Katherine F. Woolfe

Monitoring deep‐ocean temperatures using acoustic ambient noise

Measuring temperature changes of the deep oceans, important for determining the oceanic heat content and its impact on the Earths climate evolution, is typically done using free-drifting profiling oceanographic floats with limited global coverage. Acoustic thermometry provides an alternative and complementary remote sensing methodology for monitoring fine temperature variations of the deep ocean over long distances between a few underwater sources and receivers. We demonstrate a simpler, totally passive (i.e., without deploying any active sources) modality for acoustic thermometry of the deep oceans (for depths of ~ 500–1500 m), using only ambient noise recorded by two existing hydroacoustic stations of the International Monitoring System. We suggest that passive acoustic thermometry could improve global monitoring of deep-ocean temperature variations through implementation using a global network of hydrophone arrays.

Variability of the coherent arrivals extracted from low-frequency deep-ocean ambient noise correlations

Correlation processing of ocean noise can be used to develop totally passive ocean monitoring methods. Using various hydrophone pair orientations, this study investigates the frequency dependence, seasonal variability, and emergence rate of coherent arrivals from cross-correlations of low frequency ambient noise (f < 40 Hz) recorded on triangular hydrophones arrays. These arrays are located at five existing hydroacoustic stations of the International Monitoring System (IMS), situated in the deep-sound channel, and distributed across the Atlantic, Pacific, and Indian Ocean basins. For the majority of studied sites, persistent and fast-emerging coherent arrivals are reliably obtained if the axis connecting the selected hydrophone pair has a direct line-of-sight with regions of the globe containing stable and diffuse noise sources (e.g., polar-ice or seismic noise). Furthermore, for this favorable orientation, the emergence rate of coherent arrivals extracted between hydrophone pairs separated by long ranges (here ∼130 km) can be approximated based on measurements made between hydrophone pairs separated by short ranges (∼2 km) in the Atlantic Ocean. Hence, results from this study, obtained using existing hydrophone configurations of the IMS hydroacoustic stations, could be used to guide the placement of other hydrophone arrays over the globe for future long-range passive ocean monitoring experiments.

Development of the underwater acoustic prism

The acoustic prism (i.e., leaky wave antenna) has been experimentally demonstrated in air as a way to steer an emitted beam using only a single broadband acoustic source. The prism relies on a leaky, dispersive waveguide to provide a unique radiation angle for each narrowband frequency projected by the acoustic source. In air, the leakage occurs through a series of periodically spaced shunts in the waveguide. This study examines an acoustic prism design that is capable of operating underwater, where leakage occurs through the waveguide wall itself due to the much lower impedance contrast of the waveguide material in water to that in air. This results in a geometrically simpler design in the underwater case. However, shear wave effects must be considered in the design of the underwater acoustic prism. The waveguide wall is constructed out of a composite material to have a high impedance but a low shear modulus, which are both necessary conditions to decrease sidelobes in the radiated pressure field. Numeri...

Designing beampatterns with tapered leaky wave antennas

Leaky wave antennas (LWAs) have been shown to be an effective tool for frequency-steerable wave radiation in both the electromagnetic and acoustic wave regimes. LWA’s operate by modifying the impedance on a waveguide such that refraction occurs out of the waveguide at an angle corresponding to Snell’s Law. For a LWA with uniform leaking parameter across the waveguide length, that leakage angle is constant. Using analytical techniques, and by careful geometric design of the waveguide impedance, the leaked beampattern can be tailored. The process of the tapering process for an acoustic LWA is discussed here, and notional examples are presented including sidelobe reduction. [Work supported by the Office of Naval Research.]

Removing multiple scattering in underwater target detection

Recent advances have been made in the field of optics to image through multiple-scattering media in a regime where classical imaging techniques, which rely on the single-scattering approximation, fail. Using results generated by random matrix theory, it is possible to filter the response matrix generated by a source/receiver array to remove the multiple scattering components of the received signals. After removing the multiple scattering components, classical imaging techniques can be used to image with the single-scattering components. We show how this single-scattering filter can be adapted for underwater acoustical imaging in cases where multiple scattering effects are large compared to the reflection from the target. A full-wave 3-dimensional time-domain scaled model is used to characterize filter performance as a function of target strength and environmental coherence length for a shallow-water case. Preliminary results indicate that the filter can be used to detect a target located at least 2 mean f...

Optimized extraction of coherent arrivals from ambient noise correlations in a rapidly fluctuating medium, with an application to passive acoustic tomography

Ambient noise correlations can be used to estimate Green’s functions for passive monitoring purposes. However, this method traditionally relies on sufficient time-averaging of the noise-correlations to extract coherent arrivals (i.e., Green’s function estimates), and is thus limited by rapid environmental fluctuations occurring on short time scales while the averaging takes place. For instance, based on extrapolating results from a previous study [Woolfe et al., 2015], passive ocean monitoring across basin scales (i.e., between hydrophones separated by ∼1000 km) may require at least 10 weeks of averaging time to extract coherent arrivals; but such an averaging time would be too long to capture some aspects of the mesoscale variability of the ocean. To address this limitation, we will demonstrate with simulation and data that the use of a stochastic search algorithm to correct and track these rapid environmental fluctuations can reduce the required averaging time to extract coherent arrivals from noise correlations in a fluctuating medium. The algorithm optimizes the output of an objective function based on a matched filter that uses a known reference waveform to track a set of weak coherent arrivals buried in noise.

Optimized extraction of coherent arrivals from ambient noise correlations in a rapidly fluctuating medium

Ambient noise correlations can be used to estimate Greens functions for passive monitoring purposes. However, this method traditionally relies on sufficient time-averaging of the noise-correlations to extract coherent arrivals (i.e., Greens function estimates), and is thus limited by rapid environmental fluctuations occurring on short time scales while the averaging takes place. This letter demonstrates with simulation and data that the use of a stochastic search algorithm to correct and track these rapid environmental fluctuations can significantly reduce the required averaging time to extract coherent arrivals from noise correlations in a fluctuating medium.

Feasibility of low-frequency acoustic thermometry using deep ocean ambient noise in the Atlantic, Pacific, and Indian Oceans

Previous work has demonstrated the feasibility of passive acoustic thermometry using coherent processing of low frequency ambient noise (1–40 Hz) recorded on triangular hydrophones arrays spaced ~130 km and located in the deep sound channel. These triangular arrays are part of hydroacoustic stations of the International Monitoring System operated by the Comprehensive Nuclear Test Ban Treaty Organization (Woolfe et al., J. Acoust. Soc. Am. 134, 3983). To understand how passive thermometry could potentially be extended to ocean basin scales, we present a comprehensive study of the coherent components of low-frequency ambient noise recorded on five hydroacoustic stations located Atlantic, Pacific, and Indian Oceans. The frequency dependence and seasonal variability of the spatial coherence and directionality of the low-frequency ambient noise were systematically examined at each of the tested site locations. Overall, a dominant coherent component of the low-frequency noise was found to be caused by seasonal ice-breaking events at the poles for test sites that have line-of-sight paths to polar ice. These findings could be used to guide the placement of hydrophone arrays over the globe for future long-range passive acoustic thermometry experiments.

Passive acoustic thermometry of the deep water sound channel using ambient noise

Cross-correlation processing of ocean ambient noise has been proposed as a totally passive alternative to existing active methods for sensing the ocean environment such as acoustic tomography or acoustic thermometry. To this end, we investigated the spatial coherence of low frequency (f < 40 Hz) ocean noise recorded in the deep sound (SOFAR) channel to demonstrate the feasibility of passive acoustic thermometry. Continuous recordings of ambient noise obtained from hydroacoustic stations of the International Monitoring System were processed between the years 2006 and 2012. Each hydroacoustic station uses two triangular horizontal arrays separated by approximately 100 km, and each array has three hydrophones. Coherent arrivals were extracted from time-averaged cross-correlations between the two spatially separated triangular arrays. A beamforming procedure, using data-derived adaptive weights, was used to track the seasonal fluctuations of the arrival time of the coherent wavefronts throughout the 6 years. ...

Results of a scaled physical model to simulate impact pile driving

To achieve a more complete understanding of the parameters involved in the structural acoustics of impact pile driving, a scaled physical model was developed and tested. While the design of the scaled model has been presented previously (Woolfe et al., JASA 130: 2558, 2011), this presentation focuses on analysis of wall velocity data and intensity data obtained from experimental evaluation of the model. The energy contained in a control volume surrounding the pile and the energy exchanged across the surface of the control volume were estimated from near field intensity measurements. The amount of energy transferred to the fluid from the cylindrical shell structure during impact and the amount of energy transferred to the structure from the fluid immediately following impact were determined. Results indicate that the highly damped pressure waveform as observed in the water column of the scaled physical model as well as in field data is due primarily to the transfer of energy from the surrounding water back...