Francine Desharnais

Source depth and array tilt effects on seabed inversion of ambient noise

Ambient noise coherence between two vertically separated sensors in shallow water relates to the directionality of the noise field and is sensitive to the reflective properties of the surficial sediments. In earlier work, an energy‐flux model was developed to calculate the noise coherence over a multilayered seabed; in this model the surface sources were assigned a frequency‐independent dipole directivity pattern. The model was subsequently combined with a hybrid nonlinear inversion procedure to effectively search the space of geoacoustic properties that parametrize a multilayered seabed. In order to benchmark the energy‐flux model against a wave‐theoretical formulation, the issue of the unknown but finite source depth of the noise sources must be addressed. These near‐surface sources result in a frequency‐dependent beam pattern that is not dipolar above a few hundred hertz [M. J. Buckingham and N. M. Carbone, J. Acoust. Soc. Am. 102, 2637–2644 (1997)]. In this paper, we extend the energy‐flux model to in...

Feasibility Study on Surveillance and Interdiction Technologies for the Port of Halifax

This paper presents the preliminary results from the Feasibility Study on Surveillance and Interdiction Technologies for the Port of Halifax, lead by DRDC-Atlantic and performed by two independent teams of investigators. The two teams identified and specified the requirements for Detection, Deterrence and Response to water and land threats for a multimodal seaport, identified the technologies that are currently in place or will be in place in the very near future, then identified configurations of sensors, command and control elements, detection aids, and organizational infrastructure that will meet the requirements and the technologies or developments needed to deliver the recommended configurations. An Initial Operational Capability of 2011 was assumed. This feasibility study was a first step for DRDC to better understand the Port Security problem. The results from this study will be used to define the scope and structure of a larger port security project where we hope to bring all stakeholders together to reach coordinated solutions.

Broadband source localization using matched correlation processing

For many years, model‐based signal processing algorithms using Matched Field Processing (MFP) techniques have been analyzed with the goal of improving the capability of passive sonar systems for localizing quiet underwater sources. Recently, researchers at DRDC Atlantic have been investigating Matched Correlation Processing (MCP) as a faster alternative to MFP. In this method, the cross‐correlations for a source as measured with a pair of hydrophones in a horizontal array are matched with those generated with a correlation model for many candidate ranges and depths along a candidate bearing. These matches are carried out with a number of hydrophone pairs to form many ambiguity surfaces. The maximum on the average of these surfaces is assumed to yield the best estimate of the source position. By carrying out this procedure over a number of candidate bearings, a full 3‐D search for the source location is achieved. Since 2002, a number of localization trials have been carried out east of Nova Scotia, Canada....

Appraisal of an energy flux model for geoacoustic inversion of ambient noise coherence data

The vertical directionality of the ambient noise field in a shallow‐water waveguide is highly dependent on the local geoacoustic properties of the seabed and is directly related to the vertical noise coherence observed between two closely spaced hydrophones. As a result, a direct measurement of the broadband ambient noise coherence can be used to invert for sea‐bottom properties that affect propagation. In previous work, an energy flux model [D. M. F. Chapman, Proc. Inst. Acoust. 9(4), 1–11 (1987)] for computing vertical noise coherence in shallow water was extended to include the effects of multilayered geoacoustic seabeds, refraction and absorption within the water column, sensor‐pair tilt, and nondipolar radiation patterns due to near‐surface but finite source depths. This modified flux‐based coherence model was combined with a hybrid local/global nonlinear optimization scheme and used to estimate geoacoustic and source/sensor parameters for several synthetic data sets [F. Desharnais et al., J. Acoust....

Infrasound propagation and coupling between air, soil, and water

Propagation of low‐frequency sound is enhanced by very low absorption in air, water, and soil. Some examples of absorption expected in air, water, and soil are presented. Reduction in sound level between source and receiver is typically the result of spreading. In all media of interest, layering can give rise to trapped waves that decay as cylindrical waves as opposed to spherical waves. Examples of the effect of layering on propagation in the atmosphere will be given for day/night conditions. At very low frequency, scattering by rough terrain, moderate size turbulence, and waves have a reduced effect on signal amplitude and phase. Turbulence carried by the wind can result in pressure variations at the receiver that can sound like acoustic signals. An effective receiving system must deal with that wind noise. Identifying the mechanism for communications at low frequency is complicated by the exchange of acoustic energy between air, soil, and water. Some characteristics of this coupling and losses that mig...

Geoacoustic inversion of noise coherence in shallow water

It is known that the geoacoustic properties of a shallow‐water sea‐bed can be inferred from relatively simple measurements of the ambient noise coherence between a pair of vertically separated hydrophones [D. M. F. Chapman, ‘‘Surface‐generated noise in shallow water: A model,’’ Proc. Inst. Acoust. 9, 1–11 (1987)]. The design of an autonomous buoy package for acquiring geoacoustic information by this method is currently being considered by DRDC‐Atlantic in support of matched‐field localization efforts that are being developed for use with rapidly deployable arrays. Initially, vertical coherence estimates from a simple shallow water noise model were fit to measured coherences by adjusting geoacoustic parameters by a trial and error procedure. A more systematic approach involves combining noise coherence models with nonlinear global optimization methods based on matched‐coherence processing concepts to search the space of possible sea‐bed parameters more efficiently. In this paper, we report on recent effort...