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

Abstract

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