Doppler Passive Fathometry

Passive fathometry is a technique whereby broadband ambient ocean noise received on an array of hydrophones is averaged and cross-correlated to produce a sub-bottom profile [Siderius, Harrison, and Porter (2006). J. Acoust. Soc. Am. 120, 1315-1323]. Here this technique is extended to determine the vertical velocity of the array, and compensate for it, without any prior knowledge, i.e., Doppler Passive Fathometry. For Fourier transform lengths beyond a certain limit, the differing Doppler between the direct and bottom reflected paths spoils the correlation match, however it is shown by using some experimental data, where the array was known to suffer from arbitrary but near periodic motion, that compensation is possible, enabling continuing time integration. In the process, the vertical velocity becomes known. Velocity, with peak value ∼±1 m/s, is plotted against time and shown to be 90° out of phase with depth, as expected for periodic motion. Since stationary targets have already been detected by noise correlation [Harrison (2008). J. Acoust. Soc. Am. 123, 1834-1837], it is implied that the range of moving targets can also be determined.