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www.sea-technology.com May 2015 / st 39 M easuring high-frequency physical processes in the near-surface layer of the ocean—such as surface and internal waves, or turbulence—is a requirement for an in- creasing number of offshore activities. For example, oil and gas exploration projects need information about the surface waves and currents, while hydrokinetic turbine designers want to understand the turbulence within the fow. We focus here on the use of acoustic Doppler current proflers (AD- CPs) to obtain these observations. In shallow water, say less than 50 meters, wave measurements are relatively straight- forward since the ADCP can be attached to the seabed. But in deepwater and the absence of fxed structures, the only option to acquire these data requires the use of either surface or subsurface moorings. Many of our projects have required surface wave measurements in remote regions where vessel traffc and local fshing activities interfere with the long-term survival of surface buoys. So, we prefer "out-of-sight, out- of-mind" mooring techniques that position the ADCPs on a subsurface buoy. In either case, however, buoy motion can be signifcant and must be separated from the measured (appar- ent) velocity of the fuid. Our initial attempt to measure waves from a subsurface buoy employed upward- and downward-looking ADCPs. The downward- looking ADCP was used to correct the upward- looking wave velocities under the assumption that high-frequency signals in the deepest range bins were entirely due to buoy motion (similar to using bottom-track or reference- layer measurements to correct vessel-mounted ADCP data). While this approach has certain advantages, an alternative, low-cost method of directly measuring the buoy motion was desired in order to eliminate the need for the downward-looking ADCP. Motivated by this, we developed an iner- tial motion-sensing package (IMU) capable of measuring platform motions with suffcient accuracy to correct the upward-looking ADCP measurements. We have deployed the IMU in two separate projects, in both cases collecting long-term, high-quality data sets. The Deployments The frst deployment was located offshore West Papua, Indonesia, in 1,400 meters of water beginning December 2012. The wave climate at this site was mild, consisting gen- erally of 3-to-5-second wind waves, but there also was a relatively strong semidiurnal tidal current with a peak speed of about 1 meter per second (2 knots). The second deploy- ment took place off West Africa (in the Gulf of Guinea) at the shelf break in 90 meters water depth from July to Sep- tember 2014. Wave conditions at this location consisted of wind waves atop long-period swell running cross-shelf, as well as strong subtidal currents of 0.5 to 1.25 meters per second running parallel to the shelf. The subsurface foat in each deployment was a 49-inch syntactic foam sphere po- Using Low-Cost Inertial Sensors to Measure Subsurface Mooring Motions IMU Corrects Upward-Looking ADCP Measurements By Jon D. Wood • J. Anthony Schanzle • Dr. Eugene A. Terray Syntactic buoy after recovery offshore West Papua, Indonesia, showing the upward- and downward-looking ADCP confgura- tion. The IMU and additional battery packs were fxed inside the buoy well.