ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

5aUW4. Deriving scattering strengths from nonstationary time-series data: A comparison of low-frequency surface-backscattering strengths using both impulsive and coherent sources.

Roger C. Gauss

Peter M. Ogden

Naval Res. Lab., Washington, DC 20375-5350

John B. Chester

Naval Undersea Warfare Ctr., New London Detachment, New London, CT 06320

Joseph M. Fialkowsi

Planning Systems, Inc., McLean, VA 22102

The derivation of scattering strengths from acoustic field data involves applying the active sonar equation to the measured reverberation response. However, due to the typical nonstationarity observed in the direct-path reverberation time series (e.g., 10 dB/s), the choice of appropriate corresponding grazing angles becomes more problematical as the duration of the transmitted signal increases. Direct-path measurements of low-frequency (200--1000 Hz) acoustic surface scattering were made in the Gulf of Alaska in the February of 1992 using interleaved SUS and pulsed waveforms (cw, FM). Backscattering strengths were derived for pulse lengths varying between 50 ms and 2.4 s. A comparison of results revealed that scattering strengths have a nontrivial dependence on signal type and pulse length, and on the signal-processing methodology used in their derivation. General implications for deriving scattering strengths from nonstationary time-series data will be discussed. [Work supported by ONR (NRL).]