Jon Wood

Directional Wave Measurements from a Subsurface Buoy with an Acoustic Wave and Current Profiler (AWAC)

Directional wave measurements in deep water locations are intrinsically difficult to measure without the use of a surface wave buoy. Traditional acoustic Doppler current profilers do not have the appropriate data collection and processing technique to be mounted on a subsurface buoy. Nortek developed the SUV wave data collection and processing technique for measuring ocean waves from a subsurface buoy using a Nortek acoustic wave and current profiler (AWAC). In 2006 Nortek initiated a collaborative experiment to validate the SUV method and explore mooring performance by deploying two Nortek AWACs on different shape subsurface buoys offshore of Lunenburg Bay, Nova Scotia, Canada. A surface wave buoy was located nearby as an independent reference. The AWACs were deployed from September to November 2006 and measured waves over 4 m in significant wave height during three storms. The results indicate that the acoustic surface tracking (AST), used to measure non-directional wave properties, was a robust technique and worked very well with the AWACs deployed on a subsurface buoy. Greater than 99% of all AST measurements passed the quality control checks (comparable to results from a bottom mounted AWAC) and measurements of wave height and period were in excellent agreement with the surface wave buoy. The wave directional estimates were in good agreement with the surface wave buoy, but indicated clear frequency bands with increased directional uncertainty. An analysis of buoy motion suggests that the frequencies of poor directional estimates are coincident with the natural frequency of the mooring system. Guidance is offered to design a subsurface buoy which has a natural frequency outside of the wave band such that this technique may be used widely for offshore directional wave measurements.

Monitoring Suspended Sediment Plumes Using an Acoustic Doppler Current Profiler

Monitoring suspended material plumes during dredging projects or undersea construction operations is often a fundamental requirement of state and federal Section 401/404 Water Quality Certificate regulations to protect water quality and minimize potential environmental impacts. Traditional monitoring techniques typically employ in-situ turbidity monitoring and collection of water quality samples at pre-defined locations (some distance down current of the activity) and at specified times. However, sediment plumes can have complex spatial and temporal structures, specifically small length scales (order ~10s of meters) and temporal scales (order -minutes). Applying traditional sampling techniques under these conditions can result in the underestimation of suspended sediment concentrations and the collection of water quality samples that are not representative of project related impacts. Recent environmental monitoring surveys used an acoustic Doppler current profiler (ADCP) to overcome the uncertainties associated with open-water plume tracking. As a current meter, the ADCP provides real-time current data to aid in predicting the path and arrival time of a plume at a given permit compliance distance downstream. Acoustic backscatter, or the signal strength received by the transducer, provides an indicator of suspended material concentrations in the water column, allowing the ADCP to provide continuous full water depth profiles of relative suspended sediment concentrations beneath the survey vessel. This backscatter display is an essential tool to provide a high-resolution visualization of the spatial and temporal variability of the suspended material plume. As a result, the ADCP allows for improved, or targeted, sampling and in-situ data collection from a location and depth that accurately represents water quality conditions during a given project activity.

A Mooring Design for Measurement of Deep Water Ocean Waves

A recent data acquisition program obtained ocean wave measurements in high-energy wave conditions offshore of southern Australia in 1400 meters water depth. A taut-wire subsurface mooring was employed to position acoustic Doppler current profilers (ADCPs) near the surface to resolve wave height and direction across a majority of the wave spectrum. The trick to this technique was quantifying and removing buoy/sensor motion contamination from the resulting observations. Motion corrections were achieved by using tandem ADCPs configured to sample the surface wave field as well as the attenuated wave field in deeper layers, using these deeper velocities as an indicator of sensor velocity. Subsequent data analysis showed remarkable agreement between wave spectra derived from (independent) velocity, surface track, and pressure records, providing first-order confidence in the fidelity of the results. This paper provides a description of the mooring design used to obtain deepwater wave measurements and offers analysis of the buoy motion in real ocean wave conditions. Buoy motion information (resolved to 1 Hz), provided by the ADCP heading, pitch, and roll sensors combined with the downward-looking ADCP velocity data in deeper layers, was used to calculate buoy excursions and accelerations. Our analysis of the buoy motion includes both time-and frequency-domain presentation of the system response. Empirical transfer functions between the wave forcing and resultant buoy response indicates that the buoy oscillates horizontally in phase with the wave orbital motion, but that the vertical motion is constrained by the mooring line. Comparison of tilt and buoy accelerations show that the instrument tilts were small (of order 1 degree in the worst case conditions)

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.

The use of low-cost inertial sensors to improve deep water wave and current measurements from subsurface moorings

Measuring currents and waves from a subsurface mooring using ADCPs is a useful technique in regions of the world where the use of surface buoys is problematic - either because of ship traffic or theft. This paper evaluates the utility of inexpensive MEMS-based inertial sensors for measuring the platform motion. We find that these sensors are capable of providing the necessary measurements of attitude and heading, as well as the translational velocity of the platform at typical wave frequencies.