Wave height measurements using acoustic surface tracking

TLDR: In this article, a vertical, acoustic beam that detects the surface is used to measure the orbital velocities close to the surface and incorporating the Maximum Likelihood Method (MLM) estimate technique (Krogstad et al., 1988).

Abstract: Nortek has improved upon its AWAC, a current and wave measurement sensor package, by introducing a vertical, acoustic beam that detects the surface. This added functionality allows for directly measuring waves as opposed to inferring wave estimates from wave energy spectra. Traditionally, wave measurements from bottom-mounted instruments, such as the combined pressure-velocity (PUV) approach, are limited in their frequency response. This is due to attenuation of the surface signal with increasing depth. Recent advances employ the alternative solution of measuring orbital velocities close to the surface and incorporating the Maximum Likelihood Method (MLM) estimate technique (Krogstad et al., 1988). This improves the accuracy at higher frequencies. However, for deployment depths of 10 meters or deeper, these methods cannot resolve waves periods that are 3 seconds or shorter. Moreover, these bottom-mounted systems do not measure the real surface time series, which makes it difficult to calculate extreme value statistics. The following paper provides an overview of (1) the process of developing the surface track algorithms, (2) comparing with Datawell wave buoys off the coasts of Carqueiranne, France and Gabbard, UK (3) and finally we show how the same technology has been transferred from a 1 MHz to a 600 kHz AWAC to achieve similar accuracy and resolution at depths of 60 meters.