Geophysical Research Abstracts, Vol. 5, 12188, 2003c© European Geophysical Society 2003

AN INVESTIGATION OF BRITTLE FRACTUREUSING CONTINUOUS ACOUSTIC EMISSIONMONITORINGB.D. Thompson(1), R. P. Young (1,2), K. Mair (1,2)(1) Department of Earth Sciences, University of Liverpool, UK, (2) Lassonde Inst., Universityof Toronto, Canada. (B.Thompson@liv.ac.uk)

Acoustic Emission (AE) investigations of brittle rock fracture in controlled laboratoryexperiments allow a detailed 3-D examination of the spatial and temporal evolution ofmicro-fracture growth. These observations provide valuable insights into the micro-mechanical processes involved in the development of macro-scale fault structures.

Processing techniques similar to those of Earthquake Seismology are employedto locate and resolve the focal mechanisms of AE events, however at this smallscale, sampling rates of 10 MHz are required. At these high sampling frequencies,technological constraints necessitate the use of triggering logic in conventional AErecording hardware. After an event is triggered, a ’downtime’ period is experiencedwhile the waveform data is downloaded. However, the AE rate increases to thousandsper second, prior to dynamic fracture propagation, and so the inherent periods ofdowntime result in a very low proportion of events being recorded. Therefore theobservational capacities of conventional AE systems are severely limited during thecrucial period of fracture growth.

The importance of recording a complete AE data set during the critical moments offracture prompted the development and manufacture of a novel ultrasonic monitoringsystem; The Giga Recorder. This system can record up to two minutes of waveformdata onto a 40 GB Random Access Memory (RAM), sampled at 10 MHz, on16 channels. The RAM continuously rewrites, meaning at any point in time, thepreceding two minutes of waveform data are stored, and can be downloaded following

the critical experimental period. In addition, triggered events are recorded throughoutthe entire period of AE activity.

Here we present results from the initial laboratory experiments using the GigaRecorder. A series of Chevron Notched Brazilian Disk granite samples were uniax-ially loaded to failure, under a variety of loading rates. During each test, a tensilefracture propagated from the pre-cut notch, across the sample. The continuous acous-tic records associated with macroscopic fracture have been processed, in both thetime and frequency domain to yield a unique data set, demonstrating the microscopicevolution of a tensile fracture.

Future experiments will examine shear fracture and mixed mode fracture propagationon the laboratory scale, and will be compared to in-situ field scale tests at theUnderground Research Laboratory (Atomic Energy of Canada, Ltd.).