Nicholas C. Makris
William A. Kuperman
Naval Res. Lab., Washington, DC 20375
SACLANT Undersea Res. Ctr., La Spezia, Italy
The idea of using naturally occurring ambient noise to detect objects in the ocean [Flatte' and Munk, JASON JSR-85-108 (1985)] has received much attention due to experimental results in the high frequency regime [Buckingham et al., Nature 356, 327--329 (1992)]. In this paper, full-field surface-generated noise and scattering models are combined to derive an expression for the total noise field covariance of a stratified ocean waveguide with a submerged object present. The Cramer--Rao upper bound on performance is computed for some shallow water examples using conventional sensor arrays. The results, along with some simulated detections using beamforming, indicate the high frequency (~10 kHz) detection is limited to relatively short ranges due to rapid decrease in scattered to direct noise ratio. The idea may be of most practical use for remote detection at low frequency (~100 Hz). Detection is enhanced by high correlation between direct and scattered noise fields at the sensor. When direct and scattered noise fields are independent, the relatively short coherence scale of the scattered noise severely limits detection and precludes detailed imaging of the objects regardless of frequency. Standard detection techniques such as beamforming and MFP generally do not meet the upper bound on performance. [sup a)]Present address: Scripps Institution of Oceanography, La Jolla, CA.