Acoustic Telemetry - A Step Toward an Underwater Alternative
Authors: Witmer Daniel R. 
Published: December 2007

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Introduction

Acoustic systems contribute effectively to offshore operations, as has been demonstrated in exploratory drilling and deep-sea mining. Acoustic position indication, control of positioning beacons, and blowout preventer control are some established uses. As subsea oil production moves farther offshore. long-range acoustic systems are required to provide control and for telemetering at ranges up to 5 miles from a production control center. Present needs are served by an electrical cable laid along Present needs are served by an electrical cable laid along the flowline. An acoustic alternative probably would be used first as a backup in case of cable damage. If the need for a power cable is eliminated by development of wellhead-based power sources, an acoustic link could become the best means of communication. The primary benefit would be a substantial reduction in initial cost by eliminating electrical cable installation.Most basic communications technology and equipment for long-range acoustic telemetry in deep water have been successful. In shallow water, however, the task is more complex. The two greatest difficulties in achieving an acoustic link at long ranges in relatively shallow water aresignal fading because of destructive cancellation by multiple acoustic paths resulting from refraction and surface and bottom reflection, andray bending of the primary path because of water-temperature variation.

Microstructure variability of propagation paths makes it almost impossible to predict theoretically fading characteristics. This variability is caused by such phenomena as internal waves in the thermocline and phenomena as internal waves in the thermocline and heaving of the sea surface. Even a statistical description of fading characteristics is difficult because of the lack of an adequate body of in-situ data, particularly at transmission frequencies useful for offshore applications. Our program tried experimentally to seek limits for important program tried experimentally to seek limits for important environmental parameters and attempted to design a system that could operate within those limits.Our first objective was to demonstrate a generic system type that would provide command link capability at a range of more than 3 nautical miles. Our second objective was to acquire data on the limitations placed on system performance by the following environmental parameters performance by the following environmental parameters - water temperature profile, frequency dispersion, time dispersion, propagation losses of various paths, fading rate, and amplitude distribution of received signal envelope.

Choice of Generic System Type

We distinguish between the following elements of a total system: coding, and fundamental means for transferring the smallest amount of information.In this program, maximum emphasis was placed on Item 2, which includes the type of modulation. This was done to separate more easily the effects of the various medium characteristics. Being locked into a particular coding scheme for all test data would have made this difficult. In addition. a design approach that treats coding as a separate module is more adaptable for providing coding and telemetry formatting tailored to the user.

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