Turbulence Measurements From Moored Acoustic Doppler Velocimeters (ADVs): Removing Mooring Motion Using Inertial Motion Sensors

High precision, long-duration, ocean turbulence velocity measurements are typically challenging to make more than a few meters below the surface or above the bottom (i.e. from a ship or from a bottom lander). Here we describe and demonstrate a method for measuring turbulence velocity timeseries from a compliant mooring. Turbulence velocity is measured using acoustic Doppler velocimeters (ADVs), and mooring motion is measured by inertial motion sensors (IMUs) tightly synchronized with the ADV. IMU measurements of mooring motion are removed from the ADV velocity measurements in post-processing to obtain velocity timeseries in the Earth's reference frame. The accuracy of these measurements is explored in two complimentary ways: 1) velocity spectral shapes are compared with theory, and 2) independent measurements of mooring motion provide bounds on measurement uncertainty. A -5/3 spectral slope at high frequency, and low low-frequency motion indicate that this methodology provides a new and reliable approach to measuring turbulence in the interior of the ocean.