Abstract
The NEMO-SN1 (Neutrino Mediterranean Observatory - Submarine Network 1) seafloor observatory is located in the central Mediterranean Sea, Western Ionian Sea, off Eastern Sicily Island (Southern Italy) at 2036 m water depth, 25 km from the harbour of the city of Catania.
It is a prototype of a cabled deep-sea multiparameter observatory and the first operating with real-time data transmission in Europe since 2005. NEMO-SN1 is also the first-established node of EMSO (European Multidisciplinary Seafloor Observatory), one of the incoming European large-scale research infrastructure included since 2006 in the Roadmap of the ESFRI (European Strategy Forum on Research Infrastructures). EMSO will specifically address long-term monitoring of environmental processes related to Marine Ecosystems, Climate Change and Geo- hazards.
NEMO-SN1 has been deployed and developed over the last decade thanks to Italian resources and to the EC project ESONET- NoE (European Seas Observatory NETwork - Network of Excellence, 2007-2011) that funded the LIDO-DM (Listening to the Deep Ocean - Demonstration Mission) and a technological interoperability test.
NEMO-SN1 is performing geophysical and environmental long-term monitoring by acquiring seismological, geomagnetic, accelerometric, physico-oceanographic, hydro-acoustic, bio-acoustic measurements specifically related to earthquakes and tsunamis generation and ambient noise characterization in term of marine mammal sounds, environmental and anthropogenic sources. A further main feature of NEMO-SN1 is to be an important test-site for the construction of KM3NeT (Kilometre-Cube Underwater Neutrino Telescope), another large-scale research infrastructure included in the ESFRI Roadmap constituted by a large volume neutrino telescope.
The description of the observatory and the most recent data acquired will be presented and framed in the general objectives of EMSO.
Purpose
The scientific objectives of the NEMO-SN1 observatory cover a wide range of research activities in deep sea: High Energy Astrophysics, Physical Oceanography, Bio-acoustics, Environmental Sciences, Geophysics and Geo-Hazards [Favali et al., 2010] contributing to answer to the societal needs [Ruhl et al., 2011].
Consequently, the large set of sensors installed in the observatory, described in the following sections, refers to this large variety of discipline. Of particular relevance is a prototype of tsunameter specifically designed to operate in tsunami generation areas, able to detect also hydroacoustic signals that can be possible precursor of tsunami generation [Chierici et al., 2010]. The tsunameter is based on the contemporary screening by an original software of the signals recorded by the absolute pressure gauge, seismometer and accelerometer.
The Western Ionian area, where NEMO-SN1 is located, is one of the most seismically active areas of the Mediterranean and for this reason is very appropriate for the tsunameter operation assessment. In fact, some of the strongest earthquakes (M7+, in the years: 1169, 1693, 1908) caused also very intense tsunami waves.
For the first time, studies based on correlation between tsunameter alert signals, magnetometers signals and low frequency acoustic waves -recorded by deep-sea hydrophones- will be carried on, with the aim of improving reliability of the tsunami early alert system in this prone area for tsunamis. Another interesting feature of the area is the vicinity of Mount Etna, one of the biggest and active volcanoes in Europe. The roots of this volcano possibly sink down to seafloor depth but the actual extension is almost unknown and some geophysical measurements at sea, together to analogous ones on land, can strongly help to gain a better knowledge.
The NEMO-SN1 deployment area is also a key site for the oceanographic circulation between Eastern and Western Mediterranean Sea through the Messina Strait/ and the Sicily Channel. The study of the acoustic noise monitoring using signals recorded by hydrophones with very large band-width and high resolution is requested for novel high-energy astrophysics studies on acoustic neutrino detection.
The first goal of the observatory is the characterisation of the ambient noise to discriminate acoustic background (e.g., surface waves, biological sources, ships) from the bipolar (10-40 kHz) signature of high-energy cosmic neutrinos interacting with seawater [Riccobene et al., 2009]. These acoustic detectors are also used for passive acoustic detection of cetaceans to localise and fully track them. The monitoring of marine mammals can contribute to study the population trends in relation to climate changes and human impacts.
NEMO-SN1 has already contributed to individuate in the area an unprecedented and huge number of sperm whales, up to then given for disappeared, with daily detection [Pavan et al., 2009]. Used hydrophones cover, however a larger bandwidth from tens Hz to about 70 kHz, allowing detection of different marine mammals from fin whales (tens Hz) to Cuvier’s beaked whales (more than 50 kHz).
