Experimental study of geo-acoustic inversion uncertainty due to ocean sound-speed fluctuations

Acoustic data measured in the ocean fluctuate due to the complex time-varyi ng properties of the channel. When measured data are used for model-based, geo-acoustic inversion, how do acoustic fluctuations impact estimates for t he seabed properties? In May 1999 SACLANT Undersea Research Center and TNO- Physics and Electronics Laboratory (FEL), conducted a shallow-water experim ent on the Adventure Bank off the southwest coast of Sicily, Italy to asses s the effects of a time-varying ocean on acoustic propagation and geo-acous tic inversion. A favorable area for acoustic propagation was identified whi ch had slight internal wave activity and a weakly range-dependent bathymetr y with sand-like bottom properties. Oceanographic and acoustic measurements were performed continuously over a 3-day period. Broadband (0.2-3.8 kHz) a coustic signals from a bottom-moored source were transmitted over fixed pat hs and received on a moored vertical hydrophone array. During the transmiss ions extensive environmental measurements (e.g., sound speed, current, sea- surface waveheight, etc.) were made to correlate the time-varying environme ntal and acoustic data. Modeled acoustic data show time variability which a grees with the measurements. Results illustrate severe problems when modeli ng shallow-water acoustic propagation at ranges beyond a few kilometers in the frequency band considered. Further, the acoustic fluctuations in time c aused erroneous time variability in inverted seabed properties.