Chirp sub-bottom profiler for TOBI, a deep ocean survey vehicle

Abstract

The implementation of chirp sonar represents a recent upgrade step in the profiler system of SOC's deep ocean survey vehicle TOBI (Towed Ocean Bottom Instrument), a multi-sensor platform used for deep ocean scientific and commercial research and mapping. The existing profiler system consists of a seven transducer array of 110 mm diameter free-flooded rings for operation to full ocean depths. In addition novel slow waveguides on each transducer are employed to modify the acoustic directivity pattern of the individual elements. It transmits a single frequency (7.5 kHz), 500 W pulse 0.26 ms long, giving a range of up to 70 m penetration over soft sediment. Although this gives a theoretical vertical resolution of 0.2 m, ringing significantly degrades this performance. The use of a long chirp pulse reduces the effect of ringing whilst, with the use of a suitable matched filter, preserving the original resolution. Previous implementations of chirp sub-bottom profilers have relied on ship-board signal processing. This is not ideal in the TOBI case as the electrical characteristics of the 10 km tow cable can detrimentally modify any raw signals. Hence, all signal processing electronics, pulse profiles and matched filtering in this new design are located within the TOBI vehicle putting tight constraints on the size, power requirements and cable bandwidth available to the system. The use of quadrature sampling and a hardware matched filter make for a small, low cost, low power design which uniquely combines the processing performance of a ship-based system with the lower acoustic noise and smaller acoustic footprint of a deep-towed system.