Displacement during Progressive Failure Estimated from Microseismic Activity

Abstract

An acoustic emission (AE) system with eight channels of recording was used to monitor progressive failure of a weak sandstone in plane-strain compression. The recorded waveforms were analyzed for event location and source mechanism. In the quantitative assessment of acoustic emission, a localized event is idealized as a point source of displacement discontinuity. The point source can be modeled as the combination of body forces that would produce the same elastic wave-field as the defect. The assumed condition of self-equilibrium requires each body force to be paired with an equal and opposite force to form a dipole of forces called a seismic moment tensor. In this study, the seismic moment tensor of each event was evaluated and the source mechanisms were determined through a calibration of the sensors, where the first peak of the signal recorded by an AE sensor was assumed to correspond to the first peak of the normal displacement at the sensor. The microseismic events generated during progressive failure of the sandstone were characterized as being caused predominantly by shear slip. Within a certain portion of the post-peak regime where 110 events were captured, the slip displacement was 0.0217 mm from the seismic estimate and 0.0335 mm from the global measurement. This suggests that the acoustic emission technique could be used to estimate displacement along a localized zone of failure where microseismic activity takes place.