Abstract: 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.

Abstract: A method and apparatus for determining the distance from a relief borehole to a target well utilizes measurements of an electric field at the target well. The electric field is produced by injecting current into the earth near the wellhead of the target well, this current producing a target current in conductive material in the target well. The electric field corresponding to the target current is measured by means of sensors in the relief borehole, and this field measurement, together with a measurement of the alternating magnetic field at the relief well due to the target current, permits determination of the distance to the target well from the relief borehole.

Abstract: A method and apparatus for determining the distance and direction of a well from a borehole. A toroidal inductor surrounds a drill stem within a borehole. The inductor acts as a primary for a transformer, with the drill stem, the earth surrounding the borehole, and electrically conductive material at the target well forming a single loop transformer secondary. An oscillator is connected to the primary to produce an induced AC current in the secondary. Secondary current flowing in the target well produces an AC secondary magnetic field which is detected at the borehole.

Abstract: A system for measuring X and Y vector components of a composite magnetic field including static portions such as those produced by the earth's magnetic field, and time varying portions such as those produced by current flowing in a target anomaly to thereby permit determination of the distance and direction from a sensor to the anomaly is disclosed. Highly sensitive fluxgate magnetometers sense total field X and Y axis vector components of the composite magnetic field of interest and produce output signals which are fed through corresponding detector and null circuits to produce total field signals corresponding to the components being measured. These total field component signals are fed through corresponding signal conditioning circuits to equalize the time varying and static field portions of the signal components.