Acoustic emission studies on thermally and mechanically induced cracking in salt rock

Acoustic emission (AE) measurements have been carried out in two salt mines in Germany. In the Asse salt mine, microcracks were generated during heating up and cooling down of salt rock surrounding five heater boreholes. Acoustic emission sources were recorded by sensors positioned in separate boreholes around one heater borehole. The moment tensor method was applied to determine type and orientation of the sources. The moment tensors of about 2200 events indicate dominant shear cracking. The tension and pressure directions appear to agree fairly well with directions of independently measured maximum and minimum horizontal stresses. In the salt mine Bernburg, hydraulic fracturing tests have been performed aimed at investigating the stress state in rock. Acoustic emission analysis pointed out that fracture orientations depend on the orientation of the stress field at the test site. The measured pressures and the orientations of the fracture planes coincide exactly with FEM calculations of the stress field. The fracture orientation found in tests in virgin rock differs significantly from the orientation observed in the vicinity of large rooms or galleries.