Target detection and location with ambient noise.
By placing a vertical array in an ambient noise field and forming an upward and a downward beam one obtains two time series which can be cross correlated to reveal a subbottom profile of the seabed [Siderius et al., J. Acoust. Soc. Am. 120, 1315-1323 (2006)]. Here the cross-correlation approach is applied to the location in range and bearing of a point target. An experiment was designed using floats and weights mounted (and dismounted) on the same cable as the vertical array. Careful measurements were made of the location of all likely floats, ballast weights, array terminations, and so on. After suitable coherent averaging, peaks were seen at delays (correlation offsets) agreeing with the reflector positions and were shown to be absent when reflectors were removed. A trivial extension of the theory developed in Harrison and Siderius [J. Acoust. Soc. Am. 123, 1282-1296 (2008)] is used to explain the rough amplitudes of the reflections. The approach differs from "acoustic daylight" principally in having a capability to determine a target range.
Harrison
Christopher H
CH
NATO Undersea Research Centre, Viale San Bartolomeo, 400, 19126 La Spezia, Italy. harrison@nurc.nato.int