Possibilities and limitations of acoustic surface tracking

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 truncated 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 employ the maximum likelihood method (MLM) estimate technique (Krogstad et al., 1988). This improves the accuracy at higher frequencies. However, for deployment depths of 20 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 introduction of acoustic surface tracking (AST) with the vertical acoustic beam has permitted the AWAC to measure waves in deeper waters with greater accuracy and extended frequency response. This work provides a closer look at the frequency response of the AST and when it is permissible to use it to determine water level.