1. ADVANTAGES OF LOW FREQUENCY STRUCTURAL ACOUSTIC IDENTiFICATION

The Naval Research Laboratory is using its world-renowned structural acoustics facilities (originally developed for scaled submarine programs) to study the broad band (1-150 kHz) acoustic scattering from proud and buried underwater mines. The objective is to discover what information is contained in the broad-band properties of the scattered signal which might be exploited for target identification purposes. Current acoustic mine-hunting systems form acoustic images that replicate the rough geometric shape of the target. To obtain sufficient resolution, these systems must operate at frequencies that are too high for anything but time-consuming, close-in looks at the target. Even then, they often confuse mines with mine-like targets such as oil drums. In contrast, structural acoustic clues such as mine resonances, elastic wave propagation, internal structure scattering, etc., are available at lower frequencies (1-10 kHz), allowing for much longer ranges of operation as well as the construction of unique "fmgerprints " by which to identify the target as a mine. Additionally, at lower frequencies the ocean sediment is more readily penetrated by acoustic waves, creating the possibility for buried mine detection. This paper examines the feasibility of exploiting such very low frequency structural acoustic clues for long range identification ofproud and buried mines