André, M., Houégnigan, L., Delory, E., van der Schaar, M., Zaugg, S., Mas, A., Sánchez, A.M. Localising Cetacean Sounds for the Real-Time Mitigation and Long-Term Acoustic Monitoring of Noise Advances in Sound Localization, p.546-574, 2011 Résumé: Noise can have a detrimental effect on cetaceans, as well as on other marine animal species. It can cause stress and increase risk of mortality by interfering with their use of sounds in communication (social behaviour and reproduction) and in navigation (echolocation or bio-sonar to orientate and look for food). Acoustic overexposure, e.g. in areas of heavy shipping, seismic surveys, military exercises or gas exploration, can lead to hearing loss. While temporary threshold shift (TTS) represents a reversible hearing loss over time, a permanent threshold shift (PTS) results in non-reversible lesions in mammal ears, i.e. a permanent hearing loss caused by long term and/or intense exposure. Although the impact of low to mid frequency (<5kHz) acoustic pollution from the above mentioned human marine activities with regard to cetacean disorientation and death remains poorly understood, available evidence is strongly suggestive of some negative direct or indirect effects: There is an increasing mortality rate from shipping collisions, and cetacean mass strandings after military maneuvers have also been recently related with the use of active sonar, both suggesting that some populations may already be suffering from acoustic impact (i.e. TTS, PTS or blast injuries). The control of noise impact on the marine environment constitutes a scientific challenge and requires a dynamic analysis of the situation based on the parallel development of applied solutions to balance human interests and the conservation of marine species. This objective implies the ambitious synthesis of many advanced acoustic technologies that must be designed to monitor the real-time presence of determined cetacean populations in conflictive areas. Many cetacean species can be identified by their specific calls. The recording of these signature acoustic signals can reveal their presence in monitored areas. Since sound propagates efficiently in water, the detection range of these signals can be quite large, exceeding 100 km in favourable conditions for low-frequency calls far above visual detection methods. This acoustic potential to non-intrusively detect and monitor cetacean species in their environment gave rise to Passive Acoustic Monitoring (PAM) techniques, for which research is very active. The localisation of whales from their sounds in their habitats was initiated in the 1970s. This was rapidly applied to tracking whales over large distances. Advances in electronics, computers and numerical analysis now make this PAM technology more accessible and affordable to small research budgets. Various systems have been used, including radio-linked systems, drifting buoys, and arrays of autonomous recorders for versatile and long-term deployments. The goal of such PAM systems, is the continuous mapping of presence and distribution of whales over ocean basins and assessing their densities, sometimes in quasi real-time. Their performance in effectively accomplishing these tasks depends on the characteristics of the targeted cetacean acoustic signals, the environment, the type of equipment used, its deployment and configuration. This performance may significantly vary from case to case. However, in any case, PAM’s success first depends on the capacity to detect and isolate the target signals from the rest of the sounds in which they are imbedded, especially for distant sources and low signal to noise ratios (SNR). The acoustic signal source level, propagation loss, and local background levels determine detection ranges. Moreover, cetacean sounds vary considerably in time-frequency, from infrasonic calls of baleen whales to ultrasonic clicks of toothed whales, and in amplitudes among species and within a species’ vocal repertoire. The ocean noise level also exhibits considerable variability in space and time, in response to fluctuating natural sources, such as wind, ice, rain, sounds produced by various organisms and anthropogenic sources such as shipping. Sound speed structures over the water column can focus sounds from distant sources into sound channels. The 3D spatial arrangements of the sources and the hydrophones are therefore relevant to the PAM configuration.   The Laboratory of Applied Bioacoustics of the Technical University of Catalonia has developed PAM solutions to prevent ship collisions with sperm whales but also to detect, classify and track acoustic sources for the long-term study of noise effects on the marine environment at deep-sea underwater observatories. The first system, called WACS (Whale Anti-Collision System) is a passive system designed to monitor the presence of individual cetaceans or objects and transmit the real-time information of their movements to any ship concerned in preventing possible collisions or harmful operations in areas of interest. The WACS original concept is to instrument a safety corridor for marine mammals, within which cetaceans can be detected, classified, localised and their positions notified to vessels using the corridor to permit timely course alterations. WACS is based on passive acoustics detection and ocean ambient noise to locate and identify the marine mammals present in the survey area. WACS integrates two inter-correlated systems: one 3D listening system called Loc3D which allows the 3D detection of the underwater sound sources (distance, azimuth and elevation), and an azimuthal location system (locAz) of the non vocalizing marine mammals by the spatio-temporal contrast produced by the ambient noise in the survey area. The latter is fundamental, because many of the cetacean species (e.g. sperm and probably beaked whales) are silent while at or near the surface or produce very low frequency sounds difficult to detect above background noise (e.g. most baleen whales species). A simulation tool for 3D acoustic propagation was designed where a wideband 3D curved ray solution of the wave equation is implemented. This tool was developed to simulate a bi-static solution formed of an arbitrary number of active acoustic sources, an illuminated object, and a receiver all positioned in 3D space with arbitrary bathymetry. The software recreates the resulting sound mixture of direct, reverberated and echoed signals arriving at the array sensors for any array configuration and any number of sources. One object can be placed in the water column and its impact on the acoustic field at the receiver is resolved. The software simulations set bounds as for the concept viability. Detection and bearing estimates could be evaluated for vocalising sperm whales at around 5km with 200m maximum error for a single click, while silent whales at ranges of 1500m from a 4m diameter array of 32 hydrophones, where on-axis click source and ambient noise levels were respectively set to 200dBrms re 1μPa @1m (full bandwidth) and 60 dBrms re 1μPa in the 1-10kHz band. In addition to the development and use of PAM techniques for mitigation and prevention of ship collisions, the challenge to assess the large-scale influence of artificial noise on marine organisms and ecosystems requires long-term access of this data. Understanding the link between natural and anthropogenic acoustic processes is indeed essential to predict the magnitude and impact of future changes of the natural balance of the oceans. Deep-sea observatories have the potential to play a key role in the assessment and monitoring of these acoustic changes. ESONET is a European Network of Excellence of 12 deep-sea observatories that are deployed from the Artic to the Gulf of Cadiz (http://www.esonet-noe.org/). ESONET NoE provides data on key parameters from the subsurface down to the seafloor at representative locations and transmits them in real time to shore. The strategies of deployment, data sampling, technological development, standardisation and data management are being integrated with projects dealing with the spatial and near surface time series. LIDO (Listening to the Deep Ocean environment) is one of these projects that is allowing the real-time long-term monitoring of marine ambient noise as well as marine mammal sounds in European waters. In the frame of the European network ESONET and the LIDO project, vocalising sperm whales were detected offshore the port of Catania (Sicily) with a bottom-mounted (around 2080m depth) tetrahedral compact array intended for real-time detection, localisation and classification of cetaceans. Various broadband space-time methods were implemented and permitted to map the sound radiated during the detected clicks and to consequently localise not only sperm whales but vessels. Hybrid methods were also developed which permit to make space-time methods more robust to noise and reverberation and moderate computation time. In most cases, the small variance obtained for these estimates reduces the necessity of additional statistical clustering. Consistent tracking of both sperm whales and vessels in the area have validated the performance of the approach. Projet: ESONET, Network of Excellence Projet: LIDO, Listening to the Deep-Ocean Environment Projet: Noise pollution effects on cetaceans