Free descent and on bottom ADCM measurements in the Puerto Rico Trench, 19.77°N, 67.40°W

On bottom (≈2m) current velocities in the Puerto Rico Trench (≈8350m depth) were measured at 1 Hz for 75 min by acoustic-Doppler current meter at 19.75°N, 66.40°W, via untethered free-descent/ascent vehicle. The April 2008 deployment also recorded 3-axis velocity, temperature, pressure, and instrument heading, pitch, roll, and signal strength during the 153 min free-descent, and while on bottom. No data for the ascent was recorded.Signal strength was above the noise floor for the entire data set, and SNR and velocity STD were within known acceptable bounds above 7000 m. Instrument heading showed a continuous anti-clockwise rotation during descent. Doppler vertical velocity during descent is compared to the pressure time derivative and observed to exhibit extended periods of under-bias, correlated not to low SNR, but to Doppler horizontal velocity fluctuations. Doppler horizontal velocity during descent is interpreted to be tangential to rotation and includes lateral translations. Integration of horizontal velocity during descent suggests a lateral displacement of less than 30 m over the 8.35 km free-fall. Measurements made at impact indicate full functionality of the instrument at depth. Maximum horizontal velocities while on bottom varied between 1 and 5 cm/s and were directed roughly along trench axis to the W.

► We measured near-bottom hadal currents with acoustic-Doppler current meter.
► We used an untethered free-descent/ascent vehicle as the instrument platform.
► We recorded engineering data during descent that provides environmental context to future deployments of this kind.