Acoustical Measurement of Current and Vorticity beneath Ice

Abstract

An acoustical instrument has been developed to measure path-averaged horizontal current and vorticity in the subice boundary layer of the eastern Arctic during the spring of 1989. A triangular acoustic array of side 200 m was used to obtain reciprocal transmission measurements at 132 kHz, at 8, 10, and 20 m beneath an ice floe. Pseudorandom coding and real-time signal processing provided precise acoustic travel time and amplitude for each reciprocal path. Mean current along each acoustic path is proportional to travel-time difference between reciprocal transmissions. Horizontal velocity normal to the acoustic paths is measured using scintillation drift. The instrument measures horizontal circulation and average vorticity relative to the ice, at length scales characteristic of high-frequency internal waves in the region. The rms noise level of the measurements is less than 0.1 mm s?1 for velocity measurements and 0.01 f for vorticity, averaged over 1 min. Except near the mechanical resonance frequency of the moorings, the measurement accuracy is limited by multipath interference. Path-averaged horizontal velocity is compared to point measurements, and marked differences are observed due to local anomalies of the flow field. The integral measurement of current is particularly sensitive to the passage of internal waves that have wavelengths longer than the horizontal separation of the transducers. A comparison of horizontal velocity at two depths in the boundary layer shows good coherence at internal-wave frequencies, and some attenuation as the ice is approached. Relative vorticity at internal-wave length scales is dominated by horizontal shear caused by flow interaction with ice topography, and not by planetary vorticity.