Inverse Acoustical Determination of Photosynthetic Oxygen Productivity of Posidonia Seagrass
[Technical Report, Research Report]
SACLANT UNDERSEA RESEARCH CENTRE LA SPEZIA (ITALY)
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Executive Summary Mine detection in the shallow waters of the Mediterranean Sea is often limited by the background interference generated from the seabed. The interference is generally in the form of reverberation at mine hunting frequencies from the water-bottom interface. Particular to the Mediterranean Sea are fields of dense Posidonia sea grass. Posidonia affects mine hunting systems by the reverberation produced by the sea grass and oxygen bubbles. The bubbles are generated by photosynthesis and are deposited on the Posidonia leaf blades. Low frequency acoustic inversion techniques, developed to estimate the geoacoustic properties of the seabed, were successfully applied to monitoring the oxygen synthesis of the Posidonia. In this paper there are analyses of the low frequency acoustic data and its correlation with environmental observations of the solar radiation, oxygen, and other factors such assalinity and temperature as a function of depth. The most evident feature is an abrupt and marked change of attenuation and time dispersion characteristics at the onset of photosynthesis. These diurnal phenomena are attributed to bubbles of photosynthetic oxygen formed on the Posidonia leaf blades. It remains to be verified that the effects of Posidonia on higher frequency mine detection systems have a diurnal dependence. The contribution of photosynthetic oxygen to the acoustic attenuation and scattering process requires quantification. In addition to MCM, these results are also relevant to ASW in lottoral waters. Furthermore, these results may be applicable to the monitoring of the state of health of Posidonia and other seagrasses in the Mediterranean and other seas.