Surface-induced errors in target strength and position estimates during horizontal acoustic surveys.

•A smooth surface can cause serious problems for horizontal beaming in hydroacoustic surveys.•The problems can result in significant errors in the target size and depth estimates.•The reason to the problem is multipathing caused by side lobe effects and insufficient damping of the sound beam outside the nominal opening angle.•The phenomenon can be observed for both standard target and fish echo.

Horizontally-aligned, fixed and mobile, transducers are routinely deployed at depths of 0.5-0.75 m to survey the surface layer of waterbodies for fish. However, simulations and measurements demonstrate that a smooth surface can cause serious errors in the target strength (TS) and split-beam angular position estimates.Errors in TS up to 10 dB and depth position up to 0.5 m were observed. Simulations suggested that multi-path signal propagation interfered with the direct path of that signal. Furthermore, when a standard target (calibration copper sphere) was moved away from the transducer at fixed depths, the estimated TS and depth of the target started to oscillate as a function of range. The amplitude increased with increasing range. The frequency decreased from the oscillation start until a certain range where the oscillation stopped. The region of oscillation depended on both the transducer and target depth. Horizontal observations of known fish echoes behaved similarly. Experiments in a lake showed that the influence from the surface disappeared when the surface became rippled due to wind.