Comparison of Buoy-Mounted and Bottom-Moored ADCP Performance at Gray’s Reef

Abstract

Simultaneous ADCP profile measurements are compared over a 2-month period in late 2003. One set of measurements comes from a National Data Buoy Center (NDBC) buoy-mounted ADCP, the other from a bottom-mounted, upward-looking ADCP moored roughly 500 m from the buoy. The study was undertaken to evaluate the proficiency of an experimental configuration by NDBC; unfortunately, the ADCP was not optimally configured. The higher temporally and vertically resolved bottom-mounted ADCP data are interpolated in time and depth to match the buoy-mounted ADCP measurements. It is found that the two ADCP measurements are significantly different. The buoy-mounted measurements are affected by highfrequency (Ͻ10 h period) noise that is vertically coherent throughout the profiles. This noise results in autospectra that are essentially white, unlike the classic red spectra formed from the bottom-mounted ADCP observations. The spectra imply a practical noise floor of 0.045 m s Ϫ1 for the buoy-mounted system. Contamination by surface waves is the likely cause of this problem. At tidal frequencies the buoy-mounted system underestimates major axis tidal current magnitude by 10%-40%; interference from the buoy chain and/or fish or plankton are considered the most likely cause of the bias. The subtidal velocity field (periods greater than 40 h) is only partially captured; the correlation coefficient for the east-west current is 0.49 and for the north-south current is 0.64.

Environmental interference factors affecting detection range in acoustic telemetry studies using fixed receiver arrays

“…In a study conducted in isothermal and stratified waters 17 m deep off the coast of South Africa, Singh et al (2009) recorded tag detections using Vemco VR2 receivers at a distance of up to 450 m when there was no stratification and only 300 m when the water column was stratified. It is important to note that a strong inverse relationship between detections and stratification was observed in our study despite the lack of detailed stratification data based on temperature, density, or salinity profiles and the use of current data for stratification indices from a buoy-mounted surface ADCP, which provides relatively poor current data compared with bottom-moored ADCPs (Seim & Edwards 2007). Alternatively, ambient noise levels could alter sound transmission (van Walree et al 2007, Payne et al 2010 and have been observed to be loudest in the summer and quietest in the winter (Radford et al 2008), but no data were available to evaluate this hypothesis in our study.…”

Velocity Comparisons from Upward and Downward Acoustic Doppler Current Profilers on the West Florida Shelf

“…The possibility of contamination by buoy-induced motions has raised concerns over the accuracy of currents observed from a downward-looking, surface buoy-mounted ADCP. Complementing the investigations by Winant et al (1994) and Seim and Edwards (2007), this study compares ADCP observations from bottom-mounted and surface buoy bridle-mounted configurations using data from five moorings on the West Florida Shelf (WFS; Fig. 1).…”

“…In the case of a surface buoy, an ADCP may be mounted within a cage suspended below the buoy (Irish et al 1992;Seim and Edwards 2007), within the buoy bridle (Weisberg et al 1996), or within a well in the buoy hull (Winant et al 1994). The possibility of contamination by buoy-induced motions has raised concerns over the accuracy of currents observed from a downward-looking, surface buoy-mounted ADCP.…”

In Situ Observations and Satellite Remote Sensing in SEACOOS: Program Development and Lessons Learned

“…For the COMPS buoys, Mayer et al (2007) found no substantive differences for currents measured from the buoys and bottommounted instruments except in the upper few meters of the water column. However, from inter-comparison of current measurements obtained with a bottom-mounted ADCP and an ADCP mounted on a shallow-water NDBC buoy off Georgia (Buoy 41008, ~16 m depth), Seim and Edwards (2007) concluded that the instrument configuration on the NDBC buoy strongly biased the current estimates throughout the water column, likely due to surface wave contamination. The configuration was subsequently modified, and the data quality from the NDBC buoy is improved, although a thorough re-examination has not yet been conducted.…”