Jermaine L. Kennedy

LOW- TO MID-FREQUENCY SCATTERING FROM ELASTIC OBJECTS ON A SAND SEA FLOOR: SIMULATION OF FREQUENCY AND ASPECT DEPENDENT STRUCTURAL ECHOES

The scattering from roughly meter-sized targets, such as pipes, cylinders and unexploded ordnance shells in the 130 kHz frequency band is studied by numerical simulations and compared to experimental results. The numerical tool used to compute the frequency and aspect-dependent target strength is a hybrid model, consisting of a local finite-element model for the vicinity of the target, based on the decomposition of the three-dimensional scattering problem for axially symmetric objects into a series of independent two-dimensional problems, and a propagation model based on the wavenumber spectral integral representation of the Greens functions for layered media.

Acoustic response of unexploded ordnance (UXO) and cylindrical targets

A series of monostatic and bistatic acoustic scattering measurements were conducted to investigate discrimination and classification capabilities based on the acoustic response of targets for underwater unexploded ordnance (UXO) applications. The measurements were performed during March 2010 and are referred to as the Pond Experiment 2010 (PondEx10), where the fresh water pond contained a sand sediment. The measurements utilized a rail system with a mobile tower and a stationary sonar tower. Each tower is instrumented with receivers while the sources are located on the mobile tower. For PondEx10, eleven targets were deployed at two distinct ground ranges from the mobile tower system. Acoustic data were initially processed using synthetic aperture sonar (SAS) techniques, and the data were further processed to generate acoustic templates for the target strength as a function of frequency and aspect angle. Preliminary results of the processing of data collected from proud targets are presented. Also presented are the results associated with a processing technique that permits isolation of the response of an individual target, which is in close proximity to other targets.

Scattering From Objects at a Water–Sediment Interface: Experiment, High-Speed and High-Fidelity Models, and Physical Insight

In March 2010, a series of measurements were conducted to collect synthetic aperture sonar (SAS) data from objects placed on a water-sediment interface. The processed data were compared to two models that included the scattering of an acoustic field from an object on a water-sediment interface. In one model, finite-element (FE) methods were used to predict the scattered pressure near the outer surface of the target, and then this local target response was propagated via a Helmholtz integral to distant observation points. Due to the computational burden of the FE model and Helmholtz integral, a second model utilizing a fast ray model for propagation was developed to track time-of-flight wave packets, which propagate to and subsequently scatter from an object. Rays were associated with image sources and receivers, which account for interactions with the water-sediment interface. Within the ray model, target scattering is reduced to a convolution of a free-field scattering amplitude and an incident acoustic field at the target location. A simulated or measured scattered free-field pressure from a complicated target can be reduced to a (complex) scattering amplitude, and this amplitude then can be used within the ray model via interpolation. The ray model permits the rapid generation of realistic pings suitable for SAS processing and the analysis of acoustic color templates. Results from FE/Helmholtz calculations and FE/ray model calculations are compared to measurements, where the target is a solid aluminum replica of an inert 100-mm unexploded ordnance (UXO).

Volumetric Acoustic Imaging via Circular Multipass Aperture Synthesis

In this paper, volumetric imaging via multipass circular synthetic aperture sonar (CSAS) is demonstrated using an autonomous underwater vehicle (AUV). A multidimensional aperture is synthesized by performing a series of circular scans at varying grazing angles around targets and coherently combining the backscattering information from the set of scans to form high-resolution volumetric images. A data-driven technique for precision alignment of the individual scans comprising the multipass set enables synthesis of a multidimensional array. To beamform in the vertical dimension using the irregular and undersampled multipass aperture, a compressive-sensing-based approach is adopted which is similar to methods used in analogous synthetic aperture radar tomography applications but modified to accommodate for the wider fractional bandwidth of the synthetic aperture sonar (SAS) system. The modification exploits a joint sparsity assumption in the vertical scattering profile at different subbands and adapts a standard joint sparse solving algorithm to the relevant case in which the sparsity profile is common between solution vectors but the sensing matrices are different. Results are shown for a variety of targets, including proud and obliquely buried unexploded ordnance, a 2-1 solid aluminum cylinder, and a steel oil drum.

Coherent and semi-coherent processing of limited-aperture circular synthetic aperture (CSAS) data

Circular synthetic aperture sonar (CSAS) traditionally involves the coherent processing of 360 degree scattering information from acoustic targets. To obtain 360 degree scattering information, a source may circle around a central target field and constantly illuminate targets from multiple aspects. Another method of obtaining CSAS data is to fix the source location and spin a target on a rotating mount. Following data reception, a variety of methods in the Fourier or time-domain may be used to construct images. For certain targets, resonances and elastic effects can interfere with the specular portions of backscattered echoes. The time-delay associated with elastic or resonant responses destroys the uniqueness of the location to which the signal is mapped, and occasionally these resonant features can be mapped directly on top of target specular features, causing destructive interference and reduced image clarity. Destructive interference can be reduced and image clarity enhanced by incoherently summing separate images generated from sub-apertures of CSAS data. Additionally, limiting the aperture and frequency band of the pre-processed data before applying an imaging algorithm is an effective method for understanding and localizing various elastic and non-elastic target responses. In a solid 3-to-1 cylinder, for example, effects such as meridional and face-crossing rays cause well defined image features that are prominently visible when limiting the aperture to the angular portions in which these rays are the dominant elastic effects. Further analysis may be obtained by masking portions of these sub-aperture images and reversing the imaging process. This can be used to directly relate target image features to the angular frequency response (colorplot) of the target. An added benefit of this reversal process is that signals from surrounding objects, and the random noise spread throughout the image scene can be rejected by an image masking process, and the resulting time-domain information has an enhanced signal-to-noise ratio. This effect has been successfully demonstrated on data acquired in field-tests, and in controlled laboratory experiments with real and replicated Unexploded Ordnance (UXO) objects. A “Projection-Slice” based CSAS script has been tested on data acquired at sea by an unmanned vehicle, as well as in laboratory experiments from UXO objects placed on a rotational mount. The laboratory based full scale UXO datasets were acquired in a controlled environment for full 360 degree aperture in a free-field configuration. The measurements were conducted at the Naval Surface Warfare Center, Panama City Division (NSWC PCD), facility T-2069 Barge Acoustic Test Facility, which has a 31.5 feet wide, 62 feet long, and 28 ft deep vinyl linear encapsulating 423,000 gallons of isothermal freshwater. The full scale targets examined included an inert 100 mm UXO target, a machined 100 mm aluminum facsimile UXO target, and a cylinder with a notch. The targets were suspended from a rotation stage, with 360 degree rotation capability, in the water column and insonified by a broadband acoustic projector. Preliminary results have also demonstrated the ability to image high-resolution circular synthetic aperture data in which the sonar platform, Remote Environmental Monitoring Units (REMUS) 600 unmanned underwater vehicle, was programmed to circle sunken objects. [Research supported by Office of Naval Research and The Strategic Environmental Research and Development Program (SERDP) under projects MM-1665 and MM-1666.]

Acoustic scattering from underwater munitions near a water‐sediment interface.

Monostatic and bistatic scattering measurements were conducted on a set of targets near a fresh water‐sand sediment interface. The measurements were performed during March 2010 and are referred to as the Pond Experiment 2010 (PondEx10). Monostatic synthetic aperture sonar (SAS) data were collected on a rail system with a mobile tower, while a stationary sonar tower simultaneously collected bistatic SAS data. Each tower is instrumented with receivers while the sources are located only on the mobile tower. For PondEx10, 11 targets, including 6 underwater munitions, were deployed at 2 ranges from the mobile tower system. Initially, the data were processed using standard SAS techniques, and then, the data were further processed to generate acoustic templates for the target strength as a function of frequency and aspect angle. Results of the data processing from proud targets are presented. Finite element model (FEM) predictions of the scattering from an ordnance in the free field and proud on the interface ar...

Autofocusing circular synthetic aperture sonar imagery using phase corrections modeled as generalized cones

Circular synthetic aperture sonar (CSAS) is a coherent aperture synthesis technique that utilizes backscattered acoustic information from an encircled scene to generate information rich, high-resolution imagery. The aperture length required for image synthesis is much longer than in its linear synthetic aperture sonar counterpart and can result in challenging phase delay and navigation estimation constraints. Residual uncorrected phase errors manifest as focus aberrations in reconstructed CSAS imagery. This paper demonstrates that phase error in image patches can be approximated as an aspect variant linear phase shift representable as a generalized cone in wave-number space. If the geometry of the generalized cone is known, it can be applied as the spectral phase of an inverse filter for aberration correction. A method is derived for reconstructing the error cone geometry from independent estimates of its local curvatures, which are found via a series of one-dimensional line searches that maximize the focus of CSAS sub-aperture images. This approach is applied to real and simulated CSAS data containing aperture distortions, and the results successfully demonstrate estimation and correction of the underlying focus aberrations.

Comparisons of SAS processed data from ordnance replicas over various size scales: Experiments and acoustic models

Sonar has been the Navys workhorse for detection and discrimination of underwater objects from clutter through analysis of image phenomena. However, new phenomena need to be identified to accommodate a growing class of unexploded ordnance targets with more varied shapes and sizes, and difficulties increase for buried ordnance because the wave attenuation and inhomogeneity in ocean sediments make detection less predictable and high-resolution imaging more difficult. Even when imaging can be done, important image phenomena (e.g., highlight/shadow details) are lost. Therefore, modeling, data collection, and data processing/analysis has been performed at NSWC PCD to build a target response database that could be exploited both for statistical automated target recognition studies based on existing feature types and for isolation of promising physical phenomena that classifiable features could be extracted from without requiring high image resolution. Several methods to build this database have been pursued, including data collection in full-scale field experiments in the Gulf of Mexico, data collection in NSWC PCDs freshwater pond facility, data collection within a small freshwater tank using scaled targets on scaled glass bead sediments, and data generation through finite element simulations. In this paper, results from each of these methods are presented and compared to assess the tradeoffs inherent in them.

Scan geometries for three dimensional synthetic aperture sonar tomography

Synthetic aperture sonar (SAS) exploits vehicle motion and coherent multi-ping signal integration to enhance image resolution. SAS is typically used to generate high-resolution 2D sonar imagery; however the generation of high-resolution, voxel-based 3D images of targets has also been demonstrated by synthesizing multi-dimensional apertures from scans conducted at diverse grazing-angles. In the latter case, the geometry of the multi-dimensional synthetic aperture can have significant ramifications for all stages of the beamforming process, ranging from navigation refinement and beamformer design to very practical experimental issues, such as the time necessary to scan a target. In this paper, the effects that different multi-dimensional scans have on three-dimensional SAS are considered. Results of field tests for different scan geometries are shown and interpreted in light of their various benefits and detriments.

Frequency response of ordnance replicas across multiple scales

Broad-band, multi-aspect backscatter data obtained using small-scaled and full-scaled replicas of Unexploded Ordnance (UXO) are reported. Data were collected using either linear or circular rail systems. The experiments were performed in: (1) NSWC PCDs small scale test bed (less than 1/16 scaled) which has a simulated bottom, (2) NSWC PCDs Facility 383 9-million gallon fresh water test pool which has a 5-ft sand bottom, and (3) the Gulf of Mexico during GULFEX12 off Panama City, Florida. Data were processed using linear and circular synthetic aperture sonar techniques to generate both images and plots of target strength as functions of frequency and aspect angle. The results from all three experiments are compared to each other and with predictions from Finite-element (FE) analysis. These comparisons are used to assess the utility of alternative methods for generating sonar data from bottom targets of sufficient fidelity to study scattering phenomena and support development of automated target recogniti...