A system has been developed to aid in the determination of the position of marine seismic streamers. The system employs optically scanned magnetic sensors to provide magnetic heading at the point of installation in the streamer. The results of field operations indicate that the system performs satisfactorily in all conditions. Details have been provided on discrete components of the system as well as information on the installation of streamer mounted units.
From the beginning of offshore geophysical exploration to present, there has been a need for a system to more precisely locate the position of the streamer. Until recently, however, the requirement has not been sufficient to cause the development of a system with such capabilities. Since the introduction of three dimensional marine exploration techniques, there has developed such a requirement.
There have been attempts in the past to establish the position of the streamer. Earliest techniques consisted of simply towing the streamer in a straight line for a period of time considered to be sufficient to have the streamer fall in line astern of the towing vessel. An adaptation of this technique consists of attaching a tail buoy on the end of the streamer and either using visual or electronic methods to establish the location of the tail buoy. Information obtained on the position of the tail buoy indicated the position of the end of the streamer. Using this information and the assumption that the streamer will tow astern given a sufficiently long period of constant course, a basic understanding of the shape or position of the streamer can be obtained.
As the requirement developed for more accurate location of the streamer and hydrophone arrays, several devices were designed and tested under field conditions. These devices were either gyroscopic or magnetic and were designed to be placed in the streamer and provide heading of the streamer at that location. As can be understood, a gyrocompass would be the ideal solution to the problem of heading reference. It would not present the inherent problems present in a magnetic sensor of deviation caused by magnetic material in the vicinity of the sensor and variation caused by changes in the Earth's field as a function of Latitude. A gyrocompass would not experience any of these errors and would reference to true North. However, difficulties arise in the application of a gyrocompass to a marine streamer. Streamer environment cannot be considered the most suitable for satisfactory operation of a gyrocompass. The device would see temperature variations, rough handling, and perhaps the most critical, a long length of small wire for power transmission. A gyroscopic device was manufactured by a prominent gyrocompass manufacturer for installation in a streamer. The device was approximately 4 inches in diameter and approximately 3 feet long. It required 75 watts of 400 cycle power for operation. In the one field test that the author is aware of, the device developed operating problems.
