Saikat Dey

Broadband acoustic scattering measurements of underwater unexploded ordnance (UXO)

In order to evaluate the potential for detection and identification of underwater unexploded ordnance (UXO) by exploiting their structural acoustic response, we carried out broadband monostatic scattering measurements over a full 360 degrees on UXOs (two mortar rounds, an artillery shell, and a rocket warhead) and false targets (a cinder block and a large rock). The measurement band, 1-140 kHz, includes a low frequency structural acoustics region in which the wavelengths are comparable to or larger than the target characteristic dimensions. In general, there are aspects that provide relatively high target strength levels ( approximately -10 to -15 dB), and from our experience the targets should be detectable in this structural acoustics band in most acoustic environments. The rigid body scattering was also calculated for one UXO in order to highlight the measured scattering features involving elastic responses. The broadband scattering data should be able to support feature-based separation of UXO versus false targets and identification of various classes of UXO as well.

Detection and localization of inclusions in plates using inversion of point actuated surface displacements

A numerical simulation is carried out demonstrating the use of plate surface vibration measurements for detecting and locating inclusions within the structure. A finite element code is used to calculate normal surface displacement for both steel and mortar plates subjected to a monochromatic point force. The data is generated for the homogeneous plate and the identical plate within which exists a small rectangular inclusion. It is observed that when the elastic modulus of the inclusion is orders of magnitude lower than the base material, resonances of the inclusion produce large local displacements that are readily observed in the raw displacement data. For more modest moduli differences, there are no such directly observable effects. In this case, three inverse algorithms are used to process the displacement data. The first two are local inversion techniques that each yield a spatial map of the elastic modulus normalized by density. These algorithms successfully detect and localize the inclusion based on its modulus difference from that of the base plate. The third technique uses a form of the inhomogeneous equation of motion to obtain the induced force distribution connected with the inclusion. The spatial mapping of this force also successfully detects and localizes the inclusion.

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.

Structural-acoustic modeling for three-dimensional freefield and littoral environments with verification and validation

This paper describes a high-order, finite-element-based, three-dimensional time-harmonic model for large-scale exterior structural-acoustics problems. It is applicable to both freefield and littoral environments. For the freefield case, the infinite exterior is treated as a homogeneous linear acoustic medium. For littoral applications, the water or air and the sediment domains are each treated as linear homogeneous, semi-infinite half-spaces with piecewise-constant properties. Both domains admit complex-valued wave speeds to enable the inclusion of damping. The finite element formulation uses a variational statement which naturally incorporates the transmission-condition at the water or air-sediment interface. The truncation of the infinite exterior is realized using an infinite-element for the freefield case, and the perfectly-matched-layer approximation for littoral applications. Computation of the farfield quantities is done based on an integral representation which, for the littoral cases, uses efficient approximations for the appropriate Greens function. Numerical computations are presented for a series of progressively more complex problems, and are used to verify the model against analytic and other numerical solutions and validate it based on the experimental data for scattering from elastic scatterers as measured in freefield and sediment pool laboratory facilities.

Compact directional acoustic sensor using a multi-fiber optical probe.

A compact directional acoustic sensor is described which uses a two-fiber optical probe, a light emitting diode (LED), a photo-diode detector, and a slender cylindrical cantilever to the end of which is attached an optical reflector. Acoustically induced transverse displacement of the cantilever tip modulates the light reflected by it into the collection fiber, which conveys the light to a photo-detector. Directional sensitivity is achieved through the dependence of the collected light on the cosine of the angle between a line through the centers of the two fibers and the cantilever tip displacement (the sound direction). The sensor requires relatively low power, and its LED source has low levels of 1/f noise. These attributes make it a good choice for remote low frequency applications requiring long operating lifetimes. An analytic model of the acoustic response of the cantilever is constructed, which is partially verified using a finite element model and experimentally validated using measurements of the acoustic response in air. The model is used to predict to what extent and over what frequency band that response depends upon the acoustically generated flow (drag) force [Yuan et al., IEEE Sensor J. 8, 1114-1117 (2008)].

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.

Inversion methods for the detection and localization of inclusions in structures utilizing dynamic surface displacements

In this paper, we investigate the feasibility of both detecting and localizing inclusions in structures given a knowledge of dynamic surface displacements. Provided with such displacement information, the equations of motion are utilized to estimate local material parameters through inversion, as well as to indicate the locations of inclusions using a novel generalized force mapping technique. Using a finite element code, numerical simulations were carried out for the determination of the normal surface displacements for both steel and mortar rectangular plates subject to monochromatic point actuation. The data is generated for both homogeneous plates and inhomogeneous plates within which a small rectangular inclusion of differing material parameters is present, and three algorithms are applied to the calculated displacement data. The first two are local inversion techniques which provide a spatial map of the elastic modulus normalized by density, while the third technique utilizes the inhomogeneous form of the equations of motion to obtain an induced force distribution caused by the inclusion. It will be demonstrated that the algorithms can both detect and locate inclusions in structures even when the materail parameter difference of the inclusions and the background medium is relatively low.

Compact directional acoustic sensing using multi-fiber optical probes

A compact directional acoustic sensor concept is described, which uses an multi-optical fiber probe, a light emitting diode source, a photo-diode detector, and a short, slender cylindrical cantilever to the end of which is attached an optical reflector. A portion of the light exiting one fiber is collected by a second fiber after reflection from the mirror. Acoustically induced transverse displacement of the cantilever tip modulates the light collected by the second fiber, which then conveys the light to a photo-detector. Directional sensitivity is achieved by virtue of the dependence of the collected light on the cosine of the angle between the line connecting the probe fiber centers and the direction of displacement of the cantilever tip (the acoustic wave direction). An analytic model of the acoustic response of the cantilever tip is constructed, which is partially verified using a finite element-based model and experimentally validated using measurements of the acoustic response in air. The model is u...

A numerical study of defect detection in a plaster dome ceiling using structural acoustics

A numerical study is carried out to evaluate the effectiveness of using measured surface displacements resulting from acoustic speaker excitation to detect and localize flaws in a domed, plaster ceiling. The response of the structure to an incident acoustic pressure is obtained at four frequencies between 100 and 400 Hz using a parallel h-p structural acoustic finite element-based code. Three ceiling conditions are modeled: the pristine ceiling considered rigidly attached to the domed-shape support, partial detachment of a segment of the plaster layer from the support, and an interior pocket of plaster deconsolidation modeled as a heavy fluid. Spatial maps of the normal displacement resulting from speaker excitation are interpreted with the help of predictions based on static analysis. It is found that acoustic speaker excitation can provide displacement levels readily detected by commercially available laser Doppler vibrometer systems. Further, it is concluded that for 1 in. thick plaster layers, detachment sizes as small as 4 cm are detectable by direct observation of the measured displacement maps. Finally, spatial structure differences are observed in the displacement maps beneath the two defect types, which may provide a wavenumber-based feature useful for distinguishing plaster detachment from other defects such as deconsolidation.

Scattering by a cylinderical shell buried in elastic sediment

Scattering results are presented for the case of cylindrical steel targets buried in elastic sediment with sound incident from the air above. The STARS3D finite element program recently extended to layered, elastic sediments is used to compute the scattering and the resulting normal displacement at the interface since the specific focus here is detection by systems which rely on monitoring the acoustic displacements or displacement-related entities at the fluid-sediment interface. Results are compared for the scattered field produced by the cylinder buried in layered elastic sediment versus in fluid sediment and for the scattered field of a buried cylindrical shell versus a buried solid cylinder. [This work was supported by ONR.]