Broadband acoustic emission observations during fracture propagation in rock-like material

Abstract

Acoustic emission (AE) is used for monitoring fracture growth in many materials, including concrete and rock. Using a robust location algorithm allows event location from first arrival times to be determined by conventional commercial resonant sensors; however, commercial sensors modify the AE signals to a such a degree that quantitative analysis is all but precluded. Quantitative waveform analysis is required if microseismic signals are to be used to identify event magnitudes and specific fracture mechanisms. As part of a joint research effort between Shell, UIC, and UC on fracture mechanisms in rocks, we have developed a high-fidelity acoustic emission sensor specifically designed for use with rock. The sensors can be surface mounted or embedded directly into the material. This paper reports acoustic emission observations made with an array of these new high-fidelity sensors. AE signals were recorded during loading (fracture propagation) and unloading on plate grout specimens subject to splitting tests. AE events during loading were seen to originate primarily in a limited volume ahead of the crack tip, while unloading events generally came from the macroscopic crack. From first motion analysis, typical loading events appeared to be tensile in nature, while unloading events often had shear components. The median frequency and bandwidth of the loading events was seen to be greater than that of unloading events.