Frequency signatures of damage localisation

Abstract

Failure in geomaterials is often preceded by strain localisation and the related localisation of acoustic/microseismic/seismic events. Given that the localisation zone does not appear simultaneously – it is formed in a certain location and then propagates in a crack-like manner – the arrival times of the microseismic or acoustic pulses are synchronised. The aim of the study is to develop an indicator of the localisation based on the temporal synchronisation of the events. We found that the spectra of events generated by two- (2D) and three-dimensional (3D) localisation zones are shifted towards higher frequencies (‘blue shift’) as compared to the spectrum of randomly generated events. The indicator curve we propose is the relative difference between the frequency dependence of the energy of the power spectrum of arrival times of the recorded and pure random sequences of arrival times. The indicator curve for the 3D (cloud-like) localisation is always above the curve for the 2D (crack-like) localisation. The curves for these two localisation modes are above the indicator curve for one-dimensional localisation and for the randomly generated arrival times. When the pulses are recorded from a considerable distance and a number of recording stations are utilised, the comparison of the localisation indicator curves from all stations gives the information of the direction of propagation of the localisation zone provided that the length of the recording array is large enough.

Keywords:

• temporal localisation
• arrival time
• spectral energy shift
• frequency signature
• damage localisation
• geomaterial

Acknowledgements

This research was supported by ARC Discovery Grants DP0988449, DP0773839 and WA Geothermal Centre of Excellence. We are grateful to Chris Hellsten for discussions and suggestions.