Acoustic Antennas for On-Line Monitoring in Very Deep Sea

Abstract

The long-term measurement and characterization of acoustic noise spectrum in deep sea is a powerful tool to study the underwater environment and the impact of human activities on the marine ecosystem. Despite sound is extensively used in military and scientific application, there is a lack of measurement of noise level and its variation over long term, in deep sea. This is mainly due to the difficulties in performing long- term accurate wideband measurements in such a hostile environment. Following the scientific and technological success of OvDE, a pilot experimental acoustic station that acquired and transmitted acoustic data from 2000 m depth for 2 years, we propose the realization of innovative deep-sea stations for long- term on-line monitoring of acoustic signals. The innovations of proposed project are summarized in seven key points: 1) pressure resistance up to 350 bar (corresponding to -3500 m depth); 2) capability to operate for more than 5 years; 3) capability to transmit data to shore from distances of about 100 km; 4) sensitivity close to Wenz minimum noise and dynamic range of -120 dB; 5) operational range of frequencies from few Hz to about 80 kHz; 6) capability of point-source identification and tracking; 7) modularity. In order to test and operate these stations the NEMO Collaboration built two scientific infrastructures equipped with shore laboratories and underwater electro-optical cables connecting shore with deep-sea. One is located 25 km SE of the port of Catania (Sicily), already used for OvDE; the other one is located at 3500 m depth 100 km South East of Capo Passero (Siracusa, Sicily), and will be soon operative. These antennas are expected to reach good sensitivity, to allow reconstruction of source position and permit pulse shape study of acoustic signals. These tasks will be fundamental in order to perform studies on sound sources in different research fields: geophysics, acoustical oceanography, bioacoustics, astro-particle physics et cetera.