ABSTRACT

The theoretical basis for the search principle of submarine cables by magnetic techniques is explained. The magnetic search technique using the proton magneto meter is introduced as a highly accurate search method applicable to the detection of cable locations and fault points.

The development of the search system hardware and the procedure of search operation are described. The successful results of the application of finding unarmored coaxial cables buried to a depth of about 70 ~ 90 cm into the seabed at a sea depth of about 70 m and armored cables laid at some 50 m sea depth are discussed with actual data.

Furthermore, the results of simulation in the automatic detection of cables by analyzing actual magnetic field data using pattern recognition techniques are described.

INTRODUCTION

Lately more and more submarine cable systems are being buried under the seabed to a depth of about 70 cm in shallow sea areas of less than 200 m to protect the system from being damaged by fishing gears and ship anchors. However submarine cables buried below the seabed are seldom damaged. Although communication sub marine cable systems have a design life of 20 years or more and a very high reliability, but they can, though very rarely get out of order. If such trouble occurs, the cable system must be repaired as soon as possible in order to minimize the interruption time of communications.

Since the cable search operation for locating the cable route and localization of the fault point has a considerable influence on the subsequent repair operation, its accuracy and speed are considered important. When repairing a buried cable in particular, both sides of the fault point must be dug out and recovered on board. If an attempt is made to dig out the cable by hooking with a grapnel at a point too close to the fault point, the cable may unhook itself from the grapnel and fall. Conversely, if the cable is dug out at a point too far away from the fault point, the length of dug-out section becomes too great resulting in increased risk of cable breakage and fault. As a matter of course, it is reasonable to make the repaired section should be kept short as much as possible. The detection of the fault point should therefore be as accurate as possible.

Besides the localization of communication submarine cables for repair, cable localization is necessary with recent expanding development of offshore oil fields since the oil rigs and oil platforms are sometimes being constructed near communication submarine cable lines and power cable lines. Before starting such construction, lengthy search operations are required to accurately locate the cable position so as to avoid adversely affecting the existing cables.

As mentioned above, the necessity of accurately locating the position of existing submarine cables is increasing year by year. As a result, various cable search systems with different characteristics have been developed.

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