Comparison of three source types for calibrating AE sensors used in fracture coalescence tests

Abstract

Fracture coalescence tests in rocks and rock-like materials have been extensively conducted in the laboratory over the past 40 years. Its mechanism has not been fully understood. Acoustic emission monitoring and moment tensor inversion technique cast a light in investigating the accumulation of micro-meso-scale fracture (damage) before the macro-fracture-propagation-stage of fracture coalescence. To this end, reliable sensor calibration is highly warranted to ensure the AE information is readily usable in moment tensor analysis. This paper presents and compares the calibration attempts by three types of sources, namely, breaking pencil lead source, ball drop source, and the source generated by the electrically energized piezoelectric sensor. By comparing these three source types for calibration, their advantages and disadvantages are identified. The ball drop source is the most suitable source among these three types of sources for the calibration in small scale prismatic rock blocks containing preexisting flaws for fracture coalescence tests. The polarity of the pencil lead source is its major disadvantage. The source generated by piezoelectric sensor has the highest repeatability among the three. However the aperture of the sensor and the strict requirement on the over-determined condition of the calibration equation system are the major problems in using it.