Acoustic Telemetry - An Underwater Alternative
Authors: Witmer Daniel R. , Pearson Roy E. 
Published: October 2008

This paper describes an experimental acoustic investigation program conducted during 1976 and sponsored by Gulf Research and Development Co. of Houston, Texas, to determine the most effective and reliable means of communicating at long range in shallow water. An example of the use of such a system would be remote control and monitoring of subsea wellheads. A key feature of this program was the flexibility of system parameter values that was built into the test hardware. This allowed system design tradeoffs to be performed in real time. Additional benefits were the ability to determine the limiting effects of environmental characteristics on system performance. Results included the demonstration of a system based on frequency-shift keying with frequency diversity. This system was shown to be effective in operating under conditions of severe multipath fading and ray bending in depths of 200 to 400 feet at ranges of 2 to 3 nautical miles.

INTRODUCTION

The capability of acoustic systems to contribute to the effectiveness of offshore operations has been demonstrated in exploratory drilling and in deepsea mining: acoustic position indication, control of positioning beacons, and BOP control are some of the established uses. As subsea oil production moves further offshore, there is a developing requirement for long-range acoustic systems which can provide control and telemetering at ranges up to 5 miles from a production control center. Present needs are served by an electrical cable laid along the flow line. Initial use of an acoustic alternative would probably be as a backup to possible cable damage. If the need for a power cable is eliminated by the development of wellhead-based power sources, the acoustic link could become the primary means of communication. The primary benefit, of course, would be a substantial reduction in initial cost due to elimination of the electrical cable installation.

Much of the basic communications technology and equipment for long-range acoustic telemetry in deep water have been proven. 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 due to destructive cancellation by multiple acoustic paths resulting from refraction and surface and bottom reflectionRay bending due to water-temperature variation

An example of both effects is shown in Figure 1. The variability of the microstructure of the propagation paths makes it almost impossible to theoretically predict fading characteristics. This variability is due to such phenomena as internal waves in the thermocline and heaving of the sea surface. Even a statistical description of fading characteristics is very difficult, because of the lack of an adequate body of in situ data; particularly at transmission frequencies useful for offshore applications. The approach selected in the program was, therefore, to experimentally seek limits for the important environmental parameters and attempt to design a system that is robust enough to operate within those limits.