Performance of the 75 klz Long Ranger ADCP in a Low Scattering Environment

An intercomparison study in June-November 2006 assessed the performance of the Teledyne RDI 75 kHz Long Ranger ADCP (LR-75) in deep, low-scattering waters of the Gulf of Mexico. Two LR-75s and a 300 kHz Workhorse Sentinel ADCP (WHS-300) were deployed by ROV for three 1-month intervals in bottom tripods at 2000 m depth approximately 65 m away from the wellhead of an oil exploration rig. The ADCPs were oriented so that none of their acoustic beams intersected the rig\´s vertical riser. Single-ping ensembles were collected every 30 seconds in beam coordinates. The LR-75s were operated in Wideband and Narrowband mode, and with 32 m and 16 m bin lengths. Intercomparison current meters deployed on a 500 m tall subsurface mooring about 1 km away included 5 Nortek AquaDopps, an Aanderaa RCM-11, and a 1200 kHz ADCP. Most of the instruments worked and gave apparently valid data, although significant differences in instrument performance were observed. Currents were nearly homogeneous in the vertical from approximately 100 mab to 360 mab, while frequency- dependent bottom intensification was observed below 100 mab. A method of frequency-dependent intercomparison based on low-pass-filtered velocities was developed to improve the consistency of analysis results. This was necessary because the low current speeds (typically <10 cm/s) were near the noise threshold of the instruments, and because the vertical current shear was frequency-dependent. The density of acoustic scatterers was inferred from the AquaDopp current meters to be only slightly higher than that of deep Sargasso Sea water, and decreased with distance away from the sea floor. Despite the low density of scatterers, the LR-75 ADCPs gave apparently valid data out to a range of more than 350 m above bottom, as evidenced by a gradient of Echo Amplitude and high Correlation Magnitude. However, LR-75 velocities were found to be significantly low-biased in all bins, out to the farthest measured range. The speed deficit - ranged from nearly 100% in bins close to the ADCP, to more than 10% in bins several hundred meters away. The low bias error is inferred to be due to backscattering or reverberation of side lobe signals from the riser. The acoustic beam pattern of the 4-beam LR-75 is such that side lobe energy may be returned from a nearby vertical object, such as a riser, over many bins. The side lobe reflections or reverberations, although weak, are correlated with the transmitted signal but have zero Doppler shift. When such correlated "noise" is mixed with the weak backscatter signal from scatterers in the main beams, the ADCP reports a low- biased speed. Temporary removal of the oil rig\´s riser provided an opportunity to directly confirm the importance of side lobe interference. In the absence of the riser,LR-75 velocities were not low-biased compared to a reference current meter at comparable depth, whereas they were significantly low-biased immediately prior to removal of the riser. These results suggest that users of the LR-75 in a deep, low scattering environment where vertical non-moving objects are present should carefully consider these error sources and utilize ADCP beam configurations (such as disabling one beam) that minimize the strength of side lobe echoes in the other three beams and permit unbiased current measurements.