Wide Area Detection and Identification of Underwater UXO Using Structural Acoustic Senors: 4th Annual Report to SERDP MM-1513

Abstract

Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std. Z39.18Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER This project is exploring the development of a structural acoustics (SA) based sonar methodology for wide area search and identification of underwater unexploded ordnance (UXO). This approach has significant advantages over conventional acoustic approaches relying on formation of high resolution images including: diverse set of "fingerprints" leading to low false alarm rates; longer range leading to wide area coverage; and low frequency sediment penetration leading to buried target prosecution. A core element of the project is an examination of the scattering features exhibited by typical UXO targets in the SA regime using NRL's state-of-the-art underwater scattering facilities, both laboratory-based and at-sea. We have extended the original mono-static data base to include bi-static data from proud and partially buried targets, examined the special case of forward scattering and how it might be exploited, connected laboratory UXO scattering measurements to those made in St. Andrew's Bay by employing an advanced acoustic multi-path propagation model, and examined the scattering from proud and buried targets on smooth and rough sediment surfaces using an elasto-dynamic finite integration technique (EFIT) numerical simulation model. The target is placed in the plane-wave region of (1) a near-field cylindrical source at low frequency (LF) or (2) a farfield piston source at high frequency (HF) with a nearly co-located broadband short vertical receiver array. The target (which is ~ 2.7 m from the receiver) is rotated over a full 360 degrees in increments of 1 degree. iv Figure 5 -a) Broadside target strength versus frequency computed for infinite cylindrical shell with five inch rocket parameters and 3D wave correction compared to measurement. Black: measured data; red: COMSOL 2D FE calculation with 3D wave correction; green-dashed: 10log 10 (aL 2 /2λ); green-dashed-dot: wave theory rigid 2D cylinder with finite length 3D wave theory correct...

UXO Detector for Underwater Surveys Using Low-Frequency Sonar

“…It should be noted, though, that we believe that acoustic colour information could be relevant if data covering a larger range of aspect angles and frequencies would be available. This statement is supported by the results presented in the SERDP webinar on Acoustic Methods for Underwater Munitions [21].…”