Article

Deep Sound: A Free-Falling Sensor Platform for Depth-Profiling Ambient Noise in the Deep Ocean

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Ambient noise in the deep ocean is traditionally monitored using bottom-mounted or surface-suspended hydrophone arrays. An alternative approach has recently been developed in which an autonomous, untethered instrument platform free falls under gravity from the surface to a preassigned depth, where a drop weight is released, allowing the system to return to the surface under buoyancy. Referred to as Deep Sound, the instrument records acoustic, environmental, and system data continuously during the descent and ascent. The central component of Deep Sound is a Vitrovex glass sphere, formed of two hemispheres, which houses data acquisition and storage electronics, along with a microprocessor for system control. A suite of sensors on Deep Sound continuously monitor the ambient noise, temperature, salinity, pressure, and system orientation throughout the round trip from the surface to the bottom. In particular, several hydrophones return ambient noise time series, each with a bandwidth of 30 kHz, from which the noise spectral level, along with the vertical and horizontal coherence, are computed as functions of depth. After system recovery, the raw data are downloaded and the internal lithium ion batteries are recharged via throughputs in the sphere, which eliminates the need to separate the hemispheres between deployments. In May 2009, Deep Sound descended to a depth of 6 km in the Philippine Sea and successfully returned to the surface, bringing with it a unique data set on the broadband ambient noise within and below the deep sound channel. The next deep deployment is planned for November 2009, when Deep Sound will descend almost 11 km, to the bottom of the Challenger Deep at the southern end of the Mariana Trench. If successful, it will return with continuous acoustic and environmental recordings taken from the sea surface to the bottom of the deepest ocean on Earth.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... To profile the ambient noise over the full depth of the ocean, a different approach is needed. This is provided by the acoustic recording platform Deep Sound (Barclay et al., 2009), which is a free-falling (untethered) instrument platform, designed to descend under gravity and, at a preassigned depth, release a drop weight, allowing it to return to the surface under buoyancy. Two vertically aligned, broadband (30 kHz) hydrophones on board Deep Sound record the ambient noise continuously throughout the descent and ascent. ...
... dbarclay@mpl.ucsd.edu information on the engineering and operational details of Deep Sound can be found in Barclay et al. (2009). ...
... Deep Sound, shown in Fig. 1, is an autonomous, untethered, free-falling instrument platform (Barclay et al., 2009), designed to descend from the sea surface to a preprogrammed depth, which may be as great as 9 km, whereupon a disposable weight is dropped using a burn-wire release, and the platform returns to the surface under buoyancy. The rates of descent and ascent are similar at, nominally, 0.6 m/s. ...
Article
Full-text available
During the Philippine Sea experiment in May 2009, Deep Sound, a free-falling instrument platform, descended to a depth of 5.1 km and then returned to the surface. Two vertically aligned hydrophones monitored the ambient noise continuously throughout the descent and ascent. A heavy rainstorm passed over the area during the deployment, the noise from which was recorded over a frequency band from 5 Hz to 40 kHz. Eight kilometers from the deployment site, a rain gauge on board the R/V Kilo Moana provided estimates of the rainfall rate. The power spectral density of the rain noise shows two peaks around 5 and 30 kHz, elevated by as much as 20 dB above the background level, even at depths as great as 5 km. Periods of high noise intensity in the acoustic data correlate well with the rainfall rates recovered from the rain gauge. The vertical coherence function of the rain noise has well-defined zeros between 1 and 20 kHz, which are characteristic of a localized source on the sea surface. A curve-fitting procedure yields the vertical directional density function of the noise, which is sharply peaked, accurately tracking the storm as it passed over the sensor station.
... There have been other recent efforts to deploy deep-ocean moorings with sensor packages to probe the world's ocean trenches. In 2009, a team from Scripps Institution of Oceanography (SIO) deployed an untethered instrument platform equipped with a 30 kHz hydrophone (depth rated to 11 km), designed to record during controlled descent from the surface (Barclay et al., 2009). A series of descents were made with this instrument in the Mariana Trench (maximum depth of 9,000 m) and in the Philippine Sea (6,000 m), providing, at that time, the deepest ambient sound level measurements collected (Barclay et al., 2009;Barclay and Buckingham, 2010). ...
... In 2009, a team from Scripps Institution of Oceanography (SIO) deployed an untethered instrument platform equipped with a 30 kHz hydrophone (depth rated to 11 km), designed to record during controlled descent from the surface (Barclay et al., 2009). A series of descents were made with this instrument in the Mariana Trench (maximum depth of 9,000 m) and in the Philippine Sea (6,000 m), providing, at that time, the deepest ambient sound level measurements collected (Barclay et al., 2009;Barclay and Buckingham, 2010). Also, in 2010, researchers at the Lamont-Doherty Earth Observatory (LDEO) designed a deep-ocean seafloor seismometer and pressure case capable of surviving to extreme depths. ...
Article
Full-text available
We present a record of ambient sound obtained using a unique deep-ocean instrument package and mooring that was successfully deployed in 2015 at Challenger Deep in the Mariana Trench. The 45 m long mooring contained a hydrophone and an RBR™ pressure-temperature sensor. The hydrophone recorded continuously for 24 days at a 32 kHz sample rate. The pressure logger recorded a maximum pressure of 11,161.4 decibars, corresponding to a depth of 10,829.7 m, where actual anchor depth was 10,854.7 m. Observed sound sources included earthquake acoustic signals (T phases), baleen and odontocete cetacean vocalizations, ship propeller sounds, airguns, active sonar, and the passing of a Category 4 typhoon. Overall, Challenger Deep sound levels in the ship traffic band (20–100 Hz) can be as high as noise levels caused by moderate shipping, which is likely due to persistent commercial and military ship traffic in the region. Challenger Deep sound levels due to sea surface wind/waves (500 Hz to 1 kHz band) are as high as sea state 2, but can also be very low, equivalent to sea state 0. To our knowledge, this is the first long-term (multiday to week) broadband sound record, and only the fifth in situ measurement of depth, ever made at Challenger Deep. Our study indicates that Challenger Deep, the ultimate hadal (>6,000 m) environment, can be relatively quiet but is not as acoustically isolated as previously thought, and weatherrelated surface processes can influence the soundscape in the deepest parts of the ocean.
... The version of Deep Sound that was deployed during PhilSea09 is the first of three such systems that have been built to date, the two most recent, with enhanced instrumentation suites, being designated Deep Sound Mk.II and Mk.III to distinguish them from their predecessor. Since the design details of Deep Sound can be found elsewhere, 3 only its essential components will be described here. It consists of a pressure housing in the form of a Vitrovex glass sphere of 0.432 m external diameter, which contains data acquisition, data storage, and system control electronics. ...
... Even with this arrangement, however, two significant turbulence components remain: Flow-induced turbulence 13 occurs around both acoustic sensors, due to their motion through the water column and, with the sensors aligned vertically, the trailing (upper on descent and lower on ascent) hydrophone lies in the turbulent wake of the leading hydrophone and its mounting assembly. 3 Since there are no flow shields protecting the acoustic sensors on Deep Sound, the level of the wake turbulence component, in particular, is significant, affecting not only the power spectrum from the affected hydrophone but also the coherence function and the crosscorrelation function computed from the raw time series from the two phones. ...
Article
Full-text available
In 2009, as part of PhilSea09, the instrument platform known as Deep Sound was deployed in the Philippine Sea, descending under gravity to a depth of 6000 m, where it released a drop weight, allowing buoyancy to return it to the surface. On the descent and ascent, at a speed of 0.6 m/s, Deep Sound continuously recorded broadband ambient noise on two vertically aligned hydrophones separated by 0.5 m. For frequencies between 1 and 10 kHz, essentially all the noise was found to be downward traveling, exhibiting a depth-independent directional density function having the simple form cos θ, where θ ≤ 90° is the polar angle measured from the zenith. The spatial coherence and cross-spectral density of the noise show no change in character in the vicinity of the critical depth, consistent with a local, wind-driven surface-source distribution. The coherence function accurately matches that predicted by a simple model of deep-water, wind-generated noise, provided that the theoretical coherence is evaluated using the local sound speed. A straightforward inversion procedure is introduced for recovering the sound speed profile from the cross-correlation function of the noise, returning sound speeds with a root-mean-square error relative to an independently measured profile of 8.2 m/s.
... The version of Deep Sound that was deployed during PhilSea09 is the first of three such systems that have been built to date, the two most recent, with enhanced instrumentation suites, being designated Deep Sound Mk.II and Mk.III to distinguish them from their predecessor. Since the design details of Deep Sound can be found elsewhere, 3 only its essential components will be described here. It consists of a pressure housing in the form of a Vitrovex glass sphere of 0.432 m external diameter, which contains data acquisition, data storage, and system control electronics. ...
... Even with this arrangement, however, two significant turbulence components remain: Flow-induced turbulence 13 occurs around both acoustic sensors, due to their motion through the water column and, with the sensors aligned vertically, the trailing (upper on descent and lower on ascent) hydrophone lies in the turbulent wake of the leading hydrophone and its mounting assembly. 3 Since there are no flow shields protecting the acoustic sensors on Deep Sound, the level of the wake turbulence component, in particular, is significant, affecting not only the power spectrum from the affected hydrophone but also the coherence function and the crosscorrelation function computed from the raw time series from the two phones. ...
Article
Deep Sound is a free-falling high-bandwidth acoustic recording system designed to profile ambient noise from the surface to depths of 9 km. The recording platform is autonomous and descends under gravity to its preprogramed maximum depth, where a burn-wire releases weight, permitting the system to return to the surface under its own buoyancy. Two hydrophones are mounted at half meter vertical spacing allowing the noise spectrum and vertical coherence (directionality) to be obtained over four decades of frequency (10 Hz-100 kHz). The acoustic recordings are made continuously as the instrument descends and ascends along with measurements of sound speed and depth. The system's low power and large data storage capabilities allow round trips to the deepest trenches of the ocean. Alternative modes of operation include (1) synthetic aperture signal detection and (2) residence on the seabed with return to the surface at a later time. Deep Sound's design and acoustic characteristics will be described and data from initial shallow water deployments around San Diego will be reported. [Work supported by the Office of Naval Research.].
... III, into the Challenger Deep within about 25 minutes of one another. Each had a CTD profiler and multiple hydrophones on board, with system electronics contained in a 38 cm Vitrovex glass sphere pressure housing (Barclay and Buckingham, 2009). At a nominal depth of 9,000 m, during its descent to the bottom, the spherical pressure housing on Deep Sound Mk. ...
Article
Full-text available
Since HMS Challenger made the first sounding in the Mariana Trench in 1875, scientists and explorers have been seeking to establish the exact location and depth of the deepest part of the ocean. The scientific consensus is that the deepest depth is situated in the Challenger Deep, an abyss in the Mariana Trench with depths greater than 10,000 m. Since1952, when HMS Challenger II, following its namesake, returned to the Mariana Trench, 20 estimates (including the one from this study) of the depth of the Challenger Deep have been made. The location and depth estimates are as diverse as the methods used to obtain them; they range from early measurements with explosives and stop watches, to single- and multi-beam sonars, to submersibles, both crewed and remotely operated. In December 2014, we participated in an expedition to the Challenger Deep onboard Schmidt Ocean Institute’s R/V Falkor and deployed two free-falling, passive-acoustic instrument platforms, each with a glass-sphere pressure housing containing system electronics. At a nominal depth of 9,000 m, one of these housings imploded, creating a highly energetic shock wave that, as recorded by the other instrument, reflected multiple times from the sea surface and seafloor. From the arrival times of these multi-path pulses at the surviving instrument, in conjunction with a concurrent measurement of the sound speed profile in the water column, we obtained a highly constrained acoustic estimate of the Challenger Deep: 10,983 ± 6 m.
... Mk II was used to collect data during the SCE and as such this variant will be briefly described here. More complete descriptions of all the three are found in previous publications [6]- [8]. The instrument comprises a 3.6-cm-thick Vitrovex glass sphere with a 43.2-cm outer diameter and a depth rating of 9 km, four external hydrophones, a conductivity-depth-temperature (CTD) sensor, and recovery beacons, e.g., a strobe, radio beacon, and satellite beacon. ...
Article
The autonomous passive-acoustic lander Deep Sound was deployed at five locations during the Office of Naval Research (ONR)-supported Seabed Characterization Experiment, a multi-institutional field effort held at the New England Mud Patch, where the seabed is known to consist of a thick layer of silt and clay overlying a medium and coarse sand. The five deployments of Deep Sound were up to 9 h long, during which time ambient noise data, taken over an acoustic bandwidth of 5 Hz–30 kHz, were collected on four hydrophones arranged in an inverted “T” shape. Local temperature and conductivity were also recorded continuously at each location. A wave number integral model of wind-driven noise in a fluid waveguide over a two-layered elastic seabed was used to calculate the dependence of the vertical noise coherence on the geoacoustic properties in the overlying silt and clay layer, as well as the subbottom sand half-space. The modeled noise coherence was fitted to the data over the band 100 Hz–12 kHz, returning the compressional- and shear-wave speeds and densities of both layers and the thickness of the top layer.
... Finally, an innovative mobile platform has been developed for the study of ambient noise in the deep ocean. 'Deep Sound' is an untethered glass sphere which descends from the surface under gravity to a predetermined depth, at which it releases a weight and returns to the surface under buoyancy (Barclay et al., 2009). The device has been used to study depth profiles of ambient noise at depths of up to 6 km in Pacific Ocean trenches, including the depth dependence of noise from wind (Barclay and Buckingham, 2013a) and rain (Barclay and Buckingham, 2013b). ...
Thesis
Full-text available
Levels of underwater noise in the open ocean have been increasing since at least the 1960s due to growth in global shipping traffic and the speed and propulsion power of vessels. This rise in noise levels reduces the range over which vocal marine species can communicate, and can induce physiological stress and behavioural responses, which may ultimately have population level consequences. Although long-term noise trends have been studied at some open-ocean sites, in shallower coastal regions the high spatiotemporal variability of noise levels presents a substantial methodological challenge, and trends in these areas are poorly understood. This thesis addresses this challenge by introducing new techniques which combine multiple data sources for ship noise assessment in coastal waters. These data include Automatic Identification System (AIS) ship-tracking data, shore-based time-lapse footage, meteorological data, and tidal data. Two studies are presented: in the first, AIS data and acoustic recordings from Falmouth Bay in the western English Channel are combined using an adaptive threshold, which separates ship passages from background noise in the acoustic data. These passages are then cross-referenced with AIS vessel tracks, and the noise exposure associated with shipping activity is then determined. The second study, at a site in the Moray Firth, Scotland, expanded the method to include shore-based time-lapse footage, which enables visual corroboration of vessel identifications and the production of videos integrating the various data sources. Two further studies examine and enhance basic analysis techniques for ambient noise monitoring. The first study examines averaging metrics and their applicability to the assessment of noise from shipping. Long-term data from the VENUS observatory are empirically assessed for different averaging times and in the presence of outliers. It is concluded that the mean sound pressure level averaged in linear space is most appropriate, in terms of both standardisation and relevance to impacts on marine fauna. In the second study, a new technique for the statistical analysis of long-term passive acoustic datasets, termed spectral probability density (SPD), is introduced. It is shown that the SPD can reveal characteristics such as multimodality, outlier influence, and persistent self-noise, which are not apparent using conventional techniques. This helps to interpret long-term datasets, and can indicate whether an instrument’s dynamic range is appropriate to field conditions. Taken together, the contributions presented in this thesis help to establish a stronger methodological basis for the assessment of shipping noise. These methods can help to inform emerging policy initiatives, efforts to standardise underwater noise measurements, and investigation into the effects of shipping noise on marine life.
... H igh-resolution temperature data derived from measuring ocean temperature and its effects are critical to understanding global physical processes, such as ocean warming (Gouretski & Reseghetti, 2010;Bintanja & van der Linden, 2013;Cowan et al., 2013) and climate change ; such data also inform ecosystem evaluation (Miller et al., 2004) and are instructive when compared with simulation data or used to validate models (Park et al., 1999;Dowsett et al., 2013). Many different methods are used to detect ocean temperature; the methods include using the Expendable Bathythermograph instrument (Hutchinson et al., 2013), Underway conductivity-temperature-depth instrument (Rudnick & Klinke, 2007), profiling instruments (Roemmich et al., 2004;Barclay et al., 2009), underwater unmanned vehicles, or attaching a temperature chain to the mooring system (Lueck & Huang, 1999). For physical oceanography research, large numbers of ocean temperature loggers are commonly mounted on subsurface mooring systems associated with profiling systems that monitor the physical, chemical, and biological parameters of the entire water column. ...
Article
Full-text available
Abstract Monitoring seawater temperature is important to understanding evolving ocean processes. To monitor internal waves or ocean mixing, a large number of temperature loggers are typically mounted on subsurface mooring systems to obtain high-resolution temperature data at different water depths. In this study, we redesigned and evaluated a compact, low-cost, self-contained, high-resolution and high-accuracy ocean temperature logger, the TC-1121. The newly designed TC-1121 logger is smaller and more robust, and its sampling interval can be automatically changed by indicated events. The logger’s initial accuracy is ±0.002°C, and its effective resolution is about 0.0001°C. The drift error of all six TC-1121 temperature loggers during a 450-day experiment was less than ±0.002°C. They are being widely used in many mooring systems to study internal wave and ocean mixing. The logger’s fundamental design, noise analysis, calibration, and drift test are discussed. A long-term sea trial in the South China Sea was completed successfully and demonstrated the effectiveness of the logger.
Article
In September 2012, the free-falling, deep-diving instrument platform Deep Sound III descended to the bottom of the Tonga Trench, where it resided at a depth of 8515 m for almost 3 h, recording ambient noise data on four hydrophones arranged in a vertical L-shaped configuration. The time series from each of the hydrophones yielded the power spectrum of the noise over the frequency band 3 Hz to 30 kHz. The spatial coherence functions, along with the corresponding cross-correlation functions, were recovered from all available hydrophone pairs in the vertical and the horizontal. The vertical coherence and cross-correlation data closely follow the predictions of a simple theory of sea-surface noise in a semi-infinite ocean, suggesting that the seabed in the Tonga Trench is a very poor acoustic reflector, which is consistent with the fact that the sediment at the bottom of the trench consists of very-fine-grained material having an acoustic impedance similar to that of seawater. The horizontal coherence and cross-correlation data are a little more complicated, showing evidence of (a) bathymetric shadowing of the noise by the walls of the trench and (b) highly directional acoustic arrivals from the research vessel supporting the experiment.
Article
Acoustic attenuation in seawater usually has little effect on the spatial statistics of ambient noise in the ocean. This expectation does not hold, however, at higher frequencies, above 10 kHz, and extreme depths, in excess of 6 km, an operating regime that is within the capabilities of the most recently developed acoustic instrument platforms. To quantify the effects of attenuation, theoretical models for the vertical directionality and the spatial coherence of wind-generated ambient noise are developed in this paper, based on a uniform distribution of surface sources above a semi-infinite, homogeneous ocean. Since there are no bottom reflections, all the noise is downward traveling; and the angular width of the directional density function becomes progressively narrower with increasing frequency because sound from the more distant sources experiences greater attenuation than acoustic arrivals from overhead. This narrowing of the noise lobe modifies the spatial coherence, shifting the zeros in the horizontal (vertical) coherence function to higher (lower) frequencies. In addition, the attenuation modifies the amplitudes of the higher-order oscillations in the horizontal and vertical coherence functions, tending to suppress the former and enhance the latter. These effects are large enough to be detectable with the latest deep-diving sensor technology.
Article
Most measurements of ambient noise in the deep ocean have been performed using an array of hydrophones located at a fixed depth. Recently, an instrument platform known as Deep Sound has been developed, consisting of a glass sphere containing data acquisition, data storage, and system control electronics, with a pair of vertically aligned hydrophones mounted externally. Deep sound descends under gravity, jettisons a drop weight at a pre-assigned depth, and returns to the surface under buoyancy, traveling in both directions at a nominal 0.6 m/s. Throughout the descent and ascent, the hydrophones record the ambient noise over a bandwidth from 3 Hz to 30 kHz. In April-May 2009, Deep Sound was deployed to a depth of 5500 m in the Philippine Sea. The vertical coherence of the measured noise, from 1 to 10 kHz, matches accurately a simple theory of deep-water, wind-generated ambient noise, provided that the local sound speed is used in evaluating the theoretical coherence function. Moreover, the cross-correlation function of the noise, obtained by taking the Fourier transform of the coherence function, provides the basis of an inversion technique for returning the sound speed profile in the water column. [Research supported by ONR.].
Conference Paper
To measure oceanic radiance and hence, the pelagic environment of marine cephalopods, we have built a new underwater multiple camera system that consists of a glass sphere that houses 6 machine vision cameras each capable of outputting uncompressed 2 megapixel images at 60 Hz. The cameras are arranged so that their fields of view are all orthogonal, pointing along the positive and negative directions of the principal Cartesian axes (+/- x, +/- y, and +/- z). Taking all of the camera data together, the light field incident on the Omni-directional camera system can be recorded. The system was configured for autonomous operation with batteries, computers, cameras, and ancillary electronics inside the 17” glass sphere. Preliminary data, recorded in both a pelagic and benthic environment, indicate that the data acquired by the system can provide interesting insights into spatial and temporal fluctuations in underwater animal habitats.
Article
The results of recent ambient‐noise investigations, after appropriate processing, are compared on the basis of pressurespectra in the frequency band 1 cps to 20 kc. Several possible sources are discussed to determine the most probable origin of the observed noise. It is concluded that, in general, the ambient noise is a composite of at least three overlapping components: turbulent‐pressurefluctuations effective in the band 1 cps to 100 cps; wind‐dependent noise from bubbles and spray resulting, primarily, from surface agitation, 50 cps to 20 kc; and, in many areas, oceanic traffic, 10 cps to 1000 cps. Spectrum characteristics of each component and of the composite are shown. Additional sources, including those of intermittent and local effects, are also discussed. Guidelines for the estimation of noise levels are given.
Article
Both a deterministic dipole model and the law of conjugate depths are applied to study the depth dependence of the low‐frequency ambient noise in the main sound channel and, when used together, a reasonable agreement with experiment is obtained. Scattering or diffusionmodels for the region below critical depth agree well with experiment. Complete area and line source models for the ambient level are derived. Three approximate predictions of the low‐frequency attenuation come from the deep ambient depth dependence, ambient differences with site position, and ambient differences with site and depth, respectively. All three are consistent with conventional measurements and with the scattering diffusion and bottom loss mechanism.
Article
Omnidirectional ambient noise levels were measured at two deep‐water locations in the northeastern Pacific Ocean.Hydrophones were positioned throughout the water column at depths ranging from about 200 m below the surface to about 150 m above the sea bottom. Analyses of the data over the frequencies from 15 to 800 Hz show that at low frequencies the noise levels decrease with increasing depth. The decrease with depth is greater below the critical depth than it is in the sound channel. These low‐frequency noise levels, and their depth dependence, are independent of the wind speed. At higher frequencies the noise levels and the depth dependence are controlled by the wind‐generated noise. At low wind speeds there is a decrease in levels below the critical depth, but above this depth both increases and decreases in levels with depth were noted. At these high frequencies during high wind speeds the noise levels not only rise, but also fill the water column to the extent that there is little decrease in level with increasing depth, even for the region below the critical depth.
Article
During the late 1960s and throughout the 1970s, the U.S. Navy conducted a series of ocean acoustic measurement exercises to support development of systems and techniques to detect nuclear submarines. The exercises and most of the technical documentation were classified. In 2003, a project was sponsored by the U.S. Office of Naval Research (ONR, Arlington, VA) to declassify documentation and demonstrate the capability to recover acoustic data recorded on magnetic tape. One of the exercises, known as CHURCH OPAL, was selected for demonstration of acoustic data recovery. The record on magnetic tape spanned a period of ten days in September 1975 from a vertical assembly of hydrophones at a site midway between Hawaii and California. This paper presents selected excerpts from a key report (Wittenborn, 1976) on ambient noise that previously was unpublished and unavailable for general distribution. The earlier work is augmented with more complete and detailed analyses of the recovered digital data using modern analytical techniques. Data acquired from the hydrophones below critical depth enabled isolation of ambient noise due to distant shipping and local wind. The frequency band of the acoustic analyses was 30-500 Hz. The wind component of the ambient noise was evaluated at frequencies lower than reported by Wenz (1962).