Daily and annual electromagnetic noise variation and acoustic emission revealed on the Gran Sasso mountain

Acoustic emission (AE) measurements were performed at the Gran Sasso, in the underground National Laboratory, at about 1000 m above sea level and in the open, at about 2500 m above sea level. Stainless steel bars were inserted in rock-drill holes and acoustically coupled with piezoelectric sensors in order to test the existence of AE in the frequency range 0.1–1 MHz. By a very simple experimental procedure we succeeded in discriminating rare AE events due to elastic waves, from false AE (FAE) signals, due to electromagnetic noise. The reliability of this separation is confirmed by the reported amplitude and time distributions of AE events, typical of fracture dynamics and those of FAE events, similar to those of noise. While the FAE detected in the tunnel of the LNGS is clearly correlated with man made noise, in the open a systematic increase of number and intensity of FAE events exists, starting from sunset up to about midnight. The characteristics and underground or atmospheric origin of this FAE must be investigated.