Measurements of High-Frequency Acoustic Scattering from Seabed Vegetation Anthony P. Lyons and Eric Pouliquen SACLANT Undersea Research Centre, Viale S. Bartolomeo 400, 19138 La Spezia, Its@ Abstract: Acoustic experiments were conducted in order to quantifi the scattering properties of the most common Mediterranem seagrass, Posidonia Oceanica. Measurements were made over a wide range of frequencies and grming angles at several sites near the islands of Elba and Sardini~ Italy, and in Saros Bay, Turkey. Results of these measurements indicate that, for dense fields of Posidonia, mean scattering strength values are quite high (-23 dB re 1 m2) with a relatively flat grazing angle dependence. Scattering strength values showed no frequency dependence over the frequency range studied. INTRODUCTION tiowledge of the acoustic properties of seabed vegetation is required for accurate seafloor characteri~tion in many shallow water sites. As the study and modeling of acoustic scattering from seabed vegetation have been very limited to date, characteristics such as the dependence of scattering strength on grazing angle or frequency are still largely unknown. In order to quantify these properties and to aid in the understanding of the dominant scattering mechanisms of seagrasses, acoustic scattering experiments were conducted in beds of dense Posidonia at several sites near the islands of Elba and Sardinia, Italy, and in Saros Bay, Turkey. These experiments were conducted with side-scan, single beam, and parametric transducers. The oblique incidence measurements, which will be presented in this paper, were made with linear sources at frequencies of 30, 50, 80, 90 and 110 kHz with coverage over a large range of grazing angles. POSIDONIA In the Mediterranean, shallow coastal waters are ofien carpeted with beds of seagrass in water depths of 10- 30 m. The largest and most common of the marine grasses is Posidonia Oceanica. These plants play a very important role in Mediterranean marine ecosystems, helping to stabilize the seafloor, reducing the intensity of water movement along the seafloor, producing large amounts of oxygen, and providing a habitat for a variety of marine life. Posidonia grows in bundles of 5-10 ribbon like leaves spread in a fan, with densities reaching 700 bundles per square meter (1). Individual leaves are 8-11 mm in width and up to 1 m in length but are never fully rigid so that the overalI height of the plant depends on the local currents, wave action and season. Older leaves are usually completely encrusted with carbonate material (easily seen as bright spots in Figure 1). As with other seagrasses, Posidonia contains gas filled pockets running the length of the each leaf. The strong high-frequency scattering from Posidonia beds is most likely due to the gas pockets in or the encrustation on the individual leaves. FIG~E 1. Photograph of Posidonia Oceanica seagrass (field of view is approximately 50 cm x 80 cm), 1627