John N. Kemp

Acoustic normal mode fluctuation statistics in the 1995 SWARM internal wave scattering experiment

In order to understand the fluctuations imposed upon low frequency (50 to 500 Hz) acoustic signals due to coastal internal waves, a large multilaboratory, multidisciplinary experiment was performed in the Mid-Atlantic Bight in the summer of 1995. This experiment featured the most complete set of environmental measurements (especially physical oceanography and geology) made to date in support of a coastal acoustics study. This support enabled the correlation of acoustic fluctuations to clearly observed ocean processes, especially those associated with the internal wave field. More specifically, a 16 element WHOI vertical line array (WVLA) was moored in 70 m of water off the New Jersey coast. Tomography sources of 224 Hz and 400 Hz were moored 32 km directly shoreward of this array, such that an acoustic path was constructed that was anti-parallel to the primary, onshore propagation direction for shelf generated internal wave solitons. These nonlinear internal waves, produced in packets as the tide shifts from ebb to flood, produce strong semidiurnal effects on the acoustic signals at our measurement location. Specifically, the internal waves in the acoustic waveguide cause significant coupling of energy between the propagating acoustic modes, resulting in broadband fluctuations in modal intensity, travel-time, and temporal coherence. The strong correlations between the environmental parameters and the internal wave field include an interesting sensitivity of the spread of an acoustic pulse to solitons near the receiver.

Coherence of acoustic modes propagating through shallow water internal waves

The 1995 Shallow Water Acoustics in a Random Medium (SWARM) experiment [Apel et al., IEEE J. Ocean. Eng. 22, 445-464 (1997)] was conducted off the New Jersey coast. The experiment featured two well-populated vertical receiving arrays, which permitted the measured acoustic field to be decomposed into its normal modes. The decomposition was repeated for successive transmissions allowing the amplitude of each mode to be tracked. The modal amplitudes were observed to decorrelate with time scales on the order of 100 s [Headrick et al., J. Acoust. Soc. Am. 107(1), 201-220 (2000)]. In the present work, a theoretical model is proposed to explain the observed decorrelation. Packets of intense internal waves are modeled as coherent structures moving along the acoustic propagation path without changing shape. The packets cause mode coupling and their motion results in a changing acoustic interference pattern. The model is consistent with the rapid decorrelation observed in SWARM. The model also predicts the observed partial recorrelation of the field at longer time scales. The model is first tested in simple continuous-wave simulations using canonical representations for the internal waves. More detailed time-domain simulations are presented mimicking the situation in SWARM. Modeling results are compared to experimental data.

The North Pacific Acoustic Laboratory deep-water acoustic propagation experiments in the Philippine Sea

A series of experiments conducted in the Philippine Sea during 2009-2011 investigated deep-water acoustic propagation and ambient noise in this oceanographically and geologically complex region: (i) the 2009 North Pacific Acoustic Laboratory (NPAL) Pilot Study/Engineering Test, (ii) the 2010-2011 NPAL Philippine Sea Experiment, and (iii) the Ocean Bottom Seismometer Augmentation of the 2010-2011 NPAL Philippine Sea Experiment. The experimental goals included (a) understanding the impacts of fronts, eddies, and internal tides on acoustic propagation, (b) determining whether acoustic methods, together with other measurements and ocean modeling, can yield estimates of the time-evolving ocean state useful for making improved acoustic predictions, (c) improving our understanding of the physics of scattering by internal waves and spice, (d) characterizing the depth dependence and temporal variability of ambient noise, and (e) understanding the relationship between the acoustic field in the water column and the seismic field in the seafloor. In these experiments, moored and ship-suspended low-frequency acoustic sources transmitted to a newly developed distributed vertical line array receiver capable of spanning the water column in the deep ocean. The acoustic transmissions and ambient noise were also recorded by a towed hydrophone array, by acoustic Seagliders, and by ocean bottom seismometers.

Acoustic field variability induced by time evolving internal wave fields

A space- and time-dependent internal wave model was developed for a shallow water area on the New Jersey continental shelf and combined with a propagation algorithm to perform numerical simulations of acoustic field variability. This data-constrained environmental model links the oceanographic field, dominated by internal waves, to the random sound speed distribution that drives acoustic field fluctuations in this region. Working with a suite of environmental measurements along a 42-km track, a parameter set was developed that characterized the influence of the internal wave field on sound speed perturbations in the water column. The acoustic propagation environment was reconstructed from this set in conjunction with bottom parameters extracted by use of acoustic inversion techniques. The resulting space- and time-varying sound speed field was synthesized from an internal wave field composed of both a spatially diffuse (linear) contribution and a spatially localized (nonlinear) component, the latter consisting of solitary waves propagating with the internal tide. Acoustic simulation results at 224 and 400 Hz were obtained from a solution to an elastic parabolic equation and are presented as examples of propagation through this evolving environment. Modal decomposition of the acoustic field received at a vertical line array was used to clarify the effects of both internal wave contributions to the complex structure of the received signals.

An autonomous, near‐real‐time buoy system for automatic detection of North Atlantic right whale calls.

A moored buoy system for automatic detection of endangered North Atlantic right whale (NARW) upcalls was developed to provide near‐real‐time information on the presence of whales. The marine components include the WHOI buoy platform (mooring, hydrophone, power system, surface expression, and antennae) and Cornell buoy electronics (housing, analog interface hardware, GPS, embedded computer, detection engine, and telemetry hardware). Shore‐side Cornell components include telemetry equipment, server hardware and processing software, database, and interfaces for data annotation, access, and visualization. The buoy hardware/software system is capable of capturing and ranking NARW upcall candidates as 2 s, 2000 Hz sampled audio clips. GPS location, timestamp, and other metadata associated with each audio clip are bundled together and uploaded via satellite for processing. Human analysts regularly annotate incoming data, resulting in a curated database of NARW detections. Periodic “health and status” data allow ...

Ice-tethered profilers sample the upper Arctic Ocean

Studies conducted over the past decade indicate that the Arctic may be both a sensitive indicator of climate change and an active agent in climate variability. Although progress has been made in understanding the Arctics coupled atmosphere-ice-ocean system, documentation of its evolution is hindered by a sparse data archive. This observational gap represents a critical shortcoming of the ‘global’ ocean observing systems ability to quantify the complex interrelated atmospheric, oceanic, and terrestrial changes now under way throughout the Arctic and that have demonstrated repercussions for society [Symon et al., 2005]. Motivated by the Argo float program, an international effort to maintain an ensemble of approximately 3000 autonomous profiling instruments throughout the temperate oceans (see http://w3.jcommops.org), a new instrument, the ‘Ice-Tethered Profiler’ (ITP) was conceived to repeatedly sample the properties of the ice-covered Arctic Ocean at high vertical resolution over time periods of up to three years.

Drifting buoys make discoveries about interactive processes in the Arctic Ocean

Observation of the Arctic ice and ocean environment, where the heat budget of the Northern Hemisphere is largely determined, is critical for advancing our understanding of global climatic change. Because the polar environment varies greatly over the course of a year, continuous observations are needed over the entire seasonal cycle, especially during the Arctic winter and early spring when the thermal contrast between sea water and atmosphere reaches a maximum.

A near‐real‐time acoustic detection and reporting system for endangered species in critical habitats

Passive acoustics is an effective mechanism for detection and recognition of species‐specific sounds and can be a more cost‐effective approach than visual techniques for monitoring populations of rare or endangered species. A network of moored buoys has been strategically deployed in and around Cape Cod Bay to report detections of northern right whales in critical habitat. Each buoy continuously and automatically monitors for right whale contact calls and transmits detection and ambient noise data by cell or satellite phone to Cornell University on a regular basis. Each day, validated data are automatically unloaded into a Website database to provide on‐line graphical and numerical data summaries. The array of three buoys deployed in the Bay will eventually be synchronized to allow localization and tracking of individual animals. [Work supported by funds from the NOAA Right Whale Grants Program and augmented by funds from the Commonwealth of Massachusetts Division of Marine Fisheries.]

Mooring developments for autonomous ocean-sampling networks

Two general-purpose mooring designs have been developed to support autonomous underwater vehicle (AUV) operations in autonomous ocean sampling networks (AOSNs). These moorings provide two-way communications between investigators and AUVs docked on the moorings or conducting survey operations some distance from the moorings. A deep-water design that incorporates an AUV dock and recharging station was built for use in the Labrador Sea during the winter of 1997/1998. This severe winter environment required a robust design that could operate unattended for six months while isolating the dock from surface wave motion. A much lighter, easier-to-deploy design was developed for use in coastal waters to extend the nearshore AOSN operating area by extending the communications network. This coastal design has been deployed without the dock component and has typically been configured for use in a small network of moorings maintained with a small research vessel. The deep-water mooring has been deployed successfully on two occasions, for short periods of time. The coastal moorings have been deployed a number of times and have proven to be quite effective. This paper describes the two moorings in detail and provides information on their performance so that interested investigators can utilize the technology where it meets their needs.

Preliminary acoustic and oceanographic observations from the ASIAEX 2001 South China Sea Experiment

Funding was provided by the Office of Naval Research under Grant Numbers N00014-01-1-0772, N00014-98-1-0413 and N00014-00-1-0206.