaluated using the first motion of the P-wave radiation patterns. Most of the evaluated events showed significant isotropic source components, which is in good agreement with dilatation of the rock during compressional loading. The majority of the events were caused by tensile opening leading to dilatation of the rock. The tension (7) axes, which are normal to the crack plane of these tensile microcracks, were predominantly oriented radially in the cylindrical specimen, perpendicular to the maximum principal stress, which is oriented axially. The direction of the tensile opening calculated by the moment tensor evaluation coincides very well with the direction of the minimum principal stress. The applied collapsing method discovers cellular structures with a cell size in the range of a few centimetres. However, it seems that the events occur only in zones where the cell interfaces are favourably orientated in the stress field. These events are attributed to cracking at grain interfaces which occurs in rock salt under very slow creep loading above the dilatancy boundary.