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More than 400 underwater sites in the United States are potentially contaminated with unexploded ordnance (UXO)—weapons that did not explode upon deployment, which continues to pose a safety concern.

Connor Hodges, a doctoral student at the University of Texas at Austin, studies the changes in the acoustic characteristics of these UXOs after they have been subject to corrosion and biofouling to help detect them underwater.

"Many of these sites are in shallow water, potentially posing a threat to human safety, and date back several decades," said Hodges. "This long exposure to the environment leads to corrosion as well as encrustation in the form of barnacles or algal growth."

Corrosion and growth make UXOs difficult to observe with standard sonar imaging techniques, as the objects begin to lose resemblance to their original appearance and blend into their environment over time. These changes also alter how acoustic signals scatter from the objects, and the changes can become more severe over time as corrosion or organic growth gets worse.

Hodges discussed the use of acoustics for corroded UXO recovery at the of the Acoustical Society of America and 25th International Congress on Acoustics, held May 18–23.

Hodges and his collaborators tested a collection of AN-Mk 23 practice bombs—miniature bombs used for dive-bombing practice—in various stages of corrosion, which had been buried in a brackish pond on Martha's Vineyard for about 80 years. They compared the acoustics of these samples to those of pristine AN-Mk 23, monitoring the scattering response at different directions and angles.

The researchers found the change in size, shape, and material makeup of a bomb as it corrodes changes its acoustic resonance and leads to a different, weaker scattered acoustic signal than pristine bombs. The changed acoustic signature could result in the object being misclassified or undetected.

"Acoustic scattering techniques give an insight into the internal structure of the object imaged, as well as a method to 'see' into the seafloor," said Hodges, noting that using sonar to map the seafloor and detect munitions is also faster and cheaper than other techniques.

Many former military sites used for practice bombs are shifting toward public use, making UXO identification a timely endeavor.

"There is a risk of detonation if they are stepped on or otherwise disturbed," Hodges said. "This poses a larger risk to human safety in shallow waters, and UXO identification and recovery becomes vital as old sites are transitioned away from military use."

He hopes the work can help provide better predictive tools for finding UXOs in civilian environmental demining efforts and plans to study other types of munitions as well as other types of corrosion and biofouling phenomena.

"Underwater UXO can be tricky to find and recover, so it is important that this can be done safely and effectively," said Hodges. "We hope this work will ultimately help save lives."