Space-time acoustic scintillation analysis: A new technique for probing ocean flows

We present a new approach to the measurement of ocean flows. The technique exploits the coherence of the fine structure in the ocean under the influence of advection. Sound passing through this fine structure is modulated in space and time, so that the evolution and motion of the resulting pattern at a distant receiving plane contains information about the intervening flow field. The details of the fine-scale structure itself may also be recovered, to an extent determined by the complexity of the transmitter and receiver array. Two special cases of oceanographic interest are considered. First, a fully developed turbulent flow, such as that encountered in tidal channels, for which the scale of fine structure contributing to the scintillation field lies within the inertial subrange, and second, the internal wave field more generally applicable to the open ocean. We describe an experimental test of the concept. Sound traveling across a 0.66-km path in Cordova Channel, British Columbia, Canada, is detected by two closely spaced receivers. The flow speed is derived using three separate estimators and the results compared with current measurements obtained from a moored current meter. Agreement between the two types of measurement is excellent.