Evaluating the spatial resolution of an acoustic Doppler velocimeter and the consequences for measuring near-bed flows

Abstract

The acoustic Doppler velocimeter (ADV) has recently been suggested as a promising instrument for characterizing near-bed flows, particularly in the first 10 mm above the bed where many benthic organisms live. Flow characteristics in such settings often exhibit steep vertical gradients, however, so the reliable use of the ADV requires knowledge of the size and location of the instrument's acoustic sampling volume. We describe simple procedures for quantifying the vertical size of the ADV's sampling volume and assessing its height above the bed. Our results indicate that the vertical size of the sample volume for the ADV we tested was much larger than expected based on the values predicted by software configuration. Moreover, this system incorrectly reported several distances needed to accurately position the sample volume near the bed (e.g., the transmitter-to-bed distance, the sample volume-to-bed distance, and the transmitter-to-sample volume distance). We also demonstrate that incorrect assumptions about the size and location of the sampling volume can lead to inaccurate near-bed flow measurements by comparing the time-averaged flow speed profiles generated by our ADV with those obtained using a hot-film velocimeter (HFV). At heights > 10 mm above the bed, both instruments yielded similar flow speed estimates. Closer to the bed, however, the flow speeds reported by ADV were as much as 60-80% less than those from HFV. These large errors in estimating near-bed flow speeds are a direct consequence of incorrect assumptions about the centerpoint and size of the ADV's sample volume. Specifically, when the vertical size of the sampling volume is larger than its nominal size, users may mistakenly position the ADV so that the bed is included within the sampling volume, which in turn results in the underestimation of flow speeds. By validating the size and location of the sampling volume, as well as carefully monitoring signal quality parameters, users can ensure proper placement of the ADV relative to the bed and avoid erroneous measurements.