Near real-time transmission of reduced data from a moored multi-frequency sonar by low bandwidth telemetry

Abstract

Moored, internally-recording acoustic instruments can acquire continuous profiles of echoes throughout the water column, thus providing a low-cost method of studying the behavior and abundance of fish and zooplankton in oceans and lakes. Calibrated sonars with several frequencies allow some information about species composition and abundance to be deduced from acoustic backscatter data. The same instrument can be configured to look up from the bottom, down from the surface, or horizontally from a CTD cage. In this presentation we describe additional capabilities of this low-power, battery-operated, multi-frequency sonar capable of autonomously collecting data at high temporal and spatial resolution for periods of up to a year. The AZFP instrument (Acoustic Zooplankton and Fish Profiler) supports up to four frequencies in a single housing. The available operating frequencies are 38, 67.5, 125, 200, 455, 769 and 2000 kHz. The transducers are co-located, with the same nominal beam widths of 7° or 8°, except at 38 and 67.5 kHz, where the beam width is 12°. The standard AZFP can be moored at depths up to 300m, and with modified transducers as deep as 600m.