Abstract: A logging sonde injects alternating current into the well casings of a plurality of producing wells at desired subsurface depth points. The resulting magnetic fields created around the well casings of the producing wells are detected by a magnetometer in a new well being drilled to prevent drilling into one of the production wells.

Abstract: A method of measuring hardness in steel casing to detect soft spots. A magnetizing force is generated and traversed along the length of the steel casing. A flux density is created along those portions of the steel casing traversed by the magnetizing force. The flux density is monitored to detect any change in such flux density caused by the presence of a soft spot in the hardness of the steel casing.

Abstract: An electromagnetic borehole logging tool has at least two antenna arrays positioned at spaced-apart locations along the longitudinal axis of the borehole tool. Each antenna array has at least three sub arrays spaced approximately equally around the longitudinal axis of the tool. Each sub array has at least one antenna for transmitting and receiving electromagnetic energy. The three sub arrays of the first of the two antenna arrays are respectively located at approximately the same circumferential positions around the longitudinal axis of the tool as the three sub arrays of the second of the two antenna arrays. Each of the sub arrays transmits electromagnetic energy into the formations surrounding the borehole and receives electromagnetic energy upon its return to the borehole from such formations.

Abstract: The operation of a dragline machine in digging below the earth's surface is monitored to provide measurements of the variations in pull cable length, lift cable length, and main boom inclination. From these measurements, the angle of inclination of the pull cable below the horizontal is determined. The depth of the excavation is then determined from the length of the pull cable and its inclination.

Abstract: A well casing is magnetized by traversing an internal magnetizer along and within the well casing while periodically reversing the direction of the magnetic field of the magnetizer to create a plurality of magnetic flux leakage points along the well casing.

Abstract: A first well is directionally drilled to intersect a second well. The second well is magnetized along its well casing to produce a magnetic anomaly which is sensed by a magnetometer located in the first. The drilling of the first well is redirected in accordance with the magnitude of the sensed anomaly so as to cause intersection of the first well with the second well.

Abstract: In magnetotelluric exploration, channels of measurements of the earth's electric and magnetic field are digitized. Blocks of digital samples are transformed into frequency domain representations of the power spectra, including amplitude and phase, at selected harmonics. The coherence between the power spectra from different channels is determined and compared to a threshold. The power spectra which have a coherence exceeding the threshold are stacked and recorded on magnetic tape.

Abstract: A method of magnetotelluric exploration is disclosed which features multiple measurements of the earth's electric field at a plurality of locations and one or more magnetic field measurements being made simultaneously. A specific form of cable for connecting measurement apparatus with recording apparatus is also disclosed.

Abstract: In magneto-telluric geophysical exploration, the earth's electric telluric field in two orthogonal directions is determined. Electrodes are positioned on the earth's surface so that measured potentials between adjacent electrodes measures the earth's telluric field in known directions. These measurements are converted to signals representing the earth's telluric field in two specified orthogonal directions. The measurements are made between electrodes spaced along a cable which is laid on the ocean floor.