| 摘要 |
Apparatus for measuring magnetic fields while drilling includes an electrode located adjacent or in contact with a target well casing to produce an electrical current. The current including a fundamental wave component having two opposite zero crossings per cycle and an even harmonic wave component having two opposite zero crossings per cycle produces an alternating current magnetic field in the earth surrounding the target well. The target well is to be intercepted by, or to be avoided by, a relief well being drilled. A sensitive magnetometer is located within a nonmagnetic portion of the drill string within the relief well to detect the alternating magnetic field generated by the current flow in the target and to produce unambiguous output signals corresponding to the detected field and determining the distance and direction of the target well from the relief well the determination being made in synchronization with the fundamental wave component to eliminate phase ambiguity in the determination of the direction.
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