Theoretical Improvements When Using the Second Harmonic Signal in Acoustic Doppler Current Profilers

Acoustic Doppler current profilers and velocity logs are devices that compute the Doppler frequency shift undergone by a pulse after reflection by floating particles in water or by the seafloor. Using this Doppler shift, the velocity of the water currents carrying the reflecting particles or the speed of a vessel relative to the sea bottom can be estimated. The attainable performance of Doppler logs in terms of range and velocity estimate error are directly linked to the physical dimensions and geometry of the transceivers as well as the nature of the pulse transmitted. Beyond a certain transmitted power, distortion of the transmitted pulse due to nonlinear effects is significant. The second harmonic signal generated in that case can be used to estimate velocity in conjunction with the fundamental signal. It has a narrower mainlobe and a higher mainlobe-to-sidelobe ratio compared to the fundamental signal. Such geometrical properties contribute to a more localized velocity determination with less perturbations coming from scatterers away from the region of interest. Combining the velocity estimates using the fundamental and second harmonic signals also helps decreasing the velocity estimate error. For a Doppler log transmitting a pulse at 153.6 kHz using 250-W input power with a nominal range of 400 m, the attainable range for the second harmonic signal is estimated to around 221 m.