ARCTIC VOL. 68, SUPPL. 1 (2015) http://dx.doi.org/10.14430/arctic4449 Multipurpose Acoustic Networks in the Integrated Arctic Ocean Observing System Peter N. Mikhalevsky,1 Hanne Sagen,2 Peter F. Worcester,3 Arthur B. Baggeroer,4 John Orcutt,3 Sue E. Moore,5 Craig M. Lee,6 Kathleen J. Vigness-Raposa,7 Lee Freitag,8 Matthew Arrott,3 Kuvvet Atakan,9 Agnieszka Beszczynska-Möller,10 Timothy F. Duda,8 Brian D. Dushaw,6 Jean Claude Gascard,11 Alexander N. Gavrilov,12 Henk Keers,9 Andrey K. Morozov,13 Walter H. Munk,3 Michel Rixen,14 Stein Sandven,2 Emmanuel Skarsoulis,15 Kathleen M. Stafford,6 Frank Vernon 3 and Mo Yan Yuen9 (Received 26 May 2014; accepted in revised form 20 October 2014) ABSTRACT. The dramatic reduction of sea ice in the Arctic Ocean will increase human activities in the coming years. This activity will be driven by increased demand for energy and the marine resources of an Arctic Ocean accessible to ships. Oil and gas exploration, fisheries, mineral extraction, marine transportation, research and development, tourism, and search and rescue will increase the pressure on the vulnerable Arctic environment. Technologies that allow synoptic in situ observations year-round are needed to monitor and forecast changes in the Arctic atmosphere-ice-ocean system at daily, seasonal, annual, and decadal scales. These data can inform and enable both sustainable development and enforcement of international Arctic agreements and treaties, while protecting this critical environment. In this paper, we discuss multipurpose acoustic networks, including subsea cable components, in the Arctic. These networks provide communication, power, underwater and under-ice navigation, passive monitoring of ambient sound (ice, seismic, biologic, and anthropogenic), and acoustic remote sensing (tomography and thermometry), supporting and complementing data collection from platforms, moorings, and vehicles. We support the development and implementation of regional to basin-wide acoustic networks as an integral component of a multidisciplinary in situ Arctic Ocean observatory. Key words: Arctic observing systems; Arctic acoustics; acoustic tomography; cabled networks; passive acoustics; active acoustics RÉSUMÉ. La diminution remarquable de la glace de mer dans l’océan Arctique aura pour effet d’intensifier l’activité humaine dans cette région au cours des années à venir. Ces activités s’accompagneront d’une demande accrue en ressources marines et en énergie du fait que l’océan Arctique sera accessible aux bateaux. L’exploration pétrolière et gazière, la pêche, l’extraction minière, le transport maritime, la recherche et le développement, le tourisme et les activités de recherche et sauvetage mettront davantage de pression sur l’environnement vulnérable de l’Arctique. Il y a lieu de se doter de technologies qui permettront de faire des observations sur place à l’année afin de surveiller et de prévoir les changements caractérisant le système atmosphère-glace-océan de l’Arctique à l’échelle quotidienne, saisonnière, annuelle et décennale. Ces données seront utiles tant pour le développement durable que pour l’application des accords et traités internationaux relativement à l’Arctique, et elles permettront de protéger cet environnement critique. Dans cet article, nous discutons des réseaux acoustiques à vocations multiples de l’Arctique, notamment l’aspect des câbles sous-marins. Ces réseaux permettent les communications, le transport 1 Corresponding author: Leidos, Inc., 4001 N. Fairfax Dr., Suite 725, Arlington, Virginia 22203, USA; [email protected] 2 Nansen Environmental and Remote Sensing Center, Thormøhlensgt 47, 5006 Bergen, Norway 3 Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, 0225, La Jolla, California 92093- 0225, USA 4 Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 5 National Oceanic and Atmospheric Administration/Fisheries Science and Technology-PMEL, 7600 Sand Point Way, Seattle, Washington 98115, USA 6 Applied Physics Laboratory, University of Washington, Seattle, Washington 98105, USA 7 Marine Acoustics, Inc., 809 Aquidneck Avenue, Middletown, Rhode Island 02842, USA 8 Woods Hole Oceanographic Institution, 86 Water Street, Woods Hole, Massachusetts 02543 USA 9 Department of Earth Science, University of Bergen, Allégt. 41, N-5007 Bergen, Norway 10 Institute of Oceanology, Polish Academy of Sciences, Powst. Warszawy 55, 81-712 Sopot, Poland 11 LOCEAN, Université Pierre et Marie Curie, 4 place Jussieu (Tour 45-46, 5e) 75005 Paris, France 12 Centre for Marine Science and Technology, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia 13 Teledyne Web Research, 49 Edgerton Drive, North Falmouth, Massachusetts 02556, USA 14 World Meteorological Organization, 7bis, Avenue de la Paix, Case postale 23000, CH-1211 Geneva 2, Switzerland 15 Foundation for Research and Technology Hellas, Inst. of Applied and Computational Mathematics, PO Box1385, GR-71110 Heraklion, Greece © The Arctic Institute of North America 2 • P.N. MIKHALEVSKY et al. de l’énergie, la navigation sous-marine et sous les glaces, la surveillance passive du son ambiant (glace, bruits sismiques, biologiques et anthropiques), la détection acoustique à distance (tomographie et thermométrie) de même que le soutien et le complément aux données recueillies à partir des plateformes, des amarres et des véhicules. Nous sommes pour l’aménagement et l’utilisation de réseaux acoustiques régionaux à la grandeur du bassin comme composante intégrante d’un observatoire multidisciplinaire sur place dans l’océan Arctique. Mots clés : systèmes d’observation dans l’Arctique; acoustique de l’Arctique; tomographie acoustique; réseaux câblés; acoustique passive; acoustique active Traduit pour la revue Arctic par Nicole Giguère. INTRODUCTION Acoustic tomography and thermometry measure ocean temperatures and currents throughout the water column, The development and implementation of the infrastructure including the abyssal zone, to determine the heat content for multipurpose acoustic networks in the Arctic Ocean and mean circulation on regional and basin scales using would support in situ ocean observations with instru- precise measurements of travel times between acoustic mented moorings, autonomous vehicles, and floats, as sources and receivers (thermometry refers to average tem- well as with acoustics (Dushaw et al., 2010; Sagen et al., perature measurements in a two-dimensional vertical slice 2010). The OceanObs‘09 Conference Summary (Fischer et along a single path, and tomography refers to using multi- al., 2010) identifies active and passive ocean acoustics as ple intersecting paths and tomographic inversion to provide a proven technology for in situ ocean observing. Selected a three-dimensional spatial temperature map). Through moorings can be cabled to shore to provide both energy for inversion techniques, internal ocean temperature can be high-power sensors (such as vertical profilers and acous- retrieved with an accuracy of 0.01˚C over a 200 km distance tic sources) and high-speed, real-time, year-round com- (Munk et al., 1995; Dushaw et al., 2010). Average current munications to extract data from local sensors (broadband velocities are determined from the differences between seismometers and hydrophones, autonomous vehicles, and reciprocal travel times produced by simultaneously trans- floats), as well as from distant sensors that communicate mitting acoustic pulses in opposite directions along an acoustically. A fully cabled network with several landfalls acoustic path. Because of the high accuracy and integral can realistically provide the various component services nature of the data, acoustic travel times can be used to vali- needed for long-term climate change monitoring over many date and constrain numerical ocean circulation models by decades (the typical operational life of cabled systems). data assimilation (e.g., Munk et al., 1995; ATOC Consor- Acquiring and installing a cabled system can be as costly as tium, 1998; Dushaw et al., 2009; Haugan et al., 2012). In the acquiring a research ship; however, these costs are competi- Arctic Ocean, with its upward refracting sound speed pro- tive when amortized over the multi-decade lifetime of the file, the depth dependence of the acoustic modes increases system. And cabled networks provide continuous, persis- monotonically with mode number. Higher modes sample tent real-time observations year-round, which are not pos- greater depths. With the vertically stratified major water sible with ships alone. Technologies for such a system exist, masses of the Arctic basin, acoustic modes can selectively but international collaboration, coordination, and synchro- sample specific water masses, such as the AIW (Mikha- nization of funding for long-term infrastructure in the Arc- levsky et al., 1999; Mikhalevsky, 2001) and the deep Arctic tic Ocean are required to implement a basin-wide network. water. OceanObs’99 identified high-latitude regions and the The same tomography/thermometry network of acous- Arctic Ocean as key areas where ocean acoustic tomog- tic sources and receivers can also provide long-term pas- raphy should be applied (Global Ocean Observing Sys- sive acoustic listening, underwater acoustic navigation tem Steering Committee, 2000; IOC, 2000; Dushaw et al., (performing a global positioning system [GPS] function 2001). Stand-alone acoustic tomography and thermometry underwater), and low bandwidth communications provid- systems have been