Surveying of boreholes using shortened non-magnetic collars

Geometrical instruments – Indicator of direction of force traversing natural media – Borehole direction or inclination

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33313, E21B 47022

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active

RE0337080

ABSTRACT:
[When surveying a borehole using an instrument responsive to the earth's magnetic field, a length of non-magnetic drill collar is necessary to house means for measuring the magnetic field in the borehole perpendicular to the direction of the borehole axis. The instrument determines the inclination angle and the highside angle from the gravitation measurements, with these measurements and the magnetic measurements, the azimuth angle is determined. Using the method of this invention a minimum length of non-magnetic material necessary for an accurate measurement may be calculated and used.] Disclosed are method and apparatus for surveying a borehole including use of a survey instrument for making gravitational measurements from which the inclination and highside angles of the instrument may be determined. Measurements of two components of the local magnetic field perpendicular to the longitudinal axis of the instrument may be sensed with the instrument, and may be used to determine the azimuth angle of the instrument under the assumption that magnetic interference due to the pipe string in which the instrument is located lies solely along the longitudinal axis of the instrument. The accuracy of the azimuth determination may be enhanced by an iteration process. To the extent that the pipe string interference includes transverse field components at the instrument, the sensors of the instrument may be separated from such pipe string members by placing the instrument in non-magnetic material whose minimum length may be determined.

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