Monitoring the mechanical stress of rocks through the electromagnetic emission produced by fracturing

Abstract

The present note shows that the rock mass under stress and strain generates an ELF and VLF emission (EME) associated to the fracturing phenomena. On uniaxial stress this emission show two different modes. A disorderly succession of clustering EM pulses (DIS) is associated with the acoustics emission of fracturing. It is always manifested, culminates with the rupture and begins just before the 50% of the time necessary to break the rock with an constant increasing stress. In some cases, this phenomenon is preceded by a more orderly and homogeneous sequence of pulses EM (OIS), in this case not associated with the acoustics. The presence of water saturation influences the DIS EM emission. From our experimental observations results that the emission in the ELF and VLF bands is independent from the type of rock, which however condition the deformation history, and form the structural characteristics of the material. In a quarry have observed a similar phenomenon also on large stone masses under strain. These characteristics of the EME may be observed at great distances to monitor the preparation of an Earthquake; in fact radio waves as well as acoustic waves have their maximum propagation at the low frequency band.