Wave-induced biases in ADCP measurements from quasi-Lagrangian platforms

Abstract

Compact autonomous marine vehicles, both surface and submersible, are now commonly used to conduct observations of ocean velocities using Acoustic Doppler Current Profilers (ADCPs). However, in the inevitable presence of surface waves, ADCP measurements conducted by these platforms are susceptible to biases stemming from wave-coherent orbital motion and platform tilting. In typical ocean conditions, the magnitude of the bias can reach tens of centimeters per second. This paper presents analytical derivation of the depth-dependent bias formulas in the small-amplitude linear wave approximation. A variety of scenarios are considered, encompassing surface and subsurface platforms, upward- and downward-looking ADCPs, free-drifting and self-propelled vehicles. The bias is shown to be a function of the wave field properties, platform response dynamics, and the ADCP configuration (particularly, orientation and beam angle). In all cases, the wave-induced biases show parametric scaling similar to that of the Stokes drift, albeit with a number of critical nuances. Analytical derivations are validated with a semi-analytical model, which can also be used to estimate the biases for more complex measurement configurations and fully nonlinear waves. Further analysis reveals unexpected fundamental differences between the upward- and downward-looking ADCP configurations, offering insights for experimental design aimed at minimizing and mitigating wave-induced biases in autonomous oceanographic observations.