Effects of a Thermistor String Mounted between the Acoustic Beams of an Acoustic Doppler Current Profiler

Abstract

A useful extension of upward-looking buoy-mounted acoustic Doppler current profilers (ADCPs), in particular for studying surface-mixed-layer dynamics from underneath, would be to combine them with a thermistor cable for obtaining simultaneous current and temperature profiles. Measurements with an ADCP, operating at 150 kHz and sampling at 2.2 m vertical bin lengths, were carried out from a barge moored in Lake San Vincente, California, over a water depth of 45 m, to investigate the effects of a thermistor string running along the axis of the instrument between the acoustic beams and held by a small auxiliary float. The biasing effect of the thermistor string itself was only of order 1%. Doppler biasing of the horizontal currents by two different kinds of Coats (one a cylindrical hard foam float with low acoustical reflectivity, the other a hollow sphere) was investigated. The floats were suspended at 15 and 30 m. For floats at 30 m, evidence was not conclusive because of apparent interference by other reflectors. The cylindrical foam float at 15 m caused no measurable Doppler biasing, while the effect of the hollow sphere was of order 40% in the depth bin around 15 m.