Nonlinear higher harmonics and crosstalk in broadband multi-channels echosounders

One of the prominent features of nonlinear acoustic propagation is the dependence of propaga-tion speed on the acoustic pressure amplitude. A consequence is the distortion of the transmitted acoustic signal as it propagates through the medium, and hence energy leakage from the prima-ry frequency band to harmonic frequencies. When operating several broadband echosounder channels with non-overlapping frequency bands, the generated higher harmonics from one channel may overlap and interfere with the primary frequency band of others. This interference is called crosstalk and can appear as spurious targets above and/or below the main target in pulse compressed echograms and. in addition, affect the measured backscattering frequency response of targets. Here, the nonlinear propagation of frequency-modulated acoustic waves in a directional beam are modelled and suggestions are made to reduce the crosstalk for simultane-ous operation. Efficiency of the suggested method is demonstrated by measurements on the strong echo from a flat seafloor. In addition, synchronized operation of broadband echosounder channels with reduced crosstalk is confirmed by comparing the measured and theoretically es-timated target strength of a calibration sphere.