Measuring current profiles on a moored buoy through an acoustic window

Abstract

The National Data Buoy Center (NDBC) operates and maintains a network of over one hundred data buoys that provide real time meteorological and oceanographie data to the National Weather Service (NWS) for use in forecast and prediction models. Traditional measurements have included wind speed and direction, barometric pressure, air temperature and humidity, sea surface temperature and waves. Initiatives through the Integrated Ocean Observing System (IOOS) funded the enhancement of these platforms to include current profiles and salinity measurements. Early efforts at integrating the Acoustic Doppler Current Profilers (ADCP) into the buoy systems through the use of an in-line mooring cage yielded intermittent performance, with the majority of the failures being attributed to mechanical abrasion and fatigue of sensor cables across the bridle-mooring interface. Additional failures resulted from severed cables due to fishing gear entanglement and similar issues. These failures resulted in loss of real-time data pending availability of a service vessel and frequently damage to the equipment. Additionally, persistent bio-fouling would often require more frequent overhauling of the ADCP. The end result was poor long-term current profile data availability. To improve operational efficiency and data availability, NDBC applied the concept of an acoustic window to the buoy platform. Acoustic windows are used effectively on research and naval vessels. The window was developed in conjunction with a simultaneous development of an NDBC Standard Buoy which was being designed to streamline the buoy design across the major programs within the organization. Initial concerns with the buoy-mounted acoustic window were the effect of near surface bubbles on data quality and depth attenuation due to the presence of the window. The prototype was developed to determine the operational trade-off between weather-dependent data availability and reduced range of the acoustic window versus the possibility of complete data loss and a vulnerable instrument. The prototype was deployed on an engineering test buoy with an RD Instruments 300 KHz Sentinel This paper evaluates these trade-offs by examining several months of hourly data near Point Conception, Calif. In particular, the effect of sea state, wind speed, concentration of backscatters and buoy motion on data quality will be explored. The effective range of the ADCP in this new configuration is determined and compared to ranges determined from past deployments and configurations. Additionally, the design details of the acoustic well will be presented and discussed, including acoustic well-specific test procedures.