Acoustic positioning systems and passive acoustic monitoring of cetaceans with the KM3NeT underwater neutrino telescope

KM3NeT (Cubic Kilometre Neutrino Telescope) is an underwater telescope for cosmic neutrinos detection and neutrino oscillation study. There are two detectors located in France, offshore the coast of Toulon (KM3NeT-ORCA, Oscillation Research with Cosmics in the Abyss) and in Italy, offshore Capo Passero, Sicily (KM3NeT-ARCA, Astroparticle Research with Cosmics in the Abyss). Each experimental setup detects the photons produced by the Cherenkov effect due to charged particles, which derive from the interactions of neutrinos with matter. In order to correctly reconstruct the original neutrino direction, energy and the interaction type, it is necessary to know very accurately the position of the photomultipliers. So, acoustic systems are used to monitor the flexible detector geometry. The detectors consist of several vertical structures, called Detection Units (DUs), along which 18 Digital Optical Modules (DOMs), each containing 31 photomultipliers, are positioned. During the deployment of the DUs the position of all the elements is measured with an acoustic positioning system, called Navigation and Absolute Acoustic Positioning System (NAAPS), with an accuracy of about 1 m. Subsequently, through a system of acoustic emitters and receivers (RAPS - Relative Acoustic Positioning System) the position of all the sensors is refined reaching an accuracy of 10 cm, sufficient for the reconstruction of the neutrino interaction events with the requested precision. Three autonomous emitters (called “Acoustic Beacons”), not synchronized with the master clock of telescope, are installed around the detector. On each DOM an acoustic piezo sensor is present and at the base of each line there is a hydrophone. In this PhD thesis, I have tested and improved the methods that are used to reconstruct the positions of the optical modules and line bases of the detector. This was done by using simulations of progressively more realistic configurations and subsequently applying the developed and tested RAPS methods on real data. In addition to the positioning of the optical sensors, the KM3NeT hydrophones can be used for different purposes. The RAPS algorithms are also used for the identification and tracking of the cetaceans. In particular, it is possible to detect the clicks of some marine mammals, such as sperm whales, Cuvier's beaked whales and various species of dolphins. By observing the signal produced by these animals in different receivers it is possible to calculate the delay times and from these to reconstruct the position of the acoustic source. In order to have a good accuracy in the reconstruction of the cetacean positions it is necessary to know very precisely the locations of the used receivers (in particular the hydrophones). For this reason the performance of the RAPS is very important also for this goal. A statistical study on the presence of marine cetaceans in the area of the KM3NeT-ORCA experiment was then conducted. This type of research is very useful for studying the distribution, behaviour and habits of these animals. My thesis work has demonstrated that the KM3NeT acoustic system, in addition to provide an accurate positioning of the optical sensors of the detector, is able to identify the sounds emitted by various species of cetaceans, in particular sperm whales, and to reconstruct their trajectory, at least when they move close to the detector (several kilometres). A collaboration with Edgelab company was started in order to develop a mobile sound system for generic sources including marine animals. Due to the limitations imposed by the COVID-19 pandemic, the collaboration took place mostly remotely and it was not possible to carry out the tests on site. The experience gained from my participation to similar experiments, such as WhaleSafe, a European project with the goal of tracking sperm whales in the Ligurian Sea, Italy, was particularly useful. The skills obtained analysing the WhaleSafe data taken in Summer 2018 have been successfully used to apply very similar reconstruction algorithms to the data coming from the KM3NeT hydrophones. Numerous tracks of cetaceans have been reconstructed and it has been possible to evaluate the presence of different cetacean species and their movements. Thanks to the KM3NeT-ORCA acoustic receiver system, located near the port of Toulon, the study of the underwater background was performed to evaluate the impact of anthropogenic activity on the marine ecosystem. Finally, the exceptional presence of killer whale pod was observed in Genoa Pra during December 2019. I have participated in the self-organized acoustic data campaigns and performed analyses to make the repertoire catalogue of the signals in order to compare with known catalogues of killer whales around the world. This PhD was founded by Regione Liguria (D.R. n. 1917 - 25/06/2018) and it took place in co-tutorship with the CPPM (Centre de Physique des Particules de Marseille), Aix-Marseille Université, France. The collaborative work with the French institute was assiduous throughout the duration of the PhD, but the periods of attendance were severely limited by the COVID-19 pandemic.