Abstract

The purpose of the research was to investigate the feasibility of developing advanced acoustic emission (AE) monitoring for rock structure stability assessment. The equipment involves a laser interferometer based non-contact detection system to perform monitoring of AE activity in the process of rock deterioration.

Two laser based sensing units were tested in this effort, namely, the unit using photo-induced electromotive force (PI-EMF) and the one using two-wave mixing (TWM) technique. In the initial experiments, TWM technique showed superior performance and technical maturity for AE signal detection on rock surface, while the PI-EMF technique needs further development. The TWM sensing unit was also compared with conventional piezoelectric transducer in AE monitoring test on rock specimens. In these laboratory tests, TWM sensing unit showed excellent performance. The frequency band, fidelity and signal-to-noise ratio are competent for the projected use. The significance of these tests is that it demonstrated that the TWM sensing unit could detect AE signals with similar sensitivity and accuracy to that of the conventional piezoelectric transducers. Therefore, the laser-based sensing unit (TWM) can be incorporated for development of AE based rock stability monitoring system.

A series of acoustic emission tests were then conducted on specimens of rock, including weathered sandstone, shale and coal. The AE data were used for failure criteria development. The instrumentation for AE data collection has two channels: one is used for the laser-based TWM sensing unit; the other is used for conventional piezoelectric system as reference. Thirteen rock specimens (3 sandstone, 5 shale, and 5 coal) were tested under uniaxial compression for the purpose of collecting AE data for parametric analysis and pattern recognition study. Finally criteria were established for prediction of instability development in the rock specimens.

1. INTRODUCTION

  • AE/MS activity originates at locations where the material is mechanically unstable.

  • The AE/MS activity propagates through the medium undergoing attenuation as it moves farther from the source.

  • Using appropriate instrumentation, such activity may be detected at considerable distance away from the source.

  • Various characteristics of the observed AE/MS signals yield evidence of the type and degree of instability.

  • Observations obtained from a number of transducers (array) allow one to determine AE/MS source location.

1.1. Acoustic EmissionAcoustic emission, sometimes also called microseismic activity (MS), is a phenomenon which occurs as a result of various energy conversion processes inside a material. These include fracture development, relative friction between adjacent mineral grains, debounding, etc. Because the AE/MS signals of interest originate as a result of failure or defect processes, they carry direct information relative to these processes, no matter how much the dynamic behavior of the associated structure may modulate these signals. From the application point of view, the important concepts of AE/MS technique can be summarized as follows [1]:Based on these concepts, multi-channel AE/MS monitoring systems were introduced in civil and mining engineering as well as many other areas. These systems had a great impact on ground control and stability management of rock structures.

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