Design and performance of a self-contained, fan-beam ADCP

Recent field studies have shown the utility of acoustic Doppler current profilers (ADCPs) with fan-shaped transducer beam patterns (narrow in azimuth and broad in elevation) for the measurement of oceanic surface currents. This paper describes the design and performance of a self-contained, fan-beam ADCP consisting of RD Instruments 300 kHz Broad-Band electronics mated to a specially designed transducer head, and intended for long-term deployment on conventional oceanographic moorings. The instrument operated successfully during two field deployments, providing horizontal profiles of near-surface velocity with precision of about 1 cm/s and resolution of about 5 m. Smearing of the sample volume due to tilt and heading variability was small. Profiles were obtained once per minute during 20 min "burst" samples each hour. The usable horizontal range varied with wind speed from about 120 m to 190 m. The range-mean velocity from the ADCP compared well with the velocity from a nearby current meter. Distinct convergence patterns indicative of Langmuir circulation were evident in the cross-wind velocity held during strong forcing events.