| ADCPs measure water motion by transmitting sound at fixed frequency. The instrument measures the Doppler-shifted echoes backscattered from scatterers in the water column (e.g., plankton and sediment) and converts the echoes to along (acoustic-) beam velocity components. The ADCP then converts the along beam velocities to north/south, east/west, and vertical velocity components. Velocity profiles are determined by range gating the echoes so that velocities are determined at preset distances along the acoustic path (called bins).
The instrument used in this application, the RD Instruments Workhorse Mariner, is specifically designed for underway current profiling from inshore hydrographic survey vessels. The head of the transducer, which is made of molded composite plastic consists of four beams at 20° from vertical in a convex configuration. The electronics of the instrument also integrate temperature sensors, a fluxgate compass and pitch and roll sensors. It works at a nominal frequency of 600kHz to give a water velocity profile to depths of 70 meters with a vertical resolution of about 4 meters. The ADCP is equipped for bottom tracking capability to measure the ADCP speed and direction over ground. It operates under temperatures ranging from –5°C to 45°C to measure velocities in a range between 5mm/s and 20m/s with an accuracy of 2.5 mm/s.
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