Keith von der Heydt

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.

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.

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.

Vertical array receptions of the Heard Island transmissions

A long, vertical line array was deployed off Monterey, California during the Heard Island Feasibility Test to measure the modal content of the received signals. The array contained 32, equally spaced hydrophones spanning from 345 to 1740‐m depth. The multichannel data were recorded through a tether to the R/V Point Sur. The measurements had very low signal to noise ratios and indicated the cw transmission losses were approximately 140 dB for a source/receiver range of 17 000 km. Modal content was analyzed using (i) the modal extent versus depth, (ii) frequency‐vertical wave‐number spectra, (iii) modal beamforming and (iv) least squares fitting. All led to the conclusion that the modal population is surprisingly rich. There was strong evidence of population up to at least mode seven in the data.

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.]

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.

Modal Mapping Experiment and Geoacoustic Inversion Using Sonobuoys

This paper summarizes the results of an experiment whose primary goal was to demonstrate that reliable geoacoustic inversion results can be obtained in shallow water by postprocessing acoustic data acquired by Global Positioning System (GPS)-capable sonobuoys. The experiment was conducted aboard the R/V Sharp on March 5-18, 2011 off the coast of New Jersey using AN/SSQ-53F sonobuoys with a GPS capability as well as GPS-equipped research buoys originally developed under the Modal Mapping Experiment (MOMAX) project, which provided reliable geoacoustic information to which the sonobuoy results could be compared. It is shown that when low-frequency ( 500 Hz) continuous-wave (CW) signals are acquired on the two types of buoys in a colocated configuration, the geoacoustic models inferred from the sonobuoy data are very similar to those obtained from the MOMAX buoy data. The inversion results also compare favorably with bottom models for the region obtained from other experiments. This work is an important milestone toward achieving the ultimate goal of transitioning a basic research method to an operational scenario in which sonobuoy data are routinely used to infer geoacoustic parameters of the seabed.

Design and operation of automated ice-tethered profilers for real-time seawater observations in the polar oceans

Funding was provided by the National Science Foundation under Contract Nos. OCE-0324233 and ARC-0519899.

Edge wave observation using remote seismoacoustic sensing of ice events in the Arctic

As part of the Office of Naval Research Sea Ice Mechanics Initiative, a real-time monitoring and processing program for acoustic emission from ice fracture and ridge-building events was established. A wide-aperture, horizontal hydrophone array was used in combination with a vertical line array to record the acoustic signals, which were then passed through a focused beam former for real-time generation of ice seismicity maps. A number of rapidly deployable geophone arrays were used in active zones to measure the acoustic emissions in the near field for detailed seismic event analysis. During one such deployment, a highly regular transient arrival structure was recorded on all sensors located near a major lead, with a transient appearing every 5 s. These data have been processed using frequency-wavenumber analysis to show that the transients correspond to “edge waves” propagating forth and back along the edges of the lead, with the probable source being a “stick-slip” mechanical phenomenon toward the ends of the lead.