Jason D. Holmes

Broadband passive synthetic aperture: Experimental results

Bearing estimation using an acoustically short towed array can be enhanced by the incorporation of a realistic signal model. By casting the problem as a joint estimation of bearing and source frequency, the bearing estimation performance, as measured by the variance of the bearing estimate, can exceed that of the conventional array processor. Experimental results based on the radiated noise of a ferry are shown. Bearing estimation results using an array of acoustic length of approximately two wavelengths are compared to results using the same data but with a conventional frequency domain beamformer. A significant improvement in performance is demonstrated.

Nonlinear frequency-dependent attenuation in sandy sediments

This paper summarizes evidence of a nonlinear frequency dependence of attenuation for compressional waves in shallow-water waveguides with sandy sediment bottoms. Sediment attenuation is found consistent with alpha(f) = alpha(f(o)) x (f/f(o))n, n approximately 1.8 +/- 0.2 at frequencies less than 1 kHz in agreement with the theoretical expectation, (n = 2), of Biot [J. Acoust. Soc. Am. 28(2), 168-178, 1956]. For frequencies less than 10 kHz, the sediment layers, within meters of the water-sediment interface, appear to play a role in the attenuation that strongly depends on the power law. The accurate calculation of sound transmission in a shallow-water waveguide requires the depth-dependent sound speed, density, and frequency-dependent attenuation.

Results from an autonomous underwater vehicle towed hydrophone array experiment in Nantucket Sound

Shallow water experiments have been conducted in Nantucket Sound with an autonomous underwater vehicle towed hydrophone array system in an area proximate to that of a previous experiment [Frisk and Lynch, J. Acoust. Soc. Am. 86, 1928–1939 (1989)]. Transmission loss was measured, for frequencies between 220 and 1228 Hz in an ocean waveguide, under conditions of an isovelocity water column with an approximate depth of 13 m over a sandy-silty bottom. Results obtained at 415 Hz show classic isovelocity waveguide phenomena that include interference effects, mode stripping due to modal attenuation, and transmission loss proportional 15 to 10 log 10 ( R ) .

On the exponent in the power law for the attenuation at low frequencies in sandy sediments

Shallow water transmission loss measurements yield intrinsic attenuation estimates for acoustic waves in the underlying sediment, with results that are consistent with attenuation being proportional to frequency raised to a power n, with n between 1.6 and 1.87. Plausible theory suggests that n should be identically 2. The discrepancy can be explained because the inverse analysis inferences were made with the neglect of an additional attenuation mechanism where generated lower velocity shear waves carry energy downwards out of the waveguide. The shear wave effect has a weaker dependence on frequency than the intrinsic attenuation, so the apparent exponent is shifted downward.

Investigation of ocean acoustics using autonomous instrumentation to quantify the water-sediment boundary properties

Sound propagation in shallow water is characterized by interaction with the oceans surface, volume, and bottom. In many coastal margin regions, including the Eastern U.S. continental shelf and the coastal seas of China, the bottom is composed of a depositional sandy-silty top layer. Previous measurements of narrow and broadband sound transmission at frequencies from 100 Hz to 1 kHz in these regions are consistent with waveguide calculations based on depth and frequency dependent sound speed, attenuation and density profiles. Theoretical predictions for the frequency dependence of attenuation vary from quadratic for the porous media model of M.A. Biot to linear for various competing models. Results from experiments performed under known conditions with sandy bottoms, however, have agreed with attenuation proportional to f 1.84, which is slightly less than the theoretical value of f2 [Zhou and Zhang, J. Acoust. Soc. Am. 117, 2494]. This dissertation presents a reexamination of the fundamental considerations in the Biot derivation and leads to a simplification of the theory that can be coupled with site-specific, depth dependent attenuation and sound speed profiles to explain the observed frequency dependence. Long-range sound transmission measurements in a known waveguide can be used to estimate the site-specific sediment attenuation properties, but the costs and time associated with such at-sea experiments using traditional measurement techniques can be prohibitive. Here a new measurement tool consisting of an autonomous underwater vehicle and a small, low noise, towed hydrophone array was developed and used to obtain accurate long-range sound transmission measurements efficiently and cost effectively. To demonstrate this capability and to determine the modal and intrinsic attenuation characteristics, experiments were conducted in a carefully surveyed area in Nantucket Sound. A best-fit comparison between measured results and calculated results, while varying attenuation parameters, revealed the estimated power law exponent to be 1.87 between 220.5 and 1228 Hz. These results demonstrate the utility of this new cost effective and accurate measurement system. The sound transmission results, when compared with calculations based on the modified Biot theory, are shown to explain the observed frequency dependence.

An overview of unmanned underwater vehicle noise in the low to mid frequencies bands

Unmanned (autonomous) underwater vehicles offer a unique, cost-effective platform for performing ocean acoustic measurements and surveys because multiple systems can be deployed from a single research vessel. Various data surveys can be performed including on-the-bottom geo-acoustic surveys over large areas, sub-sea-surface turbulence and micro bubble structure surveys, and bi-static fish population surveys. To take advantage of the autonomous survey capabilities of underwater vehicles, sufficient signal-to noise ratio and acoustic aperture (resolution) are required for acoustic measurements. The most commonly used vehicle sonar systems provide images utilizing high frequency hull mounted arrays and sources. In the lower frequency band ( 100 Hz to 10kHz ), however, vehicle noise levels and aperture remain the two most significant challenges, especially for passive systems. Previous experimental and analytical work has shown that a towed array with synthetic aperture processing can be used to obtain the ne...

Results from the Nantucket Sound autonomous underwater vehicle towed hydrophone array experiment

Results from several experiments with an autonomous underwater vehicle towed hydrophone array system demonstrate the ability of such a system to provide rapid, cost effective and accurate characterization of the shallow water waveguide. A synthetic aperture Hankel transform of the complex pressure as a function of range was used to estimate the eigenvalues and functions. Results showed that the vehicle was capable of towing the array in a steady and stable configuration sufficient for coherent processing and formation of a synthetic aperture Hankel transform. Transmission loss calculated with propagation codes that use depth and frequency dependent geo-acoustic profiles (compressional wave speed, density and attenuation) compared well with the measured transmission loss. The ability to perform coherent processing of the data obtained by this prototype system demonstrates that the system is an efficient and accurate alternative to traditional measurement techniques. Since the vehicle has the ability to change depth and course while measuring non-acoustic data such as currents, bathymetry, and sound speed; the time required for wide area characterization is significantly reduced

Characteristics of an autonomous underwater vehicle with a towed hydrophone array

This paper presents the performance results of a REMUS AUV, a proven low noise vehicle with precise navigation, that was modified by the addition of a prototype towed hydrophone array. A quiet, acoustically sensitive synthetic aperture sonar at low frequencies required a 10 m long acoustic section of the array separated from the vehicle with a 10 m long tow cable. A drogue was used to produce an initial drag and to minimize motions at the end of the array. REMUS was designed to carry a wide variety of sensors but, the 10 meter towed array presented many challenges. Unlike vehicles with thrust vectoring, REMUS requires forward motion using three axis control to maintain constant depth without the added drag of fin control. In light of this, several hardware and software adjustments were required to ensure successful long-range missions with the system. The vehicle adjustments, along with the results from at-sea engineering trials of the vehicle-array system are presented and demonstrate the ability of the system to maintain a constant depth with a stable array tow. Effects of the added drag on the vehicle were evident in the on board recorded data, which enabled the adjustment of vehicle control through software and mechanical considerations such as attachment point of the array to the vehicle. Rapid improvement in vehicle performance in between missions was achieved on board the support ship as a direct result of the ability to view and interpret mission data in the field. Based on the engineering trials with this vehicle array system, estimates of the vehicle and array system performance including power requirements, diving capability, turning capability, effects of currents, and navigational errors are discussed

An autonomous underwater vehicle towed hydrophone array for ocean acoustic measurements

An autonomous underwater vehicle (AUV) with a towed hydrophone array (THA) can provide the capability of mobile‐single‐ship operation for both short‐range single path and long range synthetic aperture experiments. A low noise towed array for an AUV (REMUS) has been developed to demonstrate the feasibility and utility of such measurements. Previous measurements of AUV radiated noise indicated levels that would limit measurements by hull arrays providing a rational for a THA. A small‐diameter fluid‐filled prototype hydrophone array was developed and tested to ensure adequate sensitivity and system noise levels. The digital recording system (DRS) consisted of mini‐disc recorders with a band width of 20 kHz housed in a tube attached to the AUV. This combined system (REMUS, DRS, and THA) was used to conduct a proof of concept test in Dodge Pond. This paper presents the results that show, in the Sea State 0 noise field of Dodge Pond, array system self noise was less than the ambient and vehicle noise was manage...

The applicability of a small autonomous vehicle towed array system to ocean acoustic measurements and signal processing

An array was developed to demonstrate, and quantify the performance characteristics of an autonomous‐vehicle towed‐array system. This technology provides for a cost effective tool for the measurement of coherent signal propagation, depth dependent and directional noise fields and to establish quantitative limits on array performance. The tangential drag on a 40‐m length array composed of a reinforced tube with an outer diameter of 2.8 cm is extrapolated to be between 20‐28 N for diameter Reynolds number of approximately 4 104. The hydrophone‐ preamplifier groups have a sensitivity of –174±1.5 dB re 1μPa/V between 100 Hz to 10 kHz. with an adjustable spacing between 0.5 and 1.5 m. The system tow stability enabled the use of the synthetic Hankel transform to estimate the modal horizontal wave number spectrum and the identification of interface wave speeds at frequencies up to 1000 Hz. The formation of synthetic apertures combined with model based extended Kalman filter techniques was found to demonstrate b...