Acoustic characterization of submerged aquatic vegetation: military and environmental monitoring applications

The acoustic characteristics of submerged aquatic vegetation (SAV) were investigated in order to enhance the performance of mine-hunting sonars in littoral regions. The species of SAV investigated was Zostera marina, commonly known as eelgrass. In situ measurements were carried out in the shallow water eelgrass beds of Narragansett Bay, Rhode Island. Side-scan and single beam sonars were used with a Manta mine training shape as a target. The experiments clearly illustrate the backscatter and masking effects of SAV in the 100-500 kHz frequency range. These experiments also aided in determination of distribution and density of seagrass. Remote sensing of seagrass beds is important for a variety of environmental applications. Physical techniques for detection and characterization are labor and cost intensive and provide little insight into spatial distribution. Optical-based techniques are limited by water clarity and frequently result in systematic underestimation of the extent of seagrasses. Active hydroacoustic techniques have shown the ability to detect seagrasses, but the phenomenology behind detection is poorly understood. Based on the data from laboratory acoustic measurements, hydroacoustic approaches for wide area detection and mapping are discussed and several are demonstrated within areas of established eelgrass beds in Narragansett Bay.