To effectively remediate Department of Defense (DoD) sites that contain unexploded ordnance (UXO), better tools are needed to discriminate between UXO and non-hazardous items. Although great effort has been expended to detect and localize UXO in the ground and underwater, there are few devices that can inspect and identify the filler materials. A quick and safe identification would significantly lower personnel risks and remediation costs.

 The objective of this project was to utilize acoustic waves in identifying the filler materials in sealed UXO.

Acoustic waves are high frequency pressure fluctuations that travel through materials. Small sensors clamped to the outside of the ordnance send low-energy acoustic waves with a single primary frequency through the container walls and filler. The same sensors receive the return signals, which are processed to determine the characteristic acoustic properties of the filler material. To identify the filler, these properties are then compared to a database of known properties for inert filler materials. In this project, a measurement system was developed and tested on live and inert ordnance. Research efforts initially focused on the development of a test system that overcame measurement problems caused by the corrosion and variability of UXO. Together with models of the acoustic propagation, system testing provided initial indications of the reliability and discrimination capability of the technique. The acoustic techniques were then further refined to improve the signal-to-noise ratio of the received signals, resulting in increased identification reliability.

An acoustic filler identification technology with the potential to quickly identify partially uncovered UXO during site cleanup activities was developed. Prototype devices were tested on a wide variety of mortars and shells with both inert and high explosive filler materials at the Naval Surface Warfare Center Crane Division, Army Aberdeen Test Center, and Navy Explosive Ordnance Technology Division. These tests showed that, although the technology will not identify all ordnance types and filler materials, it provides a simple, low-cost way to identify some of the most common filler materials. The technology works best on fillers that are cast into the shell body and are intimately bonded to the metal walls. Plaster of Paris (POP) and cement fillers provide good signals for identification; whereas, loose sand and gravel do not. Explosive fillers do not provide good signals for identification. Although corrosion reduces the amplitude of the received signals, good signals were received for a number of highly corroded items filled with cement and POP. This reliable and safe technology is now being demonstrated through the Environmental Security Technology Certification Program.

A device based on this acoustic technique would permit DoD personnel to quickly and safely distinguish explosive and inert munitions, optimizing subsequent remediation efforts. Currently, all items that cannot be positively identified as inert must be treated as though they are explosive-filled. If efforts can be restricted to only hazardous explosive items through high confidence identification of filler materials, significant savings could be attained.