Acoustic scattering from inert underwater munitions with severe corrosion damage

More than 400 underwater sites have been identified by the U.S. Government as potentially contaminated with unexploded ordinance (UXO), posing a potential threat to human safety. While some of these munitions are more recent, some can date as far back as the 18th century, thus having the potential for severe corrosion and biofouling damage. We hypothesize that as UXO age for years or decades in the underwater environment, accumulated biofouling and corrosion will lead to an increasingly larger deviation of the acoustic scattering signature from the pristine state. To explore these relationships, we measured the free-field frequency- and aspect-dependent acoustic scattering response of recently recovered World War II era miniature practice bombs (model AN-Mk 23) that exhibit a range of corrosion and minimal biofouling, due to decades of environmental exposure in a brackish marsh. The acoustic responses of pristine, uncorroded examples of AN-Mk 23 ordnance were also measured. We found significant shifts in the aged samples’ resonance frequencies, aspect variation, and overall target strength level. Analytical material characterization and high-resolution imaging techniques were employed to determine how material loss and the formation of specific biofouling–corrosion products correlate to the modified acoustic signatures. [Work sponsored by SERDP and DOD SMART Scholarship.]