Broadband frequency dispersion in shallow water as an active classification technique

Recently, an active broadband dispersion phenomenon was observed empirically in measured data for propagation paths within the mixed layer (ML) [Dien etal. (1994)]. This so-called Wilson Dispersion Phenomena (WDP) is explained simply by noting that high-frequency energy trapped within the ML travels faster than low frequencies that diffractively ‘‘leak’’ out of the ML. Thus WDP is a useful technique for distinguishing between reflectors in the ML and reflectors below the ML. More recently it was shown that the WDP may occur in other shallow-water environments [Davies etal. (1994)]. Several sound-speed profile (SSP) environments are examined, using normal mode theory, for depth dependence of broadband frequency dispersion. A very conservative approach is taken regarding frequency reflectors. Geoacoustic properties of the bottom are not addressed. SSPs for which dispersion depth dependence is observed within the water column for a lossy bottom are labeled useful for active classification. Future research including geoacoustic properties of the sub-bottom may lead to a wider range of SSP and geoacoustic sub-bottom environments which produce dispersion depth dependence. It has been empirically observed that bottom reflectors (the primary false targets) never have significant dispersion and are spread over much longer times than reflectors within the water column. a)Work performed while on temporary leave from Neptune Sciences, Inc., Slidell, LA 70458; b)Currently stationed at Fleet Numerical Oceanographic Ctr., Monterey, CA 93943.