Hanne Sagen

Identification and quantification of soundscape components in the Marginal Ice Zone

Acoustic experiments using an integrated ice station were carried out during August 2012 and September 2013 in the Marginal Ice Zone (MIZ) of Fram Strait. The two experiments lasted four days each and collected under-ice acoustic recordings together with wave-in-ice and meteorological data. Synthetic aperture radar satellite data provided information on regional ice conditions. Four major components of the under-ice soundscape were identified: ship cavitation noise, seismic airgun noise, marine mammal vocalizations, and natural background noise. Ship cavitation noise was connected to heavy icebreaking. It dominated the soundscape at times, with noise levels (NLs) 100 km from the icebreaker increased by 10-28 dB. Seismic airgun noise that originated from seismic surveys more than 800 km away was present during 117 out of 188 observation hours. It increased NLs at 20-120 Hz by 2-6 dB. Marine mammal vocalizations were a minor influence on measured NLs, but their prevalence shows the biological importance of the MIZ. The 10th percentile of the noise distributions was used to identify the ambient background noise. Background NLs above 100 Hz differed by 12 dB between the two experiments, presumably due to variations in natural noise sources.

The Fram Strait acoustic tomography system

The deep and wide Fram Strait between Greenland and Spitzbergen is the main influx and efflux gate to the Arctic Basin. Although major resources are invested in measurements of current and temperature here (http://asof.npoar.no), the flux estimates still have significant deficiencies and errors. Our objective is to build, test, validate and use an innovated integrated observing and modeling system, including acoustic tomography, for improved monitoring of volume, heat and freshwater transports in the Fram Strait. As part of the DAMOCLES‐IP, (=Developing Arctic Modeling and Observing Capabilities for Long‐term Environmental Studies ‐ Integrated Project) a first step acoustic tomography system is to be installed in the Fram Strait between East Greenland and West Spitzbergen in August 2008. The first step tomography system consists of one acoustic source near the Svalbard shelf and one receiver array in the middle of the Fram Strait. An extended acoustic system serving both tomography and glider navigation i...

Sound speed as a proxy variable to temperature in Fram Strait

The application of ocean acoustic tomography in Fram Strait requires a careful assessment of the accuracy to which estimates of sound speed from tomography can be converted to estimates of temperature. The Fram Strait environment is turbulent, with warm, salty, northward-flowing North Atlantic water interacting with cold, fresh, southward-flowing Arctic water. The nature of this environment suggests that salinity could play an important role with respect to sound speed. The properties of sound speed with respect to temperature and salinity in this environment were examined using climatological and in situ glider data. In cold water, a factor of about 4.5 m s(-1) °C(-1) can be used to scale between sound speed and temperature. In situ data obtained by gliders were used to determine the ambiguities between temperature, salinity, and sound speed. Tomography provides a depth-averaging measurement. While errors in the sound speed-temperature conversion at particular depths may be 0.2 °C or larger, particularly within 50 m of the surface, such errors are suppressed when the depth is averaged. Using a simple scale factor to compute temperature from sound speed introduced an error of about 20 m °C for depth-averaged temperature, a value less than formal uncertainties estimated from acoustic tomography.

Seismic exploration noise reduction in the Marginal Ice Zone

A sonobuoy field was deployed in the Marginal Ice Zone of the Fram Strait in June 2011 to study the spatial variability of ambient noise. High noise levels observed at 10-200 Hz are attributed to distant (1400 km range) seismic exploration. The noise levels decreased with range into the ice cover; the reduction is fitted by a spreading loss model with a frequency-dependent attenuation factor less than for under-ice interior Arctic propagation. Numerical modeling predicts transmission loss of the same order as the observed noise level reduction and indicates a significant loss contribution from under-ice interaction.

Resolution, identification, and stability of broadband acoustic arrivals in Fram Strait

An ocean acoustic tomography system consisting of three moorings with low frequency, broadband transceivers and a moored receiver located approximately in the center of the triangle formed by the transceivers was installed in the central, deep-water part of Fram Strait during 2010-2012. Comparisons of the acoustic receptions with predictions based on hydrographic sections show that the oceanographic conditions in Fram Strait result in complex arrival patterns in which it is difficult to resolve and identify individual arrivals. In addition, the early arrivals are unstable, with the arrival structures changing significantly over time. The stability parameter α suggests that the instability is likely not due to small-scale variability, but rather points toward strong mesoscale variability in the presence of a relatively weak sound channel as being largely responsible. The estimator-correlator [Dzieciuch, J. Acoust. Soc. Am. 136, 2512-2522 (2014)] is shown to provide an objective formalism for generating travel-time series given the complex propagation conditions. Because travel times obtained from the estimator-correlator are not associated with resolved, identified ray arrivals, inverse methods are needed that do not use sampling kernels constructed from geometric ray paths. One possible approach would be to use travel-time sensitivity kernels constructed for the estimator-correlator outputs.

High-efficient tunable sound sources for ocean and bottom tomography, 15 years of operating history

In November, 2001, the R/V “Point Sur” departed from the Moss Landing Marine Laboratories pier with the first prototype of a high-Q, tunable organ pipe projector. The first test of the tunable Teledyne Webb Research (TWR) organ-pipe was successfully conducted on 11.09.2001. The TWR sound source demonstrated exceptional performance. It was coherent, efficient, powerful, and had unlimited operational depth, as well as a minimum level of high frequency harmonic content. The projector uses a narrow-band, highly efficient sound resonator, which is tuned to match the frequency and phase of a reference frequency-modulated signal. A finite element simulation shows the structural acoustics of the tunable resonator. The results of such simulation for the organ pipe with the octave frequency band are presented and discussed. This sound source was built for the Naval Postgraduate School (Monterey, CA) for studying temperature variability in the California Current. Since 2001, many deep-water ocean acoustic experiments have used this type of TWR sound source. The operating history of the sound sources is reviewed here. The first variant of the TWR sound source tuned the frequency of the resonator tube over the bandwidth 200-300 Hz. Modifications with 140-205 Hz, 500-1000 Hz and 800-1200 Hz frequency sweeps have been built. The transmission duration can vary from one second to a few minutes. In October 2002 a TWR sound source was deployed on the top of Hoke Seamount. It transmitted 135-s linear frequency sweeps for ocean acoustic tomography and RAFOS 80-s narrow-band chirps for two years until October 2004. The signal was received with a good signal-to-noise ratio at 700 km range. During 2004 and 2005 the TWR sound source system was used as a part of the instrumentation for the Meridional Overturning Variability Experiment (MOVE) of the International program CLIVAR. The sound source was deployed near the French island of Guadeloupe and transmitted tomographic signals to measure the temperature of the North Atlantic cold deep waters during 2004- 2005. After solving the engineering problems found in the first sea trials, a modified deep-water version of the sound source was built for long-term deployment.

A Comparative Study of Moored/Point and Acoustic Tomography/Integral Observations of Sound Speed in Fram Strait Using Objective Mapping Techniques

AbstractEstimation of the exchange of seawater of various properties between the Arctic and North Atlantic Oceans presents a challenging observational problem. The strong current systems within Fram Strait induce recirculations and a turbulent ocean environment dominated by mesoscale variations of 4–10-km scale. By employing a simple parameterized model for mesoscale variability within Fram Strait, the authors examine the ability of a line array of closely spaced moorings and an acoustic tomography line to measure the average sound speed, a proxy variable for ocean temperature or heat content. Objective maps are employed to quantify the uncertainties resulting from the different measurement approaches. While measurements by a mooring line and tomography result in similar uncertainties in estimations of range- and depth-averaged sound speed, the combination of the two approaches gives uncertainties 3 times smaller. The two measurements are sufficiently different as to be complementary; one measurement prov...

Transmission of a sound beam across a two‐fluid interface: Numerical results and asymptotic expressions

The transmission of a real sound beam at the interface between two homogeneous and dissipative fluid layers is considered. Numerical results are obtained using a fast Fourier transform algorithm. The results show that, at a given incident angle, the direction and displacement of the beam critically depend on the absorption coefficient and on the distance between the source and the interface. Various asymptotic formulas are also presented, which allow for a physical interpretation of the numerical results.

Ice edge ambient noise

Ambient noise data along with oceanographic, meteorological, and remote sensing data were collected during the Marginal Ice Zone Experiment in March and April 1987 in the Greenland Sea. The experiment was designed to study how ambient noise was generated by ice kinematics, ice edge eddies, and wave propagation into the ice pack. Sonobuoys were deployed in the water off the ice edge and in the adjacent ice‐field where eddies, jets, and surface waves were present. SAR images of the area were obtained during the ambient noise recordings and provided important information such as position of the ice edge relative to the sonobuoys. Advanced image processing enabled ice classification, and estimates of ice concentration and floe size distribution. These parameters provided information about how ice can influence the ambient noise level and frequency distribution. Procedures to enhance wave signature in the SAR images were used to analyze the wave pattern. Correlation between SAR and ambient noise data support t...

Monitoring the Arctic acoustic environments with the International Quiet Ocean Experiment

The northern high-latitude regions, including the Arctic Ocean, are becoming increasingly important as a result of global warming and their growing economic and political interests. Sea ice reduction is facilitating resource exploration, marine transport, and other economic activities in the regions. Warming waters lead to shifts in marine ecosystems and in soundscapes. Exploitation of resources in the Arctic is expected to grow in the coming decades, offering new opportunities for marine and maritime industries. To measure the environmental impact of ocean noise at a variety of spatial and temporal scales, the International Quiet Ocean Experiment (http://iqoe.org/) established in late 2017 a working group on Arctic Acoustic Environments. The first activities of the Working Group are focusing on identifying locations and times of existing and past acoustic studies in the Arctic Ocean, and synthesise the state-of-the-art on sounds, past, present, and future in the Arctic Ocean. WG activities at the Arctic Observing Summit 2018 (Davos, Switzerland) are linking with indigenous communities and other local stakeholders, to address emerging trends in marine transport and Arctic resource exploitation, and to plan for where/when the optimal acoustic surveys could be, and what metrics they should prioritise. The northern high-latitude regions, including the Arctic Ocean, are becoming increasingly important as a result of global warming and their growing economic and political interests. Sea ice reduction is facilitating resource exploration, marine transport, and other economic activities in the regions. Warming waters lead to shifts in marine ecosystems and in soundscapes. Exploitation of resources in the Arctic is expected to grow in the coming decades, offering new opportunities for marine and maritime industries. To measure the environmental impact of ocean noise at a variety of spatial and temporal scales, the International Quiet Ocean Experiment (http://iqoe.org/) established in late 2017 a working group on Arctic Acoustic Environments. The first activities of the Working Group are focusing on identifying locations and times of existing and past acoustic studies in the Arctic Ocean, and synthesise the state-of-the-art on sounds, past, present, and future in the Arctic Ocean. WG activities at the Arctic ...