Bottom Currents Measured with a Pulse-to-Pulse Coherent Sonar

The performance of a High Resolution Current Profiler (HRCP) using pulse-to-pulse coherent Doppler technique rather than conventional incoherent Doppler (ADCP) technique is discussed. The HRCP (300 KHz) was deployed on the bottom of the North Sea (at about 100 m depth) looking upwards. The sensor head was deployed separately form the electronics and communication was achieved by cable. The main frame holds the HRCP electronics, the Data Acquisition System, an ADCP and a CTD. The sampling rate of the HRCP was set to 4 Hz and data were collected for about 50 hours in 50 cells each 20 cm long. The HRCP gave satisfactory results with horizontal velocities of short term (order 1 s) precision of 0.5 cm/s or better. Most of the problems with HRCPs seem to be linked to the existence of high backscattering layers in the ocean reducing the harvest of good data to about 75%. Shear velocities in the range 0.4 to 0.9 cm/s and roughness lengths of order 1 cm were found. These values are comparable to those found off the northern California Shelf during CODE-1.