Monitoring Greek Seas Using Passive Underwater Acoustics

The Hellenic Center for Marine Research POSEIDON ocean monitoring and forecasting system has included passive underwater acoustic measurements as part of its real-time operations. Specifically, low-duty-cycle long-term passive acoustic listeners (PALs) are deployed on two operational buoys, one off Pylos in the Ionian Sea and the second off Athos in the northern Aegean Sea. The first step toward the quantitative use of passive ambient sound is the classification of the geophysical sources for example, wind speed and rain rate from the noise of shipping, from other anthropogenic activities, and from the natural sounds of marine animals. After classification, quantitative measurements of wind speed and precipitation are applied to the ambient sound data. Comparisons of acoustic quantitative measurements of wind speed with in situ buoy anemometer measurements were shown to be within 0.5 m s(-1). The rainfall detection and quantification was also confirmed with collocated measurements of precipitation from a nearby coastal rain gauge and operational weather radar rainfall observations. The complicated condition of high sea states, including the influence of ambient bubble clouds, rain, and sea spray under high winds, was sorted acoustically, and shows promise for identifying and quantifying such conditions from underwater sound measurements. Long-term data were used in this study to derive sound budgets showing the percent occurrence of dominant sound sources (ships, marine mammals, wind, and rain), their relative intensity as a function of frequency, and statistical summaries of the retrieved rainfall amounts and wind speeds at the two buoy locations in the Aegean and Ionian Seas.