A vertical slice of acoustic intensity and velocity [2-D images of waves and turbulence]

Phased Array Doppler Sonars (PADS) have been developed at Scripps over the past 20 years. These provide sector-scan images of radial velocity and acoustic intensity over ranges of 0.1 m (medical ultra sound) to 1.5 km (ocean surface wave measurement). Most recently we´ve deployed a 200 kHz PADS from the R/V Revelle during EquatorMix (October 2012, at 140W on the Equator). The pie-shaped measurement area was oriented to probe a vertical slice under the ship parallel to the direction of travel, so that moving averages along the plane can be formed. At the surface, our speed over ground was near 1 m/s, while near 120 m depth, the core of the Equatorial Undercurrent exceeded our speed by about 0.5 m/s; i.e., the undercurrent was moving at 1.5 m/s relative to the surface, which was nearly stationary in an Earth-referenced frame. The objective was to image descending shear-layers that often occur in the early morning before sunrise, along with any associated turbulence or internal waves. Averages can be formed moving with the flow as a function of depth, yielding focused images of acoustic backscatter strength, Doppler bandwidth, and both sine- and cosine- weighted velocity estimates. Where conditions are favorable, the last pair can be recombined to yield horizontal and vertical velocities in some detail.