A problem identified at the 2020 SERDP & ESTCP Technical Session on “Development and Use of Underwater Unexploded Ordnance (UXO) Development Sites, Follow on Discussion” is the unintended sonar detection of target lines used to facilitate relocation and recovery. Since target line is an indispensable tool in deeper water test bed sites, a comparison of the relative scattering strength of various line types and gauges would be useful to test bed managers. The objective of this project is to team with Applied Physics Laboratory at the University of Washington (APL/UW) researchers to perform a measurement of these strengths using Naval Surface Warfare Center Panama City Division's (NSWC PCD) Acoustic Test Facility (ATF) barge and provide a model-based capability to predict the detectability of these lines in selected test bed environments.
NSWC PCD’s ATF barge deck measures 30 feet wide and 60 feet long with an 832 ft2 air conditioned instrumentation area. The instrumentation area has a 2-ton bridge crane and a 2-ton monorail hoist. The 10 feet wide and 63 feet long access ramp allows forklifts and small trucks to deliver large test items directly to the barge. A vinyl liner (31.5 feet wide and 62 feet long) suspended beneath the barge holds over 423,000 gallons of clear, isothermal water and extends to the mud bottom of the pond at a depth of approximately 28 feet. A test well with a 4 feet wide and 30 feet long opening can be used to suspend test items into the water. The test range can be increased to over 50 feet by use of a 4 feet by 4 feet access hole on the deck. A positioner, with a 2-ton capacity, can be used to suspend test items in the water and can move azimuthally, vertically and horizontally under computer control. Rotational accuracy is +/- 0.04 degrees. The ATF has transducers (projectors and receivers) that cover the band of 1 kilohertz to 2 MegaHertz.
A simple set of measurements are proposed where line sections of various types and widths commonly used by divers are suspended from the barge positioner so their free-field backscatter strength as well as waveforms can be measured and recorded. The effect of entrained bubbles in braided lines will be assessed by comparing line that has been degassed under vacuum with line that has not been. Scattered signals and source replica pulses recorded will be processed into scattering form functions as a function of ka (wavenumber x line radius) by deconvolving spectral effects of the source and receiver using the source replicas. APL-UW will then incorporate these results into their Target in the Environment Response model to produce the desired capability to estimate line visibility in ESTCP test areas.
Lines used to facilitate target relocation/recovery in UXO test beds can potentially interfere with assessments of sonar-based UXO detection systems. A capability to estimate the signal-to-noise ratio of line would allow test bed managers to make informed decisions on the selection of line that provides the strength and low visibility desired at each site.