1 OCEANS 2010 MTS/IEEE Seattle, 20-23 Sep 10 Kargl et al, Acoustic Response of Unexploded Ordnance (UXO) 1 OCEANS 2010 MTS/IEEE Seattle, 20-23 Sep 10 Kargl

1OCEANS 2010 MTS/IEEE Seattle, 20-23 Sep 10

Kargl et al, Acoustic Response of Unexploded Ordnance (UXO)

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OCEANS 2010 MTS/IEEE Seattle, 20-23 Sep 10Kargl et al, Acoustic Response of Unexploded Ordnance (UXO)

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Acoustic Response of Unexploded Ordnance (UX0) and Cylindrical Targets

Steven G. Kargl and Kevin L. WilliamsApplied Physics LaboratoryUniversity of Washington

1013 NE 40th St., Seattle, WA 98105

Timothy M. Marston*Physics and Astronomy Dept.Washington State UniversityPullman, WA 99164-2814

Jermaine L. Kennedy and Joseph L. LopesNaval Surface Warfare Center

Panama City DivisionPanama City, FL 32407-7001

Research supported by SERDP and ONR

*Current Address: Naval Surface Warfare Ctr, Panama City Division



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2Technical Objective

● Understand the interaction of an incident acoustic field with UXO near a water-sediment interface over large frequency and aspect angle ranges.

● Develop an inventory of acoustic responses for various UXO, which can be used to validate propagation and scattering models.

● A central hypothesis is that the environment within which a UXO must be detected and classified alters the acoustic response of the target significantly and must be taken into account in order to develop robust detection and classification strategies.

● Monostatic and bistatic synthetic aperture sonar (SAS) data were collected from several targets. These data have been processed by standard SAS imaging techniques. Acoustic templates have been constructed using a novel SAS filtering technique for target signature separation.



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3Technical Approach: NSWC PCD Pond Facility

● 9 million gallons of fresh water

● 112 m long, 80 m wide, 14 m deep

● 1.5 m of sand sediment in the center

● Water clarity offers 12 m of visibility.



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During PondEx09, two rail systems were deployed in a perpendicular arrangement, and in PondEx10 a rail system and a stationary sonar tower were deployed. In both experiments, monostatic and bistatic measurements of the scattered acoustic field were collected. Sources and receivers cover the 1–30 and 30–50 kHz range.

The transducers were set at 20° and 40° depression angles where 20° was used for proud targets at a 10 m range and 40° was used for proud, partially buried and buried targets at a 5 m. The critical grazing angle for the sand in the pond was ~28°.

NSWC PCD Rail SystemAPL-UW Rail System

Technical Approach: Sonar Equipment



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5Technical Approach: Target Fields

PondEx09 Target Field PondEx10 Target Field

● One target in the target field

● One tower scans while other remains stationary

● Targets rotations: 0° – 80° and 0° – 280° with a 20° increment (depends on symmetry of target)

● Multiple targets in the target field

● Adjacent targets separated by 1.5 or 3 m

● Targets rotations: 0° – 80°, -80° – 80°, and 0° – 280° with a 20° increment (depends on symmetry of target)



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6Technical Approach: Targets

152 mm TP-T round155 mm howitzer projectile (empty)81 mm mortar (filled with cement)Solid steel artillery shellMachined aluminum replica of artillery shellMachined steel replica of artillery shellAluminum pipe (2:1 aspect ratio, 1 ft dia.)Solid aluminum cylinder (2:1 aspect ratio, 1 ft dia.)Two rocksSmall aluminum cylinder with a notch



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• The Comsol FEM results used environmental conditions and measured geometry from PondEx08 measurements.

• The bright observable at ~7 kHz and 40° < < 60° in the PondEx08 data is predicted by FEM. This observable is missing from an “identical” measurement during PondEx09 due to a change in the water-sediment reflection coefficient.

• Williams et al, J. Acoust. Soc. Am., 127, 3356 – 3371 (2010).

Results: Data/Model Comparison 2:1 Solid Al CylinderPondEx08

Target Strength DataPondEx09

Target Strength DataComsol™ FEM

Target Strength Result



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• Comsol™ FEM result via NURC’s AxiScat Ver 1.1model.

• Features identified using physical acoustics insight (ray acoustics).

Broadside

End-on

Results: Data/Model Comparison 2:1 Solid Al Cylinder

Face crossing Rayleigh Wave

Meridional Rayleigh Wave



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Left: Baseband pulse-compressed data with five proud targets at a 10 m range.

• Machined aluminum replica (top)• Solid aluminum cylinder• Machined steel replica• Aluminum pipe, • Real steel artillery shell (bottom)

Interference of the scattered signals from the targets.

Right: SAS images. The solid aluminum cylinder in (b) exhibits a triplet structure that was recently explained by Williams et al [JASA, 127, 3356-3371 (2010)]. For the pipe in (d), one see a strong echo from acoustic energy transmitted through the pipe and reflected from its back wall. The steel UXO in (e) and the steel replica in (c) are similar images while the image for aluminum replica in (a) is dissimilar.

PondEx10 Results: Five Targets in the Field



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10Results: SAS Filtering Algorithm to Resolve Adjacent Targets

Pulse compressed data

Time axis

Cro

ss R

ange

Target Arc

Deconvolution with a locus of closest approach arc.

Convolution with locus of closest approach arc.

Time axis

Filtered Pulse compressed data

Cro

ss R

ange

Window Image

Form a SAS Image of filtered data

The deconvolution/convolution processes are linear transformations. The processing does not alter the information in the isolated signal unless the targets are sufficiently close where multiple scattering may become important.



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11PondEX10 Results: Acoustic Template

● Machined aluminum replica (left)● Machined steel replica (center)● Real steel artillery shell (right).

The color scale is a normalized target strength. The images show the dependence of target strength on frequency and target aspect angle.

Nose of ordnance points towards the rail system at 0° and 360°, and the tail points towards the rail system at 180°. At 90° and 270°, the ordnance are in a broadside orientation.



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12PondEX10 Results: 30-50 kHz Bistatic SAS

● 152 mm TP-T round (top)● Small aluminum cylinder with notch● Solid steel artillery shell● 81 mm mortar (filled with cement)● Machined aluminum replica of artillery shell● 155 mm empty howitzer projectile (bottom)



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13Summary

● Monostatic and bistatic acoustic scattering data sets over a wide frequency range have been processed with standard SAS imaging algorithms and acoustic templates have been generated.

● Targets included 6 UXO, 3 cylindrical targets, and 2 rocks.

● SAS images and acoustic templates suggest rich elastic responses are present on the UXO and these may be used in detection and classification algorithm.

● Bistatic SAS images have been created, but the data sets have yet to be exploited in a classification scenario such as acoustics templates.

● Simulation of the machined aluminum replica, machined steel replica and the the real solid ordnance in a proud configuration are underway.

Current Research

Documents

1 OCEANS 2010 MTS/IEEE Seattle, 20-23 Sep 10 Kargl et al, Acoustic Response of Unexploded Ordnance (UXO) 1 OCEANS 2010 MTS/IEEE Seattle, 20-23 Sep 10 Kargl