This paper describes the design and actual sea test performance of the telemetry subsystem developed for the Naval Ocean Research and Development Activity Deep Towed Array Geophysical System. The telemetry equipment includes a 27 channel floating point A/D Converter, cable interface circuitry to equalize a 7,000 meter tow cable at 1.5 megabits per second (MBPS) transmission rate, and an FSK downlink control system.
The telemetry subsystem provides full duplex communication between the surface ship and a towed geophysical array used for bottom mapping in the DTAGS project. Additional applications include the collection of other types of geophysical data. Since the data is digitized at the source, transmission over very long cables with no additional transmission noise is possible using properly designed cable equalization.
The DTAGS telemetry system provides essentially error free digital transmission over the 23,000 foot tow cable. Low equivalent input noise and good channel-to-channel gain accuracy were obtained.
The laboratory and sea tests of the DTAGS telemetry system establish the usefulness of multi channel digitization of analog data in offshore geophysical data collection. The performance of the 1.5 MBPS digital uplink over a 23,000 foot cable validates the computer assisted design procedures developed for such engineering problems, and in9icates that transmission over significantly longer cables at equal or greater rates may be achieved.
The Naval Ocean Research and Development Activity (NORDA) has undertaken the development of a seismic system towed near the deep ocean bottom to determine the detailed geophysical character of the sea floor and the upper sub-bottom structure. The system, designated as the Deep Tow Array Geophysical System (DTAGS), is designed to be towed at an altitude of 100 m at full ocean depth (6000 m) with a sub-bottom penetration of 500 m. The system as depicted in Figure 1 consists of a sound source, multi-channel streamer, and telemetry system compatible with operation in the deep ocean environment. The system will provide marine scientists with high resolution data to describe the geologic, geophysical, and geoacoustic character of the deep ocean sea floor and its upper subbottom structure.
The DTAGS system operates over an acoustic band of 260 to 650 Hz with a peak source level of 201 dB/1?Pa @ 1 m. The source system, a Helmholtz resonator transducer and power amplifier system, has been tested to a depth of 4500 m. The streamer is an oil-filled hose design with 48 active groups spaced over its 1000 m length. The telemetry system is a duplex-digital configuration capable of communicating over the 7000 m coaxial steel tow cable. The uplink system operates at 1.5 M bits/sec. and the downlink command system at 10 bits/sec. These systems have been tested to a depth of 4500 m during the summer of 1983. Acquired future data will be recorded digitally in an SEG-D format.