Zachary J. Waters

Bistatic, above-critical angle scattering measurements of fully buried unexploded ordnance (UXO) and clutter

Laboratory grade bistatic scattering measurements are conducted in order to examine the acoustic response of realistic fully buried unexploded ordnance (UXO) from above-critical angle insonification, between 2 and 40 kHz. A 127 mm diameter rocket UXO, a 155 mm diameter artillery shell, a natural rock of approximately the same size, and a cinder block are fully buried in water-saturated medium grained sand (mean grain diameter, 240 μm) at depths of 10 cm below the water-sediment interface. A two-dimensional array of bistatic scattering measurements is generated synthetically by scanning a single hydrophone in steps of 3 cm over a 1 m × 1 m patch directly above the targets at a height of 20 cm above the water-sediment interface. Three-dimensional volumetric acoustic images generated from the return waveforms reveal scattering components attributed to geometric and elastic scattering, as well as multiple-scattering interactions of returns between the sediment-water interface and the buried objects. The far-field target strength of the objects is estimated through extrapolation of the angular spectrum. Agreement is found between experimental data and simulated data generated from a finite-element-based, three-dimensional time-harmonic model (2-25 kHz). Separation of the measured UXO from the clutter objects is demonstrated through exploitation of structural-acoustics-based features.

Acoustic identification of buried underwater unexploded ordnance using a numerically trained classifier (L)

Using a finite element-based structural acoustics code, simulations were carried out for the acoustic scattering from an unexploded ordnance rocket buried in the sediment under 3 m of water. The simulation treated 90 rocket burial angles in steps of 2°. The simulations were used to train a generative relevance vector machine (RVM) algorithm for identifying rockets buried at unknown angles in an actual water/sediment environment. The trained RVM algorithm was successfully tested on scattering measurements made in a sediment pool facility for six buried targets including the rocket at 90°, 120°, and 150°, a boulder, a cinderblock, and a cinderblock rolled 45° about its long axis.

Discriminating resonant targets from clutter using Lanczos iterated single-channel time reversal.

Power iterated single-channel time-reversal is extended to employ Lanczos iterations. The properties of these algorithms are studied in the presence of varying levels of noise and broadband clutter. It is shown the Lanczos iterated method possesses superior convergence properties in comparison to the standard power iterated technique. Results demonstrate that such algorithms provide an efficient means through which to isolate and extract the properties of resonant scatterers in the presence of noise and coherent interference.

Isolating scattering resonances of an air-filled spherical shell using iterative, single-channel time reversal

Iterative, single-channel time reversal is employed to isolate backscattering resonances of an air-filled spherical shell in a frequency range of 0.5-20 kHz. Numerical simulations of free-field target scattering suggest improved isolation of the dominant target response frequency in the presence of varying levels of stochastic noise, compared to processing returns from a single transmission and also coherent averaging. To test the efficacy of the technique in a realistic littoral environment, monostatic scattering experiments are conducted in the Gulf of Mexico near Panama City, Florida. The time reversal technique is applied to returns from a hollow spherical shell target sitting proud on a sandy bottom in 14 m deep water. Distinct resonances in the scattering response of the target are isolated, depending upon the bandwidth of the sonar system utilized.

Acoustic imaging and structural acoustic analysis of scattering from buried targets at above-critical grazing angles

Laboratory experiments are conducted in order to examine above-critical angle source configurations for the detection and identification of objects fully buried in water-saturated sediments. A stationary broadband spherical source (3–40 kHz) insonifies realistic unexploded ordnance (UXO), as well as objects representing both natural and man-made clutter, at several aspects from above the critical angle. Bistatic returns, received on a two-dimensional synthetic array, are processed to generate volumetric acoustic images of the objects buried in a variety of orientations. Physical acoustics based interpretations are applied in order to identify features attributed to geometric and elastic scattering processes, as well as the interaction of scattered returns with the water-sediment interface. The symmetry of images attributed to cylindrically shaped UXO is suggested as a potential feature for the discrimination of these objects from clutter. The complementary role of volumetric imaging relative to feature-ba...

Single‐channel time reversal with Lanczos iterations.

Recent investigations of array‐based time reversal techniques employing Lanczos iterations have demonstrated convergence properties superior to equivalent methods using power iterations [A. A. Oberai et al., “Efficient time reversal by Lanczos iterations,” J. Acoust. Soc. Am. 123, 3596 (2009)]. In numerical scattering simulations, time reversal techniques developed for a single‐channel transducer are applied to study echoes from a thin‐walled spherical shell. When power iterations are employed, a narrowband waveform corresponding to the dominant scattering mode of the target is selected. Lanczos iterations converge to this waveform in fewer transmissions and simultaneously identify additional higher‐order target scattering features. The performance of both techniques is studied in the presence of varying levels of stochastic and deterministic noise. [Work supported by the Office of Naval Research.]

Forward scatter and backscatter low‐frequency synthetic array measurements of the structural acoustic response from proud targets using a 48‐m‐long rail in a littoral environment.

A series of short range (25‐m) forward scatter and backscatter measurements in a littoral environment was conducted to quantify the structural acoustic response from proud targets in the 2–23 kHz frequency band. The water channel was 14 m deep in the Gulf of Mexico near Shell Island, Panama City FL. The bottom was a medium grained sand. The acoustic forward scatter response of the proud targets was measured in a bistatic configuration with the source 25 m from the target and a receiver mounted on a 48‐m‐long rail. The rail is used to position the receiver and synthetically quantify the structural acoustic forward scatter response. A second source was co‐located with the receiver on the rail, and monostatic backscatter measurements were also taken for each target. The structural acoustic response was analyzed and will be reported. The synthetic array experimental results are compared and contrasted with laboratory measurements. The laboratory measurements are convolved with water channel propagation predic...

Detection of a resonant target buried in sediment using iterative time reversal: mid‐frequency pond experiments

Iterative time reversal techniques developed at smaller scales, [Waters et al., J. Acoust. Soc. Am. 122, 3023 (2007)], are applied to the detection of a 15 cm diameter stainless steel shell buried in sandy sediment at the acoustic test pond at the Naval Surface Warfare Center ‐ Panama City Division. A mid‐frequency, directional projector is located 1.5 m above the sediment and directed normally to it. A hydrophone is located midway between the sediment and the projector. This system gives a response between 20 kHz and 200 kHz. A calibration filter is designed using the direct path response between the projector and hydrophone. This filter is applied at each time reversal iteration to prevent the time reversal technique from converging to the transducer resonance. Application of iterative time reversal allows the detection of the target at greater depths than otherwise possible due to the resonance scattering of the target. Additional experiments explore the application of filters, and the effect of differ...

Demonstration of a compact quasi-monostatic autonomous underwater vehicle based continuous active sonar

Monostatic active sonar systems are typically operated in a pulsed transmit configuration, where an acoustic source is first activated to provide ensonification of the environment and then de-activated while scattered and ambient environmental acoustic energy is collected by a receiving sensor. In applications where the transmitter and receiver are in close proximity as with a monostatic configuration, it is typically impracticable to carry out the simultaneous transmission and reception of acoustic energy. Under such conditions, during continuous transmission, the source may saturate the receive sensors, potentially resulting in an overlap of target scattering returns from down-range with the direct acoustic coupling response from the source-to-receiver as well as reverberation, effectively blinding the sonar. Here, we explore the feasibility of techniques to disambiguate lower level target returns from a continuously transmitting autonomous underwater vehicle (AUV) based sonar. We conclude via theoretical studies supported by empirical analysis that the continuous transmission and reception of acoustic energy in a shallow-water waveguide is feasible for a realistic AUV based sonar system and overview demonstrations of these techniques to detect a bottomed target-object at-sea. [Work Supported by the Office of Naval Research.]Monostatic active sonar systems are typically operated in a pulsed transmit configuration, where an acoustic source is first activated to provide ensonification of the environment and then de-activated while scattered and ambient environmental acoustic energy is collected by a receiving sensor. In applications where the transmitter and receiver are in close proximity as with a monostatic configuration, it is typically impracticable to carry out the simultaneous transmission and reception of acoustic energy. Under such conditions, during continuous transmission, the source may saturate the receive sensors, potentially resulting in an overlap of target scattering returns from down-range with the direct acoustic coupling response from the source-to-receiver as well as reverberation, effectively blinding the sonar. Here, we explore the feasibility of techniques to disambiguate lower level target returns from a continuously transmitting autonomous underwater vehicle (AUV) based sonar. We conclude via theoretic...

Towards real-time search planning in subsea environments

We address the challenge of computing search paths in real-time for subsea applications where the goal is to locate an unknown number of targets on the seafloor. Our approach maximizes a formal definition of search effectiveness given finite search effort. We account for false positive measurements and variation in the performance of the search sensor due to geographic variation of the seafloor. We compare near-optimal search paths that can be computed in real-time with optimal search paths for which real-time computation is infeasible. We show how sonar data acquired for locating targets at a specific location can also be used to characterize the performance of the search sonar at that location. Our approach is illustrated with numerical experiments where search paths are planned using sonar data previously acquired from Boston Harbor.