Time correlation between acoustic, electromagnetic and neutron emissions in rocks under compression

Acoustic (AE), electromagnetic (EME) and neutron (NE) emissions were measured during laboratory compression tests on rock specimens loaded up to failure. 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 78 kHz. EME signals were revealed by the induced current in a closed circuit due to change of the magnetic flux during specimen compression. The specimens were also monitored by means of He3 proportional counter and neutron bubble detectors. The observed EME signals were strictly correlated in time with AE signals in all the tested specimens. Bursts of AE and EME activity were observed when significant stress drops occur. The neutron emission signals were mainly detected during stress-drops in the load vs. time diagrams, or at the end of the tests when the specimens showed catastrophic behaviors characterized by snap-back instabilities. Applications of these monitoring techniques to earthquake forecasting seem to be possible.