Interpretation of acoustic backscatter measurements using a cross-correlation technique to evaluate near-bed current velocities

High resolution acoustic backscatter systems have significantly advanced the measurement of suspended sediments in the marine environment. Such systems have proved to have important advantages over other techniques when determining the fine scale temporal and spatial variations in the local suspension field. If two or more transducers are used, then independent observations of suspension parameters can be determined. Provided there is sufficient spatial coherence in the suspension field between each transducer, a cross-correlation approach can be used to determine horizontal flow velocities. The parameters can then be combined to estimate vertical profiles of horizontal flux, which is a necessary requirement for developing sediment transport models. In this study, measurements were used from a field experiment, where the flow was dominated by turbulent rectilinear currents. A pair of 2 MHz transducers pointing vertically down were aligned with the main horizontal flow direction, and profiles of the backscattered signal strength from sediments in suspension were recorded. The two backscatter profile measurements were compared using a cross-correlation technique, and estimates of the current velocity profile were evaluated. The velocity measurements obtained from this new procedure have been compared with other methods for determining horizontal flow, and the results from these observations are discussed.