Correlation Between Acoustic and Other Forms of Energy Emissions from Fracture Phenomena

Abstract

In the present investigation, acoustic (AE), electromagnetic (EME), and neutron (NE) emissions were measured during laboratory compression tests on rock specimens loaded up to failure. All the signals were acquired by a National Instruments Digitizer with eight channels simultaneously sampling. The aim was to find a time correlation between these three different forms of energy emission from rocks under compression. The tests were performed on magnetite and basalt specimens at constant displacement rate. AE signals were detected by applying to the specimen surface a piezoelectric (PZT) transducer with resonance frequency of about 150 kHz. EM signals were revealed by the current induced in a closed circuit due to change in the magnetic flux during specimen compression. The specimens were also monitored by means of a He³ proportional neutron detector. During the tests were first detected the AE signals, and then the EM emission. All the recorded signals were correlated to the load vs time diagrams. The EM signals were obtained, in particular, during the typical snap-back instabilities, which characterize the load versus displacement diagrams of brittle materials such as rocks in compression. Neutron emission signals were generally identified at the end of the tests. As a matter of fact, neutron bursts usually occur when the behaviour of the specimen in compression is particularly brittle. Applications of these monitoring techniques to earthquake forecasting seem to be possible.