A multi-frequency echo sounder for use within a towed undulating vehicle to study oceanic zooplankton abundance

A compact multi-frequency sonar operating over 250 kHz to 2 MHz has been developed that fits to a towed undulating vehicle. This combines the ability of high-frequency acoustics to sample distributions of zooplankton size ranges <1 mm to >1 cm with the surface to 500 m depth coverage of a towed vehicle. The transducer array incorporated seven broad-band elements, with novel features to increase bandwidth and reduce extra attenuation. A minimum of circuitry was placed in the underwater unit, all of the signal processing taking place using commercial off-the-shelf hardware onboard the ship. Initial results from the Gulf of Oman illustrate frequency dependent backscatter from small zooplankton and also from occasional single targets such as larger zooplankton, micronekton and fish.