Phased-array Doppler sonar measurements at the equator: Currents or swimming?

Abstract

A 64-channel 200 kHz phased array Doppler sonar (PADS) was deployed on the Equator at 140 W, sampling a vertical slice of the ocean, from 9 m to 200 m depth by 100 degrees, twice per second. The instrument was operated for two nearly continuous time-series, Oct. 10–20 and Oct.24 to Nov. 3, 2012. While the PADS was operated off the Starboard side of the R/V Revelle, a “Fast-CTD” (FCTD) was simultaneously operated off the port stern, sampling to 250 m every 2.5 min. Headway was maintained at less than 1 m/s relative to the surface flow (which was small, in contrast to the ~1.5 m/s undercurrent at 100 to 130 m depth). Because of this slow headway, motile scatterers were able to develop ship-centric swimming patterns, particularly at night. This, in turn, introduced some subtly non-physical characteristics in the estimated vertical velocities in particular. Animations of the backscatter intensity reveal a variety of scales and speeds of the scatterers. Visual nighttime inspection revealed a preponderance of small squid (< 30 cm) with the occasional large predator passing rapidly through (not identified). In spite of this biological interference, reasonable profiles of horizontal velocity were produced via a simple de-spiking selector.