Investigation of fracture processes using moment tensor inversion technique

The development and progress of fractures is accompanied by the radiation of acoustic waves due to microcracking. In the transient waveforms of acoustic emissions a huge number of information about fracture processes and the state of the material is inherent. Signal-based analysis of acoustic emissions allows, amongst other studies, for a fracture mechanics based investigation of failure. From the spatial distribution of seismic energy, an inversion on a system of equivalent forces – the so-called seismic moment tensor – is performed. The inversion is simplified by the assumption of a point source and the neglection of near field effects, which means that the source volume is small compared to the distance between source and receiver. With the help of the moment tensor, arbitrary and complex source mechanisms can be described. For an interpretation of the results, the moment tensor is decomposed into simple fracture modes on the basis of an eigenvalue analysis. Furthermore, the appropriate radiation pattern of particle motion of the moment tensor is plotted on a sphere enclosing the source, revealing a 3Dvisualization of the stress field. These results and additional data from further investigations can be combined in a comprehensive representation of the failure. Basic principles of moment tensor inversion and some examples of experimental data and their analysis will be presented. The results will be discussed with regard to the influence of near field effects.