NEMO – OνDE Observatory

The NEMO-OvDE Observatory positioned from 2005 to 2006 in the Gulf of Catania

From January 2005 to December 2006, a monitoring station called "NEMO - Ovde" (Neutrino Mediterranean Observatory - Ocean ν(n)oise Detection Experiment) was connected at 2100 m deep in the submarine site off the Catania harbor. The OvDE station has been equipped with 4 acoustic sensors (hydrophones) in order to monitor submarine background noise. The INFN's interest in the propagation of sound in the sea arises from the need to develop an acoustic positioning system for the KN3NeT telescope and to study the possibility of neutrino acoustic detection. NEMO-OvDE provided for the first time the opportunity to monitor the various underwater acoustic sources in a wide range of frequencies. The first studies in the area of environmental and anthropogenic noise were carried out and passive acoustic monitoring of different cetacean populations in the area was made possible. In particular, it was possible to carry out first studies on the presence and population dynamics of the sperm whale (Physeter macrocephalus, an example of sounds recorded) and dolphin eco-localization activity (Delphinidae), as described in the works published in the international scientific journals PloS ONE and Scientific Reports.

The NEMO-SN1 multidisciplinary observatory

The NEMO-SN1 Observatory, cabled node of the European Research Infrastructure EMSO ERIC, was installed in May 2012 at Catania's underwater site at a depth of 2100 m (Favali et al., 2013). The observatory operation -from June 2012 to May 2013- allowed, for the first time, the continuous and long-term monitoring of the underwater noise generated by ship traffic and the characterisation of the acoustic environment of the Ionian Sea. The presence of the fin whale (Balaenoptera physalus) in the area was monitored continuously for the first time, by revealing the typical signals emitted at frequency of about 20 Hz, thanks to NEMO-SN1 hydrophones sensitive at very low frequencies (50 mHz - 1 kHz) (Sciacca et al., 2015). The presence of impulsive sounds generated by airguns was also acoustically revealed (Sciacca et al., 2016). In addition, as described in a recent study published by Viola et al., (2017), acoustic noise levels have been measured and correlated with ship movements, by using a dedicated AIS antenna. The study area is characterised by the proximity of medium-sized ports and NEMO-SN1 is located close to commercial shipping lanes. The experimental acoustic data were compared with the results of a model based on AIS data to evaluate the contribution of ship noise to the background in six consecutive 1/3 octave bands, including the 1/3 octave frequency bands centred at 63 Hz and 125 Hz, as required by the Marine Strategy Framework Directive (2008/56 / EC).

SN1 Observatory working from 2012 to 2013, during refurbishment at LNS laboratory of the harbour of Catania	A fin whale sighted in the Gulf of Catania	Spectrogram of a typical 20 Hz signal sequence of a fin whale

Acoustic antennas SMO

The SMO - FIRB 2008 (Submarine Multidisciplinary Observatory) project started in 2009 thanks to the collaboration between INFN, INGV, University Sapienza Roma, Università di Roma 3 and CIBRA. The project aimed at developing new technologies to be integrated into the KM3NeT submarine laboratory. SMO aimed at testing new technologies for the detection of high energy neutrinos, for the acoustic positioning of the KM3NeT towers, for the study of underwater geophysical events and for research in bioacoustics and environmental monitoring.

Thanks to the SMO project, the submarine site of Catania hosts from June 2012 the station SMO - OvDE, an acoustic antenna made up of a frame equipped with 4 synchronized hydrophones arranged in a tetrahedral configuration. The SMO Observatory is connected, like the others operating in the area, to the shore laboratory of the port of Catania via an electro-optical submarine cable, consisting of 10 optical fibers and 6 electric conductors.

A further three-dimensional acoustic antenna was installed on board the NEMO Phase 2 detector, a prototype of a detection unit for the KM3NeT high-energy neutrino telescope. The antenna is made up of 10 broadband hydrophones with the same characteristics of those installed on the SMO-OvDE station of Catania. The sensors were installed so to configure a 3D acoustic array of about 500 m in height. The SMO data acquisition system has been fully integrated into the electronics of the detector. The SMO array allowed a successful test of the acoustic positioning system for the KM3NeT teslescope (Viola et al., 2013, 2014). A new technology allowing GPS time stamp of acoustic data in situ with an accuracy better than 1 microsecond was also developed for SMO. All acoustic data acquired at sea were transmitted to the shore laboratory in real time.

The acoustic station SMO-OvDE before deployment off the port of Catania in 2012	Example of daily spectrograms processed by SMO data. The figure shows the average noise recorded on 22-02-2017. The spectrogram reports the average noise measured every second within 24 hours. On the top, the noise recorded in the entire frequency range 1 Hz - 96 kHz available. Below, the same data is shown in the 1 Hz - 1 kHz band.

NEMO Phase 2 prototype

NNEMO Phase 2, a demonstrator for the KM3NeT neutrino detector, was installed at 3500 m depth in the submarine site of Portopalo of Capo Passero, as part of the NEMO collaboration activities and the KM3NeT consortium (Adrian-Martinez et al., 2016). NEMO Phase 2 was deployed in March 2013 and has been operational until June 2014. The NEMO Phase 2 demonstrator is made up of a series of arms each 10 meters long, connected together to form a structure about 500 m high; the arms were equipped with a total of 32 optical sensors, needed to capture the Cherenkov radiation radiated from the charged particles produced by the interaction between high energy neutrinos and water. In addition to the NEMO Phase 2 optical sensors, it has been equipped with environmental monitoring sensors (CTD probes for pressure, conductivity and temperature analysis, flow meters, etc.). Observatory sensors have allowed us to study the daily rhythm of bioluminescence and correlate it to underwater currents.