Long-term measurements of acoustic background noise in very deep sea.

The NEMO (NEutrinoMediterraneanObservatory) Collaboration installed, 25km offshore the port of Catania (Sicily) at 2000m depth, an underwater laboratory to perform long-term tests of prototypes and new technologies for an underwater high energy neutrino km3-scale detector in the Mediterranean Sea. In this framework the Collaboration deployed and successfully operated for about two years, starting from January 2005, an experimental apparatus for on-line monitoring of deep-sea noise. The station was Equipped with four hydrophones and it is operational in the range 30Hz–43kHz. This interval of frequencies matches the range suitable for the proposed acoustic detection technique of high energy neutrinos. Hydrophone signals were digitized underwater at 96kHz sampling frequency and 24bits resolution. The stored data library, consisting of more than 2000 hours of recordings, is a unique tool to model underwater acoustic noise at large depth, to characterize its variations as a function of environmental parameters, biological sources and human activities (ship traffic, etc.), and to determine the presence of cetaceans in the area.