166 O. Borla et al. As regards the neutron emissions (NE), original experimental tests were performed by Carpinteri et al. on brittle rock specimens [29–36]. Different kinds of compression tests under monotonic, cyclic and ultrasonic mechanical loading have been carried out fully confirming the hypothesis of piezonuclear reactions, giving rise to neutron emissions up to three orders of magnitude higher than the background level at the time of catastrophic failure. These phenomena have important implications also at the Earth’s crust scale. Recent neutron emission detections by the authors [37] and other Russian researchers [38–41] have led to consider the Earth’s crust as a relevant source of neutron flux variations. In this work the authors, after summarizing the main laboratory experimental tests on gypsum samples, describe the preliminary results acquired at a gypsum mine situated in Northern Italy (Murisengo, Alessandria) and related to the evaluation of acoustic and nuclear phenomena. The monitoring system, based on the simultaneous acquisition of the various physical quantities, control the structural stability of the mine carrying out, at the same time, the environment monitoring for the seismic risk evaluation. Taking into account the relationship between AE, EME, NE and seismic activity, it will be possible to set up a sort of territorial database station that could be at the base of a warning network. This warning system could combine the signals from other stations to prevent the effects of seismic events and to identify the earthquakes’ epicentres. Similar networks are being utilized all over the World in countries like Mexico, Taiwan, Turkey, Romania and Japan [42]. However, these already established seismic monitoring systems are usually based on the kinematic quantities of the ground motion: displacement, velocity, and acceleration. The last, in particular, is proportional to the inertial forces transmitted by the ground shaking to the masses. On the other hand, the use of AE, EME, NE will represent a huge step forward, not only for their monitoring capabilities during the earthquake, but also for their forecasting potentialities before the event. Neutrons therefore appear to be as the most advanced earthquake precursor (up to 2 weeks before) [37–41]. An integration of AE/EME/NE data with CO2 and Radon variations, that are considered as additional seismic precursors [43], is currently planned and the ad-hoc instrumentation is under installation and testing. 21.2 Material and Methods 21.2.1 Acoustic Piezoelectric Transducer The AE activity emerging from the compressed specimens was detected by a piezoelectric (PZT) transducer glued on the external surface, resonant at 78 kHz, which is able to convert the high-frequency surface motions due to the acoustic wave into electric signals (the AE signal). The transducer sensitivity in the low-frequency range was measured by placing it on shaker excited by frequencies in the range 0–10 kHz (white noise). The result of this calibration at low frequencies was 1.2 V/(mms 2). Resonant sensors are more sensitive than broadband sensors, which are characterized by a flat frequency response in their working range, and then they can be successfully used in monitoring of large-sized structures [6, 7]. 21.2.2 Dedicated Loop and Telescopic Antennas EM signals were monitored using a device, calibrated according to metrological requirements, constituted by three winding loops with different number of turns that can be positioned around the monitored specimen. The working principle is based on the induction Faraday’s law. It states that the electromagnetic force (voltage) in a closed circuit (loop) is proportional to the change of the magnetic flux in the windings section. In fact, the three coaxial coils with an increasing number of turns are capable to perform the measurement from very low (Hertz) to high (MHz) frequencies in the magnetic field. The first coil, constituted by 5 turns, works in a frequency range from 300 kHz to 4 MHz. The other two coils constituted by 125 turns and 500 turns, work in the frequency range from 0 to 20 kHz, and from 0 to 1 kHz, respectively. Each turn is realized by a 0.2 mm copper wire, mounted on two coaxial PVC tubes embedded in a two components resin, in order to allow a large range of measurements. In addition, a telescopic antenna was tested in view of a permanent use at the gypsum mine. The employed antenna, having a maximum length of 125 cm, was coupled with an Agilent DSO1052B oscilloscope (50 MHz, 2 channels) that allows appropriate monitoring of EM signals with frequencies up to tens of MHz.
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