Deep water wave measurements from subsurface buoys

We describe various methods of correcting wave measurements obtained from upward-looking acoustic Doppler current profilers (ADCPs) mounted on subsurface buoys. Subsurface buoys are forced by both horizontal currents and surface waves, and so the resulting signals also include translational and rotational motions, which must be removed to maximize the accuracy of the results. We describe our most recent experience where estimates of buoy motion were obtained from an inertial motion unit ('MU) consisting of tri-axial accelerometers, rate gyros, and magnetometers. The platform motions were validated by comparing to independent motion estimates from a colocated downward-looking ADCP. Careful synchronization of the 'MU and ADCP signals and rotating velocities into a fixed geographic reference frame allows us to subtract these motions from the upward-looking wave velocities, surface track, and pressures, and calculate wave height and directional spectra. Another critical adjustment was required for discretization errors arising from spatial changes in wave velocity between the ADCPs opposing beams, which becomes significant for higher-frequency waves. Since most of the translational movements of the buoy were in the horizontal plane (order ~50 cm/sec), with very little motion observed in the vertical plane (order ~ 5 cm/sec), corrections are more important for horizontal velocities than the vertical component. Wave height spectra derived from horizontal and vertical velocities, surface track, and pressure, were in remarkable agreement once corrections were applied. The effect of platform motion on the mean wave direction, which rely exclusively on the ratio of north and east velocities, also was small. We regard it good practice to correct for the full 3D velocity of the ADCP in order to maximize confidence in the resulting wave spectra. MEMS-based inertial sensors, of the kind used here, provide an excellent and low cost way of acquiring these data.