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Remotely measuring subsurface water currents from imagery of the wave fieldhas become a much-used technique. We study the biases and errors in suchmeasurements due to spectral leakage, and suggest mitigating procedures.Deviations between peak values in the three-dimensional wave spectrum and theknown dispersion relation in quiescent water are extracted and interpreted ascurrent-induced Doppler shifts, from which the sub-surface current is inferred.The use of discrete Fourier transforms, however, introduces spectral leakagebetween nearby frequency bins. Analysing synthetically generated wave dataadhering to realistic input spectra we show that although no current is in factpresent, spurious currents can be "measured" which can amount to a significantfraction of the phase speed at the spectral peak. We analyse the effects ofdata tapering, method of Doppler shift extraction, limited wavenumber andfrequency resolution, peakedness and angular width of the input spectrum, andaverage misalignment between waves and Doppler shift velocity direction. Thenarrower the input wave spectrum in frequency and/or direction, the greater thebiases become. The use of a window function reduces the severity in nearly allcases, yet mitigates the effects of limited resolution more effectively inspace than in time. When a current is present the absolute biases remainessentially unchanged, when waves and currents are roughly aligned taperingmuch alleviates the concomitant biases, whereas in the case of a cross-current,biases remain significant even for tapered data.

Biases from spectral leakage in remote sensing of near-surface currents