Doppler sonar measurements from FLIP

Abstract

A pulse-to-pulse incoherent Doppler sonar has been developed at the Marine Physical Laboratory for use in the observation of oceanic internal gravity waves. Mounted on the Research Platform FLIP, the sonar transmits at frequencies between 65 and 90 kHz at a peak power of −32 kW. During preliminary tests, measurements have been made out to a range of 1.6 km, depths of 1.2 km, with a range resolution of 25 m. The sensitivity of the sonar measurements to a given internal wave group depends on both the orientation of the sonar beam and the direction of propagation of the wave packet. Since the vertical angle of wave propagation is a function of frequency, a predictable pattern can occur in sonar derived estimates of the interal wave frequency spectrum. These can be used to infer a limited amount about the azimuthal directional properties of the wavefield. A more direct approach is to use multiple sonars, operating simultaneously but pointed in different directions. A four sonar system is currently being constructed for use on FLIP. This should permit accurate estimates of azimuthal directionality of waves up to kilometer scales.