Project contract no. 036851

ESONET

European Seas Observatory Network

Instrument: Network of Excellence (NoE)

Thematic Priority: 1.1.6.3 – Climate Change and Ecosystems

Sub Priority: III – Global Change and Ecosystems

AOEM D9 - Final report on design of the acoustic network for acoustic tomography, underwater navigation and passive listening in the Fram Strait.

Due date of deliverable: 30 November 2010 Actual submission date: 10 January 2011

Start date of project: July 2009 Duration: 17 months

Organisation name of lead contractor for this deliverable: NERSC Lead authors for this deliverable: Hanne Sagen, and Stein Sandven, Revision [draft 1 date 10 January ] Revison [final date 23 February 2011]

Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006) Dissemination Level PU Public PP Restricted to other programme participants (including the Commission Services RE Restricted to a group specified by the consortium (including the Commission Services) CO Confidential, only for members of the consortium (including the Commission Services) x

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TITLE: Final report on design of the acoustic REPORT IDENTIFICATION network for acoustic tomography, ESONET – AOEM – D9 underwater navigation and passive listening in the Fram Strait.

CLIENT : CONTRACT

SIXTH FRAMEWORK PROGRAMME EESSOONNEETT CCOONNTTTRRAACCCTTT GRANT AGREEMENT NO.:

(036851)

• AOEM Demo mission • AWAKE project • ACOBAR project

CLIENT REFERENCE AVAILABILITY FFRRAAMMEEEWWOORRKK PPRROOGGRRAAMM 6 6 EESSOONNEETT Open report within the ESONET consortium. FFRRAAMMEEEWWOORRKK PPRROOGGRRAAMM 7 7 AACCOOBBAARR Polish-Norwegian Research Fund (AWAKE)

Contributing investigators AUTHORISATION Nansen Environmental and Remote Sensing Center, Norway: Hanne Sagen, Stein Sandven Bergen, 23 February 2010 Alfred Wegners Institute, Germany: Eberhard Fahrbach, Agnieszka Beszczynska-Möller, Olaf Klatt Scripps Institution of Oceanography (SIO), USA: Peter Stein Sandven F. Worcester Woods Hole Oceanographic Institution, USA / Teledyne Webb Research Corporation, USA: Andrey Morozov Acknowledgment to the University of Bergen, the Norwegian Coast guard, and in particular to the crews onboard RV Håkon Mosby and KV Svalbard.

AOEM D9 – Final report February 24, 2011

2 CONTENT Executive summary ...... 2

Introduction...... 3

Acoustic characteristics in the Arctic environment...... 3

Acoustic characteristics in the Fram Strait marginal ice Zone...... 9

Acoustic navigation and tracking of Lagrangian systems ...... 13

Accuracy of positioning and timing...... 21

Acoustic communication...... 22

Passive acoustics ...... 23

Acoustic technology. ...... 24

Summary and Conclusion...... 26

References ...... 28

Executive summary Acoustic infrastructure and measurements can contribute to fill the significant gap in ocean observations in the Arctic. An acoustic network can measure the acoustic travel times to derive heat content and mean circulation on a regional or basin scale in minutes or hours respectively, provide an underwater “GPS” system for navigation and timing for under-ice Lagrangian systems, and provide information about ice dynamics, earthquakes, and marine mammals through passive listening. Furthermore, the need for a low frequency acoustic navigation system for gliders and floats in the Arctic (Lee and Gobat, 2006) coincide to a large extent with the requirements for the acoustic thermometry system (Sagen et al. 2010, Dushaw et al. 2010). It is therefore cost effective to develop and implement a multi purpose system in the Arctic, which take care of both navigation and provide thermometry data. Furthermore, a cabled acoustic network in the Arctic would provide basin wide measurements in real time and year round. Providing continuous data availability in fixed critical locations the cabled network can observe episodic events such as eddies or the passage/influx of warm or cool water masses when they happen to permit researchers to deploy/redeploy/direct other assets such gliders, unmanned systems, ice-tethered or moored platforms to monitor, track, analyze and study the event. Technologically there is no problem to integrate acoustic sources and receivers into cabled networks. One of the acoustic sources in the Fram Strait acoustic network is recommended to install in the planned cabled network in the Fram Strait. Passive acoustic systems are easy to implement in cabled network, and we recommend including a cluster of minimum three vertical hydrophone arrays for advanced detection and localization and tracking of marine mammals in connection with a cabled network in the Fram Strait. To proceed to an operational acoustic network in the interior Arctic co-ordinated actions on the international level has to be taken across disciplines. The international ANCHOR (Acoustic Navigation and Communication for High-Latitude Ocean) group of experts was established to coordinate the interoperable acoustic infrastructure the high Arctic (Lee and Gobat, 2006). Implementation of cabled systems in the Arctic can only be developed through international collaboration. The Svalbard Integrated Observing System can offer opportunities to develop a system in the European sector of the Arctic. European efforts to establish an acoustic network infra structure covering the Arctic have to be coordinated with Russian, Canadian and US initiatives and interests.

This report forms the baseline of preparation of a publication in referee journals.

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3 Introduction. The Marginal Ice Zones and the interior Arctic Ocean under the ice is severely under sampled due to lack of regular observing systems. A future sustainable Arctic Ocean observing system will need to combine data from several sensors on ice tethered platforms or underwater moorings with satellite data and models using data assimilation (Lee et al, 2010; Sagen et al., 2010, Dushaw et al., 2010). During the International Polar Year (IPY) 2007-2009, new technologies for ocean observations such as acoustic thermometry/tomography, oceanographic sensors on ice tethered buoys, floats and gliders operating under the ice. Except for ice-tethered buoys, these technologies need an acoustic network to become operational in the Arctic.

The relevant capabilities of an acoustic network is to • Measure the acoustic travel times, to derive heat content and mean circulation on a regional or basin scale in minutes or hours respectively, • Provide an underwater “GPS” system for navigation and timing for under-ice Lagrangian systems, • Provide information about ice dynamics, earthquakes, and marine mammals through passive listening.

The objective of AOEM - WP 5 – Acoustic network – is to “define and design a cabled acoustic network for acoustic tomography, acoustic navigation of gliders and floats and for passive listening of human activities and marine mammals in the Fram Strait”. This report describes important features and characteristics of acoustic propagation in the Arctic influencing the design and capability of a future acoustic network, acoustic tomography in the Arctic, acoustic navigation in the Arctic, passive acoustic systems, technological state of the art, and future perspective of a cabled acoustic observatory covering the entire Arctic Ocean is defined. Acoustic characteristics in the Arctic environment Achievements in acoustic monitoring, communication and navigation depend on fundamental knowledge how acoustic signals, in particular low and mid frequency sound, propagate and behave in the Arctic environment. The ice cover and the oceanographic conditions in Arctic waters have strong impact on the propagation of acoustic signals. First of all, sound propagates slower in cold Arctic water (1460 m/s) than in warmer temperate water (1500 m/s).

Second, a strong surface duct, with cold and fresh water, underneath the Arctic ice cover, characterizes the interior Arctic Ocean. This has a strong impact on how the acoustic signal propagates through the water masses. If the acoustic source is located inside the duct a large portion of the acoustic energy above the cut-off frequency is trapped inside the duct, see for example in figure 1. Acoustic energy at frequency below cut-off will not sense the surface duct. Furthermore, the trapped acoustic energy, at launch angles near horizontal axis (+/-14 degrees) is attenuated due to repeated interaction with the rough underside of the ice, while the energy launched at steeper angles are refracted to the deeper water masses and bounces the surface much less frequent. If the source is positioned underneath the surface duct primarily mode 2 and higher modes are excited, and therefore less energy goes into the surface duct and more energy penetrates into the deeper water masses. In both cases, the steepest and deepest going rays interact with both the sea floor and surface, causing reflection and scattering losses at two boundaries. The bathymetry can cause severe stripping of the acoustic signal propagating through the ocean. The source receiver configuration and geographical position is therefore important for the acoustic insonification of the different watermasses found in the Arctic oceans. This was studied numerically in the EU project Acoustic monitoring of ocean climate in the Arctic - AMOC.

Figure 2 show examples of numerical calculation of acoustic travel times carried out in the AMOC project (Johannessen et al. 2001), the acoustic signal travels 700 km from a source to a receiver once a month for 100 years. The travel times were calculated by feeding a 100 year long time series of range dependent monthly mean oceanographic fields from the HOPE ice-ocean model into the ray trace model RAY. Four different configurations of source and receivers were considered. In the two plots to the left the receiver is positioned below the duct at 500 m and in the plots to the right the receiver is positioned inside the duct. In the upper two

AOEM D9 – Final report February 24, 2011

4 plots the source is positioned at 50 m, well inside the surface duct, and by comparison it is clearly seen that in this case there is far more arrivals inside the surface duct than below it. Furthermore, a traceable inter-annual variability is only present when considering the receiver below the duct (plot to the left). In the two lower plots the source is positioned at 500 m, well below the surface duct. By comparing the plot to the left, receiver below the duct, with the plot to the right, the receiver is inside the duct, we observe the same trends and inter annual variability both inside and below the surface duct. At the same time we see much less arrivals inside the duct than underneath the surface duct. The observed inter-annual variability in the two cases is similar to inter annual variability of un-trapped acoustic energy in the case of source inside the duct. This show that when the acoustic energy is produced underneath the surface duct the acoustic energy is not trapped within the duct.

Furthermore, Figure 2 show that he configuration of the sources and receivers are essential for detection of the climate signal. The increasing temperature in the intermediate and deeper water masses is clearly pick up by decreasing travel times for the first arrivals, when the source depth is 500 m, both at a shallow (60 m) and deep receiver (500 m). If the source is at 60 m within the duct only the inter-annual variability is observed, while the steady warming of the intermediate and deeper water masses is not detected either at the lower or deeper receiver. When a source is positioned within a 200-250 m thick low speed surface channel most rays are trapped. In mode theory energy for a source in the surface channel goes into mode 1 (see figure 1). In all travel time versus time plots in Figure 2, except in the case of source and receiver in the duct, annual and decadal variability is significant for the later arrivals. This is due to variable thickness of the surface channel changing the refraction and turning point depths of the rays and thereby the distance of propagation. However, thinning of the sound speed channel is also an important climate signal to detect.

When sound is trapped within the duct the sound it is repeatedly interacting with the sea ice cover and flavoured by the acoustic reflectivity of the sea ice, dependent on floe thickness, density, shear and longitudinal sound speeds, and roughness. Figure 3 presents reflection loss at the interface of smooth ice for different ice thicknesses using the OASES software package to calculate the reflection coefficient as function of grazing angle and frequency for different ice thickness and elastic properties of the sea ice. The main finding using this simple approach, see Figure 3, demonstrates that the reflection loss at frequencies below 100 Hz is insensitive to changes in ice thickness, while it is very sensitive to this at frequencies above 100 Hz. Scattering from the rough ice cover is a very complex problem and several model approaches has been used. If the roughness is small compared to the acoustic wavelength and stationary in time and space a perturbation approach can be used. To get a feeling the acoustic wavelength at 100 Hz in seawater with sound speed near 1500 m/s is 15 m, and 1.5 m at 1000 Hz. A roughness of the order of 1-2 m is therefore small compared to the wave length at 100 Hz. In this case the boundary conditions given at a rough surface can be transformed to a mean surface. Rayleigh first used this for a free surface boundary. The approach was later generalized by Kupermann and Schmidt (1989) to rough surfaces between solid/fluid and fluid-fluid layers, and implemented in the OASES model (Schmidt, 1997). OASES treats scattering with the Kirchoffs approximation (infinite correlation length and small roughness) and non-kirchoffs scattering corresponding to infinite correlation length. Within AMOC a few calculations were carried out introducing small-scale roughness on the underside of the ice and varying the impedance between water and ice. This small study showed that the reflection losses at frequencies below 100 Hz as modeled by OASES are slightly sensitive to changes of the under ice roughness. The sensitivity to changes in the shear and longitudinal wave speeds in the sea ice was also clearly observed.

Fricke, 1991 used a finite difference solution to study the acoustic scattering from the elastic ice. His approach was to model scattering from elemental features either in open water or frozen into ice, In the work by Fricke, 1993, the importance of including the elastic properties of the sea ice is clearly illustrated by calculating the scattering form a fluid keel and from a elastic keel. In a later work Fricke and Unger, 1995 used this to estimate the transmission loss over long distances, and he found a fairly good agreement with measurements. The most important results from the above simulations are that a thinning of ice will cause a reduction of reflection loss at increasing frequencies, and the importance of including elasticity in the description of the ice cover to calculate the losses. This indicate that the changes in Arctic ice conditions most likely have pushed the optimal frequency to higher frequencies compared to the situation in the early 1980s.

AOEM D9 – Final report February 24, 2011

5 Successful trans-Arctic acoustic thermometry experiments demonstrated the unique capability of underwater acoustics to measure large-scale changes in temperature and heat content of the Arctic Ocean. The 1994 TAP (Transarctic Acoustic Propagation) experiment revealed, for the first time, basin-scale warming of the Arctic Intermediate Water (Mikhalevsky et al., 1999), which was confirmed by submarine measurements (Gavriliov & Mikhalevsky, 2002). The ACOUS (Arctic Climate Observations Using Underwater Sound) experiment in 1998– 1999 detected an extraordinary warm and wide mass of Atlantic water crossing the Nansen Basin north of the Franz Victoria Strait in August-December of 1999, which would have been extremely difficult and expensive to observe by conventional oceanographic means (Gavrilov and Mikhalevsky, 2006). This experiment also showed that the received acoustic energy was correlated with integral path-average ice thickness changes, which could provide means for continuous remote observation of basin-scale changes in the Arctic sea ice thickness. To summarize, the propagation paths and attenuation of the sound depends on source and receiver configuration relative to the vertical stratification of the ocean, and sea ice characteristics (thickness, roughness and elastic properties) in a complex way. This makes the acoustic field sensitive to the characteristics of the surface duct, the roughness of the underside of the sea ice, the thickness of sea ice and elastic properties of the sea ice (e.g. Fricke 1991, Sagen, 1998, Gavriliov & Mikhalevsky, 2006). It is therefore of great interest to reveal the environmental information embedded in the acoustic signals. The most elaborated and mature acoustic methodology for environmental and climate monitoring is acoustic thermometry/tomography (e.g. Munk et al. 1995, Dushaw et. al. 2010). The method builds on measurements of acoustic travel times between acoustic sources and receivers and inversion of techniques. The method has been successfully tested for basin wide monitoring using a 20 Hz signal. Thinning of the mean ice thickness during the last decades in the interior Arctic and the simulations above indicates sources frequency between 70-100 Hz can be used. It is however, unsure how large impact his has and if the thinning of the ice may cause a larger under ice roughness due to “softer” ice. This will need a test sources using frequencies in different ranges. However, an increase of frequency to 70-100 Hz will significantly reduce the size of the sources significantly and they will be cheaper to produce and easier to handle during deployment and recovery. The technological approach for providing a reliable low frequency source (70-100 Hz) with accurate clocks and mooring motion system exists at Teledyne Webb Research.

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6

Figure 1. The upper illustration show the sound speed profile in the Arctic causes a split of the acoustic modes composing the acoustic energy at 20 Hz, so that the different modes senses the different layers of the water masses. This makes the inversions “easy” and favours the concept of acoustic tomography. The lower illustration shows the configuration of sources and receivers during the two trans arctic thermometry experiments in the Arctic Ocean in 1994 and 1999 (Peter Mikhalevsky, 2010)

AOEM D9 – Final report February 24, 2011

7 Figure 2. This figure summarizes the major result from the EU project AMOC (Johannessen et al. 2001). A 100 year long (1950- 2050) anthropogenic scenario for the Arctic was calculated using the HOPE model at Max Planck Institution. The spin-up period for the climate model was from 1950 to 1975. Model resolution was around 40 km in horizontal with 40 layers in the vertical. The upper left Hovmöller diagram show the vertical temperature profiles averaged over a 700 km section along the TAP B section from 1950 to 2050. The monthly mean-range dependent oceanographic fields (temperature and salinity) were used as input to the RAY model to produce travel times for each detected eigenray as function of time. The upper plots show travel time versus time (months) for for source depth 60 m, and lower plots are similar but for source depth 500 m. To the left the receiver depth is 500 m am to the right the receiver depth is 60 m.

AOEM D9 – Final report February 24, 2011

8

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Figure 3. The reflection loss as function of frequency and grazing angle calculated for ice thickness of 0.5, 1.0, 2.0, 3.0 m. The material constants are Cc=3600 m/s, cs=1800 m/s, ac=1.0 dB/wavelength as=2.5 dB/wavelength, density=0.92 g/cm3. Sound Speed in water is set to 1440 m/s. Critical angle of reflection for the compressional waves are 61.3 degrees, and critical angle of shear waves 36.9 degrees. The white/pink colour corresponds to close to total reflection (Sagen, 1998).

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9

Acoustic characteristics in the Fram Strait marginal ice Zone. The deep and wide Fram Strait, between Greenland and Spitzbergen, is the main passage through which the mass and heat exchange between the Atlantic and Arctic Ocean takes place: on the eastern side of the strait the northward West Spitzbergen Current (WSC) brings Atlantic water to the Arctic Ocean whereas on the western side the southward East Greenland Current (EGC) brings cold water from the Arctic back to the Atlantic ocean. The topographic structure of the Fram Strait leads to a splitting of the WSC into at least three branches as schematically shown in Fig. 1. The East Greenland (EGC) current carries cold fresh water southwards, for a large part across the shallow continental edge at less than 200 m depth. The large-scale circulation pattern in the Fram Strait are thus horizontally stratified. However, the Fram Strait circulation pattern is more complex due to mesoscale eddies, with typical diameter 20 km and extending down to some 600 m depth, transecting the strait at irregular intervals, with a passage time scale of about a month. Another, pronounced feature of the Fram Strait is the highly variable and complex transition zone separating the interior Arctic from the open ocean called the Marginal Ice Zone (MIZ). Associated with the MIZ we find strong horizontal gradients in oceanographic fields due to fronts between cold and warm water, and waves in open ocean propagates into the ice is associated with the MIZ. Superimposed on this comes the tidal currents and internal waves. Dynamic processes in the ocean and changes in the sea ice will certainly impact the acoustic propagation. The impact of fronts and eddies on acoustic propagation in the MIZ has been studied during the MIZEX and SIZEX programmes during the 1980s and1990s (Mellberg et al, 1987) and in the AMOC programme (Hobæk et al. 2001).

During the AMOC project (Johannessen et al. 2001) the Fram Strait obtained special attention as a key area in climate monitoring. Two different acoustic systems was addressed one thermometry concept with two moorings one with a source and one with a receiver array; and a more complex system for tomography based on scintillation concept requiring 8-9 moorings. Due to the feasibility consideration the prime interest was the simplest system, and a acoustic sensitivity study to temperature changes was carried out using oceanographic fields form the HOPE model (as for the Arctic). In Figure 4 the results from the HOPE model and the acoustic simulations are shown. The upper plot show the horizontally averaged temperature profile between the source and receiver (200 km) as function of time (Hovmöller diagram). The source and receiver is positioned along the from 79 N within the WSC, and the averaged temperature profile has a much stronger seasonal and annual variability than in the interior arctic. This results in a much higher variability in the acoustic travel times. Despite this; the ocean warming signal results in a clear reduction in travel times. However, averaging of the arrivals may enable us to pick up the climate signal earlier than when considering the “raw” data (Johannessen et al. 2001). To understand the climate changes in the Arctic Ocean it is important to quantify the heat and mass transport through the strait, and it is important to measure the impact of the meso-scale currents and to determine the influence of the recirculation. Standalone acoustic systems provide averaged temperature and current information along the defined acoustic tracks well suited for climate monitoring. In order to obtain information about the impact of meso scale features on heat and mass transports it is not sufficient to use a stand alone tomography system. The AMOC study concluded that the tomography system has to be combined with ocean circulation models through data assimilation (Johannessen et al. 2001). The integral nature of the data from tomography is best employed in conjunction with numerical ocean models and data assimilation (eg. Munk et al, 1995; Dushaw et al, 2010, Sagen et al. 2010). In 1997 a section of standard oceanographic moorings across the Fram Strait was establish to monitor the ocean volume and heat fluxes through the strait. The system consists of 17 moorings with a total of 40 instruments [7]. Estimates of the transports over 13 years (1997-2010) indicate a mean northward transport of 12 Sv (1 Sv = 106 m3/s), mostly in the West Spitsbergen Current and a southward transport of 14 Sv, mostly in the East Greenland Curren. The spatial resolution of the moorings, which varies from 10 to 30 km, is not sufficient to resolve the meso-scale variability and estimate the volume and heat transport by the recirculation current. Therefore, the estimated transports (and their variability) have significant uncertainty e.g. 40 % in the WSC and above 100 % for the EGC.

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10

Figure 4 (Johannessen et al. 2001). These two panels summarize the major Fram Strait result from the numerical study carried out under the EU project AMOC. A 100 year long (1950- 2050) anthropogenic scenario for the Arctic was calculated using the HOPE model at Max Planck Institution. The spin-up period for the climate model is from 1950 to 1975. Model resolution was around 40 km in horizontal with 40 layers in the vertical.

The upper diagram show the vertical temperature profiles averaged over a 200 km section across the eastern part of the Fram Strait as function of depth and time for Anthropgenic run for the period 1950 - 2050.. The lower panel shows the arrival times as function of time. Travel time across 200 km of the eastern Fram Strait based on anthropogenic warming Scenario and a source depth of 500 m. The reduction in travel time over the 100 year period is 1.65 sec

More recently, the DAMOCLES EU/FP6 project (2005-2010, http://www.damocles-eu.org/), has taken the first steps towards an integrated data and model system in the Fram Strait, combining high-resolution ice-ocean models (3.5 km horizontal resolution), ocean acoustic tomography, gliders and traditional oceanographic moorings through data assimilation. A single track acoustic thermometry experiment was carried out in the Fram Strait from 2008 – 2009 using state of the art source technology and receiver technology jointly developed at Scripps Oceanographic Institution, USA and at Teledyne Webb Research Cooopration, USA. Acoustic sweeps from 190 Hz to 290 Hz were transmitted every 3 hours for a year, and the signals were received 130.010 km away on a 700 m long vertical array with eight hydrophones spaced by 96 meters. Example of raw and uncorrected data from all the hydrophones in the receiver array is shown in Figure 5. The 80 s of raw data in the left panel ha a periodic feature in it, this has nothing to do with the acoustic signal, but corresponds to seismic investigations carried out by the university of Bergen in the Fram Strait. To be able to see the tomographic signal pulse compression has to be carried out, and the tomographic signal is detected by several arrivals in the to panles to the right. Data from each of the hydrophones has been collected, quality checked, pre-processed involving detection of arrivals, correction for mooring motion and clock drift. After pre-processing the data has

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11 been used for inversion, ocean model validation and assimilation. Preliminary results of the data analysis demonstrate the use of acoustic data for model validation and tomographic inversions to temperatures. Acoustic travel times are currently undergoing testing for assimilation in to ocean models under the ongoing ACOBAR project.

Figure 5. Upper figure: The single track experiment 2008-2009 is shown as a yellow line overlaid a map and a synthetic aperture radar (SAR) image. Lower three panels: Acoustic receptions 00:11:18 on 29 August 2008 at 8 receiver depths from 300 to 1000 m.

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12

B

A

Upper: Quality checked acoustic arrivals corrected for mooring motion and clock drift. Centre: Arrival times based on 30 consecutive outputs from the matched filter have been averaged to provide less noisy data. Lower: Model predicted waterborne acoustic arrivals.

Figure 6. A) Modelled travel times as compared to the observed acoustic observations from the hydrophone at 989 m from 22 September 2008 to 1 August 2009. The predicted arrivals were less than 100 ms earlier than observed arrivals, which indicate that the model is less than 0.5 0C too warm along the acoustic track through the year. The warming in the Fram Strait occurs too late in the summer compared to observations. B) This graph show the result of data oriented inversions of the acoustic travel times Skarsoulis et al., 2010. The inversions rely on EOFs calculated on the basis of temperatures from the oceanographic mooring array.

Triangle acoustic experiment 2010-2012. The single track acoustic experiment is followed up by a 2-year triangle tomographic experiment from 2010 to 2012 (http://acobar.nersc.no, Sagen et al., 2008, 2010, Figure 7). In August/September a 3 D tomography, ambient noise and glider navigation experiment started in the Fram Strait by the Nansen Center and the Alfred Wegner Institute in collaboration with Scripps Institution of Oceanography, Woods Hole Oceanographic Institution and Geophysical Institute, University in Bergen (Figure 2). The system was deployed from RV Håkon Mosby and KV Svalbard in August/September 2010. Two moorings is positioned in the MIZ and frequently covered with ice. Those two moorings are recovered after two years during summer 2012. A partial recovery of the source near Svalbard (A) and the receiver mooring (D) will take place September 2011. Mooring A and D will be redeployed immediately after change of batteries. During the experiment the sources will produce 60 s long signals sweeping from 190 Hz to 290 Hz every 3 hour every other day for two year. Moreover, the three sources will produce 80 s long narrow banded sweeps at 260- 261 Hz every six hours for glider navigation during selected time windows during fall 2010 and fall through winter 2011/2012. The triangle experiment 2010-2012 is designed to provide two-way travel times along each of the sides of the triangle (red lines). In addition one way travel times along sections between each of the corner of the triangle and the long vertical receiver array in the middle of the triangle (red lines). Acoustic data from six tracks will be available for analysis in summer 2012. In ACOBAR acoustic data from six tracks will be assimilated using the TOPAZ model system (http://topaz.nersc.no), Sagen et al. 2008. It is the first time that an acoustic system is deployed to serve both acoustic tomography and positioning and navigation of gliders and floats. This experiment will demonstrate the uniqueness and strength of acoustic multipurpose system concept, and brings us over to the requirements and potential of acoustic navigation of Lagragian systems, acoustic communication, and passive systems.

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13 Figure 7. The acoustic system was deployed with RV Håkon Mosby and KV Svalbard in august/September 2010. The triangle in the SAR image is formed by the transceiver moorings in A, B, C and in the middle the receiver mooring D is positioned. The acoustic moorings are separated by 88km upto – 300 km. The satellite image show how the tomographic moorings (yellow crosses in the triangle) and the RAFOS sources (yellow crosses outside the triangle) are positioned in the MIZ. This is what makes the system unique and important both for navigation of gliders in under the sea ice and out again, and for the acoustic tomography. Each mooring records low frequency ambient noise, and represents a unique and forefront passive acoustic monitoring system of marine mammals.

Acoustic navigation and tracking of Lagrangian systems . The ARGO float program is the most extensive oceanographic program ever. Through international collaboration the net of 3000 ARGO floats provide oceanographic profiles from the world oceans (http://www.argo.ucsd.edu/), except from the high latitudes. Each ARGO float has a life-time of 2-5 years depending on the number and kind of sensors. In ice-free waters ARGO systems will surface usually every 10- day to send data and update their position and internal timing via satellite communication. ARGO floats are untracked during the drifting phase between each profiling. This makes the ARGO floats unable to resolve several meso-scale phenomena. Narrow banded acoustic signals (RAFOS) have been used for several decades to track drifting float. RAFOS floats do not profile as they seek to maximize the Lagrangian content of a float track and focus on revealing the structure and statistics of fluid motion. The RAFOS floats are used to get a clear picture of how fluid moves around in the Oceans. By equipping the ARGOS floats with acoustic receiver system, and introducing an operational acoustic tracking systems in the world ocean, the benefit of the drifting phase of the ARGO floats would increase.

Floats and gliders are the only tool for elucidating meso scale processes in the Arctic and Antarctica. However, in ice-covered areas gliders, AUVs, and profiling floats cannot rise to the surface to use satellite based navigation (GPS) and data telemetry via IRIDIUM or ARGOS communication satellites. Therefore, under ice drift phase, in the order of several months, is expected in the Arctic and Antarctica. In order to operate underwater platforms in the polar regions, it is necessary to develop acoustic navigation and telemetry systems (Lee & Gobat, 2006). Under ice navigation of gliders has been successfully used in the Davis Strait mid frequency sources at 780 Hz (Lee et al. 2010). The navigation of gliders builds on acoustic tracking as used for RAFOS floats. An underwater acoustic tracking and navigation system need a series of nested acoustic subsystems of different frequencies, each of which will consist of a small number of acoustic sources serving many inexpensive

AOEM D9 – Final report February 24, 2011

14 receivers carried by floats, gliders and other autonomous platforms. Basin wide acoustic navigation systems would consist of a network of sources at frequencies below 100 Hz. The feasibility of basin wide acoustic network was successfully tested as part of the ACOUS thermometry experiment using 20 Hz source (Gavriolov and Mikhalevsky, 2006), but it was not exploited for navigation of gliders or tracking of floats. During the last decade several regional acoustic networks has been implemented and used at several locations at high latitudes, such as for RAFOS float tracking in the Lofoten basin (Søyland, 2010, personal communication), glider navigation in the Davis Strait (Lee, et al 2010), glider navigation in the Fram strait (Sagen et. al. 2010, Figure 4), and RAFOS float tracking in the Weddell Sea (Klatt et al, 2007, Figure 3). The majority of the regional acoustic systems uses organ pipe sources to generate a 260 Hz standard RAFOS signal. One exception is the Davis Strait experiment where RAFOS signals have a center frequency at 780 Hz. While the regional networks mentioned above are based on sources integrated in bottom-mounted moorings, Advanced Ice Tethered Platforms (AITP) are under development for regional implementation in the central Arctic Ocean (Dickson, 2008, Sagen et al. 2008). The AITPS are currently equipped with SOFAR 1560-Hz and 780-Hz sound sources to provide acoustic navigation and communication for gliders and floats operating under ice. The AITPs are planned to be deployed in clusters of 3 to 5 covering areas of 100 km by 100 km up to 200 km by 200 km, depending on acoustic frequency. Recent results obtained under the ACOBAR indicate that the typical ranges for 780 Hz is 160 km in the interior Arctic (Jean Claude Gascard, 2010). As previously shown in this report the acoustic propagation characteristic are very sensitive to the source receiver configuration in an Arctic environment. In near future, the acoustic sources will be mounted to the CTD profiler, which scans temperature, salinity and pressure from surface to 1000 m depth. In this way acoustic propagation conditions and optimal receiving conditions are obtained in real time, allowing the CTD profiler carrying the acoustic source to park at an optimal depth for transmission of navigation signals to glider and floats. The profiler transmits data to a surface buoy, using inductive modem, providing near real time capability through satellite communication (Jean Claude Gascard, 2010).

In December, 2002, an array of ten sound sources was installed by Alfred Wegners Institute in the Weddell Sea (Figure 8). The system was recovered in February 2004. The sources transmitted a standard 260 Hz RAFOS signal twice a day. Several floats equipped with acoustic receivers were deployed and tracked acoustically using receptions from the 10 sources. Acoustic tracking depends on good acoustic receptions from at least 3 different sources. In Figure 8.b the receptions obtained from each source by a NEMO float is plotted as function of date (independent of year). From the plot we see that the quality of the received signals is better for small distances than for longer distances, which is not surprising. But we see also a seasonal time-dependency of the quality. From July to November the transmission is worse compared with the other period, in particular for distances larger than 600 km where you can see the black dots, but also for smaller distances where more blue dots in winter than in summer are visible. This behavior is due to reflection bounces of the sound-signals at the sea ice surface causing loss of acoustic energy due to reflection and scattering. During winter-time the ice coverage along the tracks was around 80 %, while it was more or less ice free during summertime. To conclude from figure 58, the RAFOS sources in the Antarctica have a range of at least 500 km in winter-time and 900 km in summer conditions (See Figure).

In comparison with the Arctic the Antarctic sea ice smoother and younger. We therefore expected that the acoustic ranges in the Arctic are less than 500 km for sources at 260 Hz. To know this for sure we will need to measure the acoustic ranges for 260 Hz in the Arctic environment under different ice conditions.

In 2008 a tomographic sweeper source was deployed in a mooring at 400 m in 1480 m water depth in the Fram Strait. During a field experiment acoustic measurements were made both in ice and in open water as shown in Figure 9. In open water, the small vertical hydrophone array attached to a 100 m long cable was launched from a small motor-boat operating out from the ice breaker KV Svalbard. At recording sites in the ice pack, the recordings were made either from ice floe close to the ship or from the small boat close to KV Svalbard. There was no source signal detected under the ice. This is partly due to high noise levels from the ship, partly because of cable strum, and finally because the hydrophone array to be inside the typical Arctic surface duct. However, the receptions in open water are detectable and sharp at distances up to 130 km where the long vertical mooring

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15 array was located. In Figure 5 the tomographic signal is received at the different receiver depths are plotted. We see that the signal to noise ratio is good at all depths, but we also notice that there is multipath arrivals coming in very close. This is important to be aware of when detecting and selecting the arrival time and acoustic paths to be used for ranging.

In 2009 two RAFOS source moorings was deployed at two different locations, see R19 and R14 in Figure 10, but only the R19 source was working. After deployment a listening program was carried out in open sea. In total 18 positions were visited (plotted in Figure 10), and the signals was obtained with a very high signal to noise ratio at all ranges up to 228 km, including the locations near the ice edge at R16 Se selected stations in Figure 10.

Three tomographic sources configured in a triangle ABC were deployed in August/September 2010 from RV Håkon Mosby and KV Svalbard. Each source is at 400 m. The system will be recovered in 2012. This is a unique system, which provide both the standard narrow banded (1.5 Hz) navigation signal every 6 hours every day during selected time periods and the broad banded (100 Hz) tomographic signal every 3 hour every other day. Two additional RAFOS sources at 800 m are positioned well inside the ice edge.

During the KV Svalbard cruise in 2010 the signals from this system was been detected at 100 m below sea surface at locations both inside the ice edge, see Figure 11, and outside the ice edge, see Figure 12. This is a very good sign as it is expected that the signal from the sources at 400 m and 800 m will be most difficult to detect within the 200-250 m deep surface duct.

Institute of Marine Research, in close collaboration with Thomas Rossby at Rhode Island University has an ongoing acoustic float program in the Nordic Seas with 7 sources deployed in the Lofoten basin (Søiland et al. 2007). About 10 floats were deployed in 2010. RAFOS signals from the sources in the Fram Strait have been identified by the floats in the Lofoten basin (Henrik Søyland, 2010). Besides showing that the ranges in open water is more than 1000 km in open ocean, this show that the Fram Strait sources can be used to track floats drifting northwards towards the Arctic Ocean.

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16

Figure 8 a). Within the Weddell Sea Alfred Wegner installed an array of ten sound sources in December 2002 and recovered in February 2004. The range of the sound signals is about 500km during winter and 900km in summer. The circles in the figure correspond to 500km. Each sound source emits one pong per day.

Figure 8 b). The y-axis depicts the distance of the float from the respective sound source, the x-axis the date of the transmission independent from the year. Each dot represents a received sound signal and the color of the dots depicts the correlation height of the received signals, a measure of the quality of the transmission. Black points stand for - no useful signal received. All other colors can be used for the subsurface tracking, but the more the color is to the red side - the better is the transmission.

Figure 8 b). A NEMO float with RAFOS receiver was deployed in the Weddell Gyre. The White line in the map corresponds to the acoustically tracked subsurface trajectory. The float was deployed in open water (red labels), and then it drifted under ice for 240 days (black labels). Under-ice profiles were interim-stored over winter and geo-located using the RAFOS array. Normal profiles in red (dots and numbers). Stored profiles are in black (dots and numbers)

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17

Figure 9. During the DAMOCLES experiment, 2008-2009, one source and one receiver mooring was deployed in the Fram Strait. The blue dot in the ASAR image refers to the position of the receiver array. Red dot shows the source position. A short acoustic array was deployed from a small boat using a 100 m long cable, and several listening stations were visited during the KV Svalbard cruise in 2008. Yellow is location of acoustic recordings. In the lower plot the recordings has been run through pulse compression to detect the arrival times. The x-axis shows the elapsed time since the transmission started. The y-axis is the unnormalized cross-correlation between the signal and the replica (190 to 290 Hz linear FM sweep in 60 s).

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18

Figure 10. As part of the ACOBAR project a RAFOS source was deployed at point 19 at 800 m below the surface. In the lower plot the recordings has been run through pulse compression to detect the arrival times. The x-axis shows the elapsed time since the transmission started. The y-axis is the unnormalized cross- correlation between the signal and the replica (190 to 290 Hz linear FM sweep in 60 s).

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19

Figure 11. The deployment plan of the tomographic sources in A, B, C, D, and the RAFOS sources in FSQ1-2. FSQ2_2 (not working) and FSQ3_1.

Sonogram obtained by the unmanned drifting listening station deployed by WHOI for 2 days during the KV Svalbard cruises. The listening hydrophone is at 100 m. The listening station was 25 km away from C source from which we can clearly see the signal. We can also see the RAFOS 1.5 Hz sweep at 260 Hz generated by FSQ1_2 source. The distance between the RAFOS source and listening station is 59.1 km. In both cases the acoustic path is through ice- covered region. Note that depth and distance for the RAFOS 1 source in Figure is not correct.

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20 Figure 12. The Tomographic sources overlaid the SAR image from 15th of September 2010. The white flag indicated the recordings made on the 16th of September.

FRAM Stait 2010 Tomography Sources 400 Fram Strait 2010 Tomography Sources On the 16th of September the three tomography sources were

350 all observed in a recording by a Source A Source B Source C hydrophone at less than 100 m 200-300Hz 190-290Hz 210-310Hz below the small boat. The listening station was 90.6 300 km from A-source, 245 km from B source and 187.2 km from the C mooring. AC track is partially 250 covered with ice, while the other two tracks are only in open water. Frequency (Hz) 200

150

100 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 Time from beginningTime of recording (sec)

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21

Accuracy of positioning and timing The navigation of gliders and tracking of acoustic float builds on the same positioning approach. The glider and float is equipped by a hydrophone to receive the acoustic signal, and a computer with performing a correlation of the received signal to the known RAFOS signal to detect of the travel time of the signal. How accurate the signal arrival is detected depends on signal strength and the capability to resolve the arrivals. The latter depends on the signal bandwidth, the wider the better. Sources in moored navigation systems usually send out narrow banded sweep from 259.375 Hz up to 260.898 Hz in 80s. The narrow bandwidth of 1.5 Hz results in an error of 1.5 km/sec*/ 1.5 Hz = 1 km error. In the Davis Strait a 40 s long, 5 Hz wide sweep signal at 780 Hz is used. The 780 Hz source have a limitation of 1.5km/s / 5.0 Hz = 0.3 km. In drifting local networks formed by Acoustic Ice tethered platforms (AITPs) a 20 s long sweep from 1555 Hz up to 1565 Hz. The 1555 Hz source have a limitation of 1.5km/s / 10.0 Hz = 0.15 km. By using the 100 Hz wide tomography signal from the Webb sweeper source would reduce the limitation caused by bandwidth to 1.5 km/s /100 Hz = 0.015 km.

The arrival time is now given relative to the internal clock system. (Note: The further processing is equal but performed internally in the glider and in hind cast in the case of floats.) To calculate the accurate travel times requires accurate clocks both in source and receiver. Webb RAFOS sound sources, the RAFOS floats and gliders uses TCXO Seascan clocks which suppose to have ~ 10-07 - 3.0*10-08 stability in water. In ideal case this gives 1- 1.5 second a year. Sometimes it is 3 sec / year. Due to the clock instability in the RAFOS sources (O(1s)) there is an error increasing with time towards at least 1.5 km/sec*1 sec = 1.5 km during an year. A good Rubidium give 10-10 s, and near 0.03 s/year, corresponding to 0.045 km during a year. For a long time accurate atomic clocks meant high power consumption. Currently, Teledyne Webb Research is integrating low consumption (20 mW) atomic clock (10^-10) in source electronics. Power consumption is close to Seascan clock.

A Webb source and an atomic clock would provide a accuracy of 60 m. However, the precision of real system is limited by spread in travel time and time spread due to scattering caused by internal waves, tides, current flow turbulence and small-scale ocean inhomogeneities. This small scale ocean speed fluctuations are hardly predictable and difficult to measure by acoustical inversion. The time scale of such fluctuations can reach ~ 60- 70 msec and will limit the accuracy of range estimation by the order of 100 meters.

At this point we need the sound speed profiles between source and receiver to calculate the range corresponding to the measured travel time. This is a source of errors introduced because the details of the sound speed field are usually unknown. This source of error is difficult to eliminate or estimate. Using an averaged sound speed profile could be reduced by repeated transmissions, this is however not practical in a long-range navigation system. Due to battery capacity and the power consumption for sources used in a long range acoustic network the navigation signal goes off only 2-4 times a day. In the more local network constituted by several sources in a net of AITPs, the sources transmit every 15 minute. This improves the accuracy caused by environmental conditions.

To obtain the position, accurate arrival times and corresponding ranges are in general needed from 3 or more sources to perform triangulation. In a system consisting of several sources, each source is identified by the time the signal is sent. For glider navigation the final step is to calculate the difference between the target position and the estimated position one find the navigational parameters such as bearing and distance from target.

System requirements for an improved long range joint acoustic navigation and thermometry/tomography have been defined, see Duda et al. (2006). As above, their study conclude that the accuracy of kilometres accuracy for basin-wide navigation achieved using narrow banded RAFOS signals is reduced to less than 100 meters of accuracy by using broadband acoustic tomography signals for navigation. Based on this we strongly encourage to use the broadband signal produced by the Webb sweeper sources instead of the narrow banded RAFOS signal. Improved accuracy is useful if the data from floats and gliders is used for process study, validation of high resolution ocean models or assimilation into high resolution models. Before, implementing an operational system of acoustic system in the Arctic it is essential to know how far away from the sources the signal at

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22 different frequencies can be detected. Acoustic communication. Simple acoustic communication is already extensively used for example acoustic releases in bottom-mounted moorings, and when we interrogate acoustic transponders placed at the sea floor. Furthermore, a new generation of acoustic communication devices, acoustic modems, has been developed the last 5-10 years. The benefit of using acoustic communication within ocean observatories or between different observatories is that the need of cables can be significantly reduced. However, acoustic communication has its clear limitations in particular to the amount of data to send due to the highly variable properties of the ocean waveguide properties in space and time. In 2007 a report was prepared by IFREMER for the ESONET project where several modems from different providers were compared [1.20]. It was detected large differences in the capacities of the modems. In this report the modems were tested in a temperate environment. Here we just summarize a few experiences reported by our partners in ACOBAR, AOEM, and AWAKE project to illustrate that acoustic communication is not yet “plug and play”.

Propagation of acoustic signals (amplitude and travel time) is strongly influenced by spatial and temporal variability in the oceanography (internal waves, tidal current, mesoscale features), in addition to strong bathymetric effects. This will of course influence the acoustic communication. The most difficult area to get modems to work seems to be in the ice-covered regions, this is due to the strong stratification of the ocean causing a bunch of multi paths with arrivals that overlaps in time such that the information is not always obtained. Therefore, even if a communication set-up h is developed and implemented in a particular area, for a particular use, works; do not necessarily imply that it will work in another season or in any another area. The communication has to be designed for the problem on hand including ranges, typical oceanographic characteristics, modem platform (mooring, glider) and amount of data to be sent. Typical set ups are data transmissions between instruments and surface units within a mooring (ranges of some 1000s meters), between different moorings (ranges of 10s of km), between a moored unit and for example a glider, and commands to very distant moorings.

University of Bergen uses modems in moorings in the Weddell Sea, Barents Sea and Færøy channel. So far only results from the Weddel sea is available. In this case standard instruments from Aanderaa Data were interfaced with a commercially available modem. Instruments and modems were deployed within a mooring deployed in a partly ice covered region (Svein Østerhus, 2010, personal communication). During data download the data transmission was delayed by poor signal to noise and propellers had to be stopped and the surface unit was lowered between the ship and ice floe to be protected from drifting ice floes. Range limitations were significant [1.21]. However, a large amount of the oceanographic data was successfully downloaded.

Long-range horizontal transmission of oceanographic data between mooring in the Fram Strait has been tested by AWI within ACOBAR and DAMOCLES. It showed that it is very hard to get the modems to work; however some data was transferred. The instability in data delivery is claimed to be due to variable tilt of the modem due to mooring motion. Attempts to solve this have been made by the manufacturer, and results of this are currently under evaluation (Beszczynska-Moeller, 2010, personal communication)

Under the DAMOCLES project, NERSC used acoustic modems in acoustic data from tomography moorings in the Fram Strait for data download with ship born acoustic modem ([1.22], [1.23]). These modems suffered from hardware and software bugs, and signal to noise ration was reduced due to noise ships. Furthermore, the amount of data from a tomography mooring is very high, and the acoustic data has to be internally processed in the mooring. With the bit rates available in this environment with this specific modem, the ship had to stay on position for at least 78 hours to download data, corresponding to engineering data and a few days of pre- processed receptions from the 8 hydrophones. Based on the experience of current modems capabilities in data transfer rates it was concluded that acoustic modem for transfer of acoustic receptions is not yet practical. A cabled system provides the best data transfer both within passive and active acoustics. Other alternatives are to use pop-ups, winches, or gliders/AUVs equipped with modems.

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23 To summarize, acoustic communication is not a easy task; and success depends on careful testing of instruments in the laboratory to check both hardware but also check the interface between the observing sensors/instruments and the modem it self. In addition the sound propagation conditions in the area of deployment should be investigated to choose the optimal configuration of the system and to be aware of the environmental limitations of the communication. Passive acoustics Ambient noise is generated by natural physical sources such as sea ice, ocean waves, seismic activity, rain, gas bubbles in the ocean; marine mammals and by human activities. In the interior Arctic the main natural sound generating mechanisms are ridging, break up of sea ice, and thermal cracking. Ambient noise in the Marginal Ice Zone is mainly caused by wave interaction with different types of sea ice, internal ice stress, thermal cracking and break-up of ice floes (e.g. Makris and Dyer, 1986, Lewis and Denner, 1990). After the ambient noise is generated is flavoured by the propagation conditions determined the ocean stratification and the boundary conditions at the sea surface (ice or sea state in the open ocean) and the sea floor. Accordingly, the ambient noise field contains a lot of information about the marine environment, which we would like to extract.

There are several operational systems for passive listening both for civilian and military purposes, but none in the Arctic Ocean. The most extensive system is operated by the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO), who operates eleven passive hydroacoustic monitoring stations around the world (six hydrophone and five Tphase stations using seismographs on small islands), which routinely detect, locate, and characterize manmade and natural underwater events including explosions, volcanoes, earthquakes and marine mammal vocalizations (Dushaw et al. 2010). Three acoustic observatories was implemented on the Australian continental shelf as part of the Integrated Marine Observing System (IMOS) (Gavrilov, et al. 2009) to study the natural sources of noise in the ocean such as earthquakes, underwater volcanoes and ice breaking processes in Antactica. In the Arctic Basin there is no operational permanent systems for passive acoustics, but several scientific systems has been deployed for time limited periods, in particular during the 80s and 90s. One example was the study of seasonal variation and impact of tidal currents on ambient noise in the Barents Sea studied in a 1-year long experiment (1992-1993) by NDRE and NERSC (Johannessen et al. 1992). More recently, in 2008, two passive recording systems were deployed in the Fram Strait (personal communication, Sue Moore, 2009) in order to monitor marine mammals and anthropogenic sound. In 2010 an acoustic autonomous system, developed by NAXYS AS, within the WIFAR project funded through NFR-PETROMAKS program and Total, was deployed for 2 years operation as part of the Fram Strait acoustic network. The aim of the deployment is to listen to ambient noise generated by dynamic process in the ice with particular focus to reveal the period of waves-in- ice, to provide benchmark data of ambient noise and to observe the vocalization of marine mammals in this remote area. Furthermore, in 2012, low frequency ambient noise data will be available from each of the five tomographic vertical hydrophone arrays in the Fram Strait, each array consisting of 4 hydrophones. This data set will be the most extensive continuous recording of ambient noise, providing information about whales, walruses, seals, and anthropogenic sound over a period two years.

A network of acoustic listening arrays can be used to detect vocalisation from several types of animals (Nieukirk et al., 2004; Stafford et al. 2007). Methodologies for localization of the sources implies an inversion problem, the oldest of which is based on measured arrival time differences at pairs of hydrophones and location of the sound source (animal) on a hyperboloid. The method requires that the same acoustic signal can be recorded at 4-5 hydrophone locations to avoid ambiguous location solution (Spiesberger, 2001). A more advanced inversion technique compares the measured pressure field to that predicted by a propagation model (replica generation). Localization is determined by repeated model predictions on a single multi hydrophone array using an exhaustive search over assumed source positions. Statistical measures are used to find the localization with the highest probability along with error estimates. A matched field approach, based on normal mode theory, was used by Thode et al. (2000) using data from a 48 element tilted vertical array off the Channel Island. Another,

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24 extensive and promising approach uses ray trace models (Skarsoulis et al., 2005,2006; Tiemann et al., 2004). Tiemann et al., 2004 showed that inclusion of range dependent acoustic models could improve the localization accuracy significantly, especially in regions with complicated bathymetry and shallow water.

Passive acoustics would comprise a few hydrophones and a small controller/processing unit with modest power consumption and is easily integrated into platforms. By mounting autonomous acoustic recorders on fixed moorings, floats, gliders, and AUVs, permits systematic measure of the seasonal occurrence of vocal cetaceans (Mellinger et al., 2007), and provides a window into the large-scale seasonal movements and habitat selection of Arctic marine mammals (Moore and Huntington, 2008). However, acoustic recordings from large arrays of hydrophones, will produce a large amount, which will be demanding to obtain in real time unless the system is connected to a cabled network. It would make sense to implement a cluster of vertical arrays in the vicinity of the cabled network in the Fram Strait for environmental monitoring and for localization and tracking of marine mammals in this region.

Acoustic technology. The organ pipe sources used in ACOBAR/DAMOCLES, commercially available from the Webb Research Corporation, USA, produce sweeps with a exactly 100 Hz wide band width starting at around 200 Hz moving continuously up to around 300 Hz (Webb and Morozov, 2006). The time duration of the transmissions is adjustable. The source can be programmed to produce CW signals or RAFOS signals. On axis maximum level is 190 dB // 1 mPa at 1 m from the source. To obtain accurate clock and positioning of the source in the water column, the sound source is integrated with STAR electronics, developed by Scripps Institution of Oceanography, in one pressure housing as one unit. The sound source is generally rated down to 2000 m, but can be rated to deeper depths on request. Existing, low and mid frequencies, sources usually require a modest amount of maintenance except for the change of batteries. Depending on if the sources are old or new the energy efficiency varies a lot, but in any case the acoustic sources need significant energy to produce the low frequency signals for long-range propagation. This of course impact the maintenance costs. Change of batteries would have to be done every 1-2 year depending on how often the sources are transmitting. Change of batteries requires full recovery of the moorings, and the sources available are between 500-1000 kg. Recoveries and deployments need to be handled by ships with cranes and winches. Also, changing batteries is best handled onshore, and replacement sources have to be available or the deployment and redeployment has to be done in two separate cruises. The experiment schedules for the tomographic transceiver and standalone receiver arrays are controlled by the Simple Tomographic Acoustic Receiver (STAR) technology, which is developed by Scripps Oceanographic Institution. The standard STAR provides a precise clock, using a two-oscillator system (MCXO plus Rubidium). This time keeping concept provides a precision/stability better than 3 ms over a year. Furthermore, the STAR system together with four acoustic transponders surrounding each mooring location provide a long-baseline acoustic navigation system to measure the position of the control unit with an accuracy of 0.5 - 1 m. The four transponders are deployed 1.0-2.0 km away from the anchor position of each mooring, the distance depends on the distance from navigation transducer on the mooring down to the sea floor. A standard STAR comes with a tube containing the electronics and lithium batteries. The standard STAR electroics supports 4 hydrophones each with a separate cable, the length of one individual cable can not exceed 300 m. Usually the hydrophones are spaced by 1.5 wavelengths to be able to detect both arrival time and arrival angle through array processing. However, in DAMOCLES a long vertical aperture is chosen to increase the amount of information available for the inversions and assimilation. A 686 m long vertical receiver array is obtained by joining two individual STARs tail –by – tail, where the hydrophones on each STAR are spaced by 96 m. The upper STAR instrument is at 300 m, and the first hydrophone is positioned 7 m below the STAR navigation transducer. Correspondingly the vertical array consists of 8 hydrophones at 307 m, 403 m, 499m, 595 m, 691m, 787m, 883 m, 979m. It is a clear limitation of the system that only 4 hydrophones can be used in a standard STAR technology. The standard STAR is prepared for communication through acoustic modems through RS232 connection, however the amount of data from acoustic measurements has to be reduced significantly due to limited data transfer rates in acoustic modems. Ambient noise can be studied by the STAR, but the sampling rates used is 1000 Hz or 2000 Hz and this limits to low frequency ambient noise.

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25 Recently a flexible vertical hydrophone array (DVLA) was developed at Scripps Oceanographic Institution. This achievement will revitalize the Arctic acoustic research significantly. The array can be up to 1000 m long, and can include 99 hydrophones/logger systems freely distributed along a mooring wire, which is used as an inductive modem used to send commands and synchronization signals to the individual hydrophone/logger systems. The system can be used both in shallow water and in deep water just be adjusting the wire length and the number and spacing of the hydrophone units. This concept, provide a totally new capability to map the arrival time structure as a function of depth, to do decoupling of the acoustic field into modes, and advanced beam forming. In short and dedicated experiments the system DVLA can be attached to a surface buoy with communication capabilities and in long term experiments it can be included in a under water mooring. The mobile and flexible test range will bring new knowledge important for development of innovative acoustic technologies for the Arctic and optimalisation of the tomography array, very-long range communication and navigation systems. A successful short-term Pilot Study/Engineering Test of the DVLA was conducted in the Philippine Sea during April-May 2009. A large-scale, one-year-long, acoustic propagation experiment in the Philippine Sea, for which six acoustic transceivers and a water-column-spanning Distributed Vertical Line Array (DVLA) receiver were deployed during April 2010, and is now in progress ( P. Worcester, 2010). The main limitation is that the DVLA cannot provide data through modem connection.

It would be a great benefit if the acoustic sources and receivers were implemented into a cabled network as this would provide power for the sources and real time capability for a huge amount of data. During the most extensive acoustic experiment in the North Pacific Ocean from 1996 till 2006, part of the Acoustic Thermometry of Ocean Climate (ATOC) project and the North Pacific Acoustic Laboratory (NPAL) project, one of the sources and receivers was cabled to shore, providing near-real time data (Dushaw, et al 2010, http://aog.ucsd.edu/publications/po_posters/CLIVARPoster_ATOC.pdf). There is no technological problem to integrate acoustic the state of the art sources and receivers into cabled networks. However, several acoustic transceiver moorings are needed, and they have to be configured to cover a large geographical area. Therefore, a acoustic system for tomography and navigation most likely will consist of both cabled and un-cabled moorings.

Acoustic listing systems are commercially available with different complexity. The main differences are found in clocks used for time keeping, number of hydrophones, hydrophone sensitivity and frequency domain, and depth rating.

To summarize the acoustic technology providing acoustic travel time measurements for tomography and signals used by gliders and floats for navigation is mature and can be integrated into a cabled network.

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26

Summary and Conclusion. Background. The Marginal Ice Zones and the interior Arctic Ocean under the ice is severely under sampled due to lack of regular observing systems. Whereas satellites provide basin-wide atmospheric and sea ice parameters in daily intervals, observations of the interior of the Arctic Ocean still come mainly from expeditions with research vessels, ice stations and aircrafts which are scattered in space and time. A future sustainable Arctic Ocean observing system will need to combine data from several sensors on ice tethered platforms or underwater moorings with satellite data and models using data assimilation (see http://www.oceanobs09.net/documents/OceanObs09-Conference_Summary-draft_24OCT10.pdf. During the International Polar Year (IPY) 2007-2009, new technologies for ocean observations such as acoustic thermometry/tomography, oceanographic sensors on ice tethered buoys, floats and gliders operating under the ice (Dushaw et al., 2010, Lee et al, 2010; Sagen et al., 2010).

We know that these observation systems have their limitations and advantages, in particular in the marginal ice zones. This is why we use an integrated system in the ongoing ACOBAR (2010-2012) for the Fram Strait, including high resolution models, tomography (low spatial resolution, but covers large volumes nearly instantaneously, high temporal resolution and not depth limited); gliders (high spatial resolution, but moving slowly causing low temporal resolution and the glider is depth limited); ice tethered platforms (need a stable and not to dynamic ice field) and fixed oceanographic moorings (point or profiling measurements with limited spatial resolution/representation, but high temporal resolution and not depth limited). These observing systems provide complementary data sets, and will hopefully, in combination with models and satellites, provide us with representative and useful 4 D ice-ocean fields for the Fram Strait. Our primary focus in AOEM WP 5 is on the acoustic network.

Capabilities of acoustic networks. An acoustic network can measure the acoustic travel times, to derive heat content and mean circulation on a regional or basin scale in minutes or hours respectively, and provide information about ice dynamics, earthquakes, and marine mammals through passive listening. Furthermore, the same network of acoustic sources provides an underwater “GPS” system for navigation and timing required for under-ice for gliders and floats in the Arctic. Gliders and floats can provide oceanographic fields at a high resolution in space, complementing the horizontally averaged acoustic measurements at a high temporal resolution. In this way acoustic infrastructure and measurements can contribute to fill the significant gap in ocean observations in the Arctic, including the marginal ice zones, by means of a limited number of transceiver moorings. It is therefore cost effective to develop and implement a multi purpose system in the Arctic as outlined in Figure 13.

Cabled moored networks. In the interior Arctic drifting local acoustic networks (<100 km) consisting of ice- tethered platforms (Lagrangian) with surface units can provide data in real time. However the lifetime of the ice- tethered platforms depend on how stable the ice conditions are. Keeping in mind the ongoing thinning of the sea ice in the Arctic and the last years minimum in sea ice extent, a larger area of the Arctic sea ice has become much more dynamic and variable. This is not in the favor of long term operating ice-tethered systems. In areas with drifting and dynamic sea ice, as in the Marginal Ice Zones, the only technical solution is underwater moorings, floats and underwater vehicles. Underwater moorings, of any kind, in the Arctic have a major problem in achieving real time capability, as they cannot be attached to surface units placed on a drifting ice floe.

Technologically there is no problem to integrate acoustic sources and receivers into cabled networks. A cabled acoustic network consisting of a modest number of moorings in the Arctic would be a robust and manageable observing system. This would provide basin wide measurements in real time and year round. Providing continuous data availability in fixed critical locations the cabled network can observe episodic events such as eddies or the passage/influx of warm or cool water masses when they happen to permit researchers to deploy/redeploy/direct other assets such gliders, unmanned systems, ice-tethered or moored platforms to monitor, track, analyze and study the event.

AOEM D9 – Final report February 24, 2011

27 Recommendations. On the European level our recommendation is to install one acoustic transceiver into the planned cabled network in the Fram Strait or north of Svalbard (See Figure 1). However, to establish an acoustic network this cabled transceiver has to be combined with un-cabled transceiver moorings. Furthermore, we recommend to include a cluster of minimum three hydrophone arrays into the acoustic network for advanced detection and localization and tracking of marine mammals. The Svalbard Integrated Observing System can offer opportunities to develop a system in the European sector of the Arctic. To proceed towards an operational acoustic network in the interior Arctic co-ordinated actions on the international level has to be taken also across disciplines. The international ANCHOR (Acoustic Navigation and Communication for High-Latitude Ocean) group of experts was established to coordinate the interoperable acoustic infrastructure the high Arctic (Lee and Gobat, 2006). Implementation of cabled systems in the Arctic can only be developed through international collaboration. European efforts to establish an acoustic network infra structure covering the Arctic have to be coordinated with Russian, Canadian and US initiatives and interest. Actions to establish international network are in progress both on European and US side.

Environmental assessment report has been developed for the ACOBAR, project see http://acobar.nersc.no for a summary. A final environmental assessment will also include the interior Arctic.

Figure 13. To the left the envisioned basin-wide mooring grid in the Arctic Ocean measuring the Arctic Ocean heat content in a couple of hours (tomography) and serving as a underwater “GPS” system for floats and gliders. The drifting ice tethered acoustic platforms are additional components of this high accuracy regional positioning system. The acoustic network provides a required infrastructure to operate gliders and floats under sea ice in the Arctic Ocean. The upper right panel shows the Fram Strait acoustic network for tomography, positioning of gliders and floats and passive acoustics overlaid on a satellite image. This system was implemented in the Fram Strait under the European projects DAMOCLES and ACOBAR in August/September 2010 (http://acobar.nersc.no). Transceiver moorings (labelled A, B, C) receiver mooring (D) and two RAFOS sources (green crosses) constitute the acoustic system. The first under ice glider mission is planned for summer 2011 using the existing acoustic network. The acoustic network and glider operations are co-located with the array of oceanographic moorings across the strait at 78°50’N. The right lower panel shows positions of moorings overlaid on the temperature distribution in Fram Strait (red colouring indicates warm Atlantic water and blue depicts cold Arctic waters). Transceiver moorings also record low frequency naturally generated sounds, and represents the unique and forefront passive acoustic monitoring system of ice dynamics, earthquakes, and marine mammals.

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