Deriving Sub-Surface Flows from Two-Dimensional Power Spectral Density Techniques

The determination of near surface currents in wavy environments has typically been accomplished through the use of three-dimensional power spectral density techniques (3D PSD). While 3D PSD techniques have been proven to be and accurate robust methods of extracting near surface currents in wavy environments, their successful application requires, at minimum, 256 independent images of ocean waves and the resolved flow represents a temporal average over the duration of the images. A two-dimensional power spectral density technique (2D PSD), which is based on calculating the cross-spectrum between two images is developed and compared with the 3D PSD technique. In contrast to the 3D PSD technique, the 2D PSD algorithm is capable of accurately determining near surface flows from fewer images and to deeper depths. The 2D PSD technique is also capable of determining velocity time series and spectra. Results are presented from imagery collected from an unmanned aerial vehicle over the Gulf of Mexico and implications for the study of wind drift and bathymetry are discussed.