Measurement of deepwater ocean waves from a subsurface mooring

We measured wave height and directional spectra in the open ocean from a subsurface mooring in 1400 meters water depth. Six months of data were recovered successfully at a site south of Australia where high-energy waves (with periods greater than 10 seconds) dominated the spectrum. Nondirectional wave height spectra were estimated from measurements of velocities made with an upward-looking ADCP mounted at the top of the mooring. Additional estimates of the nondirectional spectrum were obtained from a pressure sensor inside the ADCP, and from echo-location of the surface derived from the acoustic backscatter intensities measured by the ADCP. Mooring motion was removed using measurements from a second downward-looking ADCP mounted on the bottom of the float. Wave direction was determined from the velocities measured by the upward-looking ADCP using a variant of single point UVW processing, which permitted heading variations to be compensated continuously. We verified that the resulting horizontal and vertical velocities within the frequency bands containing appreciable wave energy are in quadrature, and that their coherence is close to unity. This paper describes the technique and assesses the quality of the resulting wave spectra and moments.