David R. Barclay

Depth dependence of wind-driven, broadband ambient noise in the Philippine Sea

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 depth-dependence of rain noise in the Philippine Sea

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.

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

Ambient noise in the deep ocean is traditionally monitored using bottommounted or surface-suspended hydrophone arrays. An alternative approachhas recently been developed in which an autonomous, untethered instrument platform freefallsundergravityfromthesurfacetoapreassigneddepth,whereadropweight isreleased,allowingthesystemtoreturntothesurfaceunderbuoyancy.Referredto as Deep Sound, the instrument records acoustic, environmental, and system data continuously during the descent and ascent. The central componentof 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

On the spatial properties of ambient noise in the Tonga Trench, including effects of bathymetric shadowing

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.

Novel fiber-based integrating sphere for luminous flux measurements

Traditional integrating spheres, which use a single detector to measure luminous flux, have a number of drawbacks associated with the sources of error caused by baffling, nonideal topology, and variations in surface reflectance. In this article we address the potential drawbacks of many traditional integrating spheres and present a new instrument which achieves accurate measurements without the use of baffles, by relying on optical fibers to distribute measurement points around the interior of a highly reflective structure.

Laboratory Measurements of Coarse Sediment Bedload Transport Velocity Using a Prototype Wideband Coherent Doppler Profiler (MFDop)

AbstractA prototype wideband coherent Doppler profiler (MFDop) was tested for measuring bedload velocity of different gravel and coarse-sand-sized fractions (d = 1–32 mm) in the laboratory. The sediment was spread out on a smooth-surface tray, and motion was initiated by tilting the tray at angles of α = 20°–39° from the horizontal. Particle velocities downslope (u), cross slope (υ), and vertical to the tray (w) were determined for different MFDop parameter settings, such as monostatic/bistatic configuration, acoustic beam angle, and pulse length. Video observations of bed particle velocity were made for comparison to the acoustic measurements. Velocities estimated using the MFDop equal to, on average, 71%–74% of the velocities determined using the video observations. Standard deviations ranged from 21% to 35%, including observed irregular motion. Three stages of sediment motion were observed: (i) single particles moving with u ≤ 5 cm s−1, (ii) varying motion of particles and particle groups with predomin...

Ambient noise in the Mariana Trench.

In November 2009, ambient noise measurements were made in the Mariana Trench from the surface to a depth of 9000 m using the instrument platform Deep Sound. Deep Sound is a free‐falling acoustic recorder designed to descend from the ocean’s surface to a pre‐assigned depth where it drops an iron weight and returns to the surface under its own buoyancy. The ascent and descent rate is 0.6 m/s, resulting in an 8 deployment time to 9 km. The instrument recorded the continuous ambient noise time series over the bandwidth 5 Hz–30 kHz on four hydrophones mounted with vertical and horizontal spacings. Environmental data were recorded on a CTD and were used to calculate the local sound speed. Power spectra of the ambient noise were calculated as a function of depth, while vertical and horizontal coherence were calculated and used to infer information on the directionality of the noise field. The spectral levels of ambient noise over the measured range of frequency were found to increase with depth. [Work supported ...

Estimating channel capacity and sonar performance in the changing arctic

Underwater acoustic processes including ambient noise, propagation, reverberation, and scattering have been studied for over half a century in the Canadian Arctic. Despite this, realistic predictions of communications and sonar performance have proved challenging due to the complex impact of ice cover on these processes, and the relative scarcity of sound speed profiles and information about the seabed. This challenge has been exacerbated in recent years because the rapid change in environmental conditions is reducing the relevance of many historical records. A key component of the present effort is to extract site specific model inputs from the environmental data contained in the literature, with an informed weighting toward more recent measurements. Site specific modelling was enhanced using inputs from recent year-long ambient noise recordings. Model inputs were also influenced by specific source and receiver hardware being developed for use in future Arctic experiments. In this paper, low frequency sonar and communication performance estimates in the frequency band 20–250 Hz, based on site-specific environmental inputs and this low frequency hardware, are presented for several geographical regions of strategic relevance in the Canadian Arctic.Underwater acoustic processes including ambient noise, propagation, reverberation, and scattering have been studied for over half a century in the Canadian Arctic. Despite this, realistic predictions of communications and sonar performance have proved challenging due to the complex impact of ice cover on these processes, and the relative scarcity of sound speed profiles and information about the seabed. This challenge has been exacerbated in recent years because the rapid change in environmental conditions is reducing the relevance of many historical records. A key component of the present effort is to extract site specific model inputs from the environmental data contained in the literature, with an informed weighting toward more recent measurements. Site specific modelling was enhanced using inputs from recent year-long ambient noise recordings. Model inputs were also influenced by specific source and receiver hardware being developed for use in future Arctic experiments. In this paper, low frequency so...

Short-term fluctuations in the ambient noise field at an Arctic chokepoint horizontal line array

The Arctic is a region known for high variance in both ambient noise levels and local sound propagation conditions, and is currently experiencing an historic increase in shipping traffic, resulting in an acoustic environmental that is changing so rapidly that the sparse ambient noise data available from decades-old studies have been rendered obsolete. Renewed interest in Arctic acoustics have provided fresh noise data at northern sites, and in the summer of 2015 Defence R&D Canada collected a continuous two-week recording of the 48-hydrophone Northern Watch horizontal line array at Gascoyne Inlet. Over the ice-free study period, numerous ship passes, weather events, and anomalies were observed in the high-quality acoustic recordings. In this paper, spectral fluctuations, directionality, and correlation with ship tracks and weather observations in the noise data are analyzed to help improve the applicability of operational arctic noise models supporting sonar performance.The Arctic is a region known for high variance in both ambient noise levels and local sound propagation conditions, and is currently experiencing an historic increase in shipping traffic, resulting in an acoustic environmental that is changing so rapidly that the sparse ambient noise data available from decades-old studies have been rendered obsolete. Renewed interest in Arctic acoustics have provided fresh noise data at northern sites, and in the summer of 2015 Defence R&D Canada collected a continuous two-week recording of the 48-hydrophone Northern Watch horizontal line array at Gascoyne Inlet. Over the ice-free study period, numerous ship passes, weather events, and anomalies were observed in the high-quality acoustic recordings. In this paper, spectral fluctuations, directionality, and correlation with ship tracks and weather observations in the noise data are analyzed to help improve the applicability of operational arctic noise models supporting sonar performance.

Estimating muddy seabed properties using ambient noise coherence

During the Seabed Characterization Experiment, a multi-institutional field effort held at the New England mud patch, the autonomous passive acoustic lander Deep Sound made a series of ambient noise measurements from the seafloor. The instrument platform carried four hydrophones, arranged in an inverted ‘T’ shape with three spaced in the horizontal and two in the vertical, and landed on the seafloor with the bottom phones 30 cm above the interface. Pressure time series, vertical and horizontal noise coherence (directionality), were recorded continuously for periods of 9 hours over the acoustic bandwidth of 5 Hz to 30 kHz, along with the local temperature, conductivity, and depth. An analytical Pekeris waveguide noise model was fitted to the data in order to determine the bulk sound speed, sheer speed, density, and frequency dependent attenuation in the bottom fluid half-space. Acoustic properties of the mud were determined by comparing the data to the output of a range independent noise model, featuring a realistic multi-layered seabed. [Research supported by ONR.]During the Seabed Characterization Experiment, a multi-institutional field effort held at the New England mud patch, the autonomous passive acoustic lander Deep Sound made a series of ambient noise measurements from the seafloor. The instrument platform carried four hydrophones, arranged in an inverted ‘T’ shape with three spaced in the horizontal and two in the vertical, and landed on the seafloor with the bottom phones 30 cm above the interface. Pressure time series, vertical and horizontal noise coherence (directionality), were recorded continuously for periods of 9 hours over the acoustic bandwidth of 5 Hz to 30 kHz, along with the local temperature, conductivity, and depth. An analytical Pekeris waveguide noise model was fitted to the data in order to determine the bulk sound speed, sheer speed, density, and frequency dependent attenuation in the bottom fluid half-space. Acoustic properties of the mud were determined by comparing the data to the output of a range independent noise model, featuring a ...