Measurement of directional wave spectrum with a modular acoustic velocity sensor

Abstract

Directional wave spectra, valuable to describe the sea-state, can be computed as the correlation of horizontal velocity with pressure measured at a single point. Pressure was used instead of the vertical component of velocity for the correlation since the deployment height of 70 cm above the bottom attenuated the vertical wave velocity component. A Modular Acoustic Velocity Sensor (MAVS) with integral pressure sensor was deployed on the bottom in 10 meters depth in Buzzards Bay, Massachusetts during February 2000 in conjunction with the ASIS Multi-Spar Buoy, providing surface wave measurements, and other wave sensing instruments. MAVS was attached to a bracket on the corner of a sled resting on the bottom that contained a fan beam Acoustic Doppler Current Profiler (ADCP) for measurement of directional wave spectra from beneath the surface. The purpose of the experiment was to observe fetch-limited waves. The deployment site, near Nye Ledge, is about 7 km from shore in all directions except southwest. Waves with periods ranging from 5 to 12 seconds were measured propagating north to northeast when strong winds blew from the southwest. Velocities and pressure were sampled at 1.4 Hz for 14 minutes every 4 hours. The observations of velocity were corrected for tilt and compass orientation of the sensor and stored as Cartesian components of velocity along with digitized output of a 20 decibar Omega pressure sensor. Although the deployment of the instrument was 25 days, the data record from MAVS was limited to 15 days by battery and data capacity. This technique of directional wave spectral measurement from a simple vector current meter with pressure sensor demonstrates a capability that can be used in moored arrays for wave refraction studies over topography or for shoaling waves and fetch limited waves as a function of distance from the shore. The low cost of the instrument used in this experiment permits such an array to be deployed with moderate cost.