Abstract: A borehole drilling and survey assembly includes a drill string of magnetic material, a non-magnetic drill collar, and a non-magnetic rotary drilling system including a drill bit. A near-bit survey instrument is located in the rotary drilling system at a fixed distance from the junction of the drill string and the drill collar. A second survey instrument is located in the drill collar at a fixed distance from said junction. The survey instruments measure the local values of the component of the earth's magnetic along the borehole axis, and these values are processed to remove the magnetic effects of the drillstring. The survey instruments optionally also measure gravity vector components to enable borehole heading to be derived.

Abstract: A borehole drilling and survey assembly includes a drill string of magnetic material, a non-magnetic drill collar, and a non-magnetic rotary drilling system including a drill bit. A near-bit survey instrument is located in the rotary drilling system at a fixed distance from the junction of the drill string and the drill collar. A second survey instrument is located in the drill collar at a fixed distance from said junction. The survey instruments measure the local values of the component of the earth's magnetic along the borehole axis, and these values are processed to remove the magnetic effects of the drillstring. The survey instruments optionally also measure gravity vector components to enable borehole heading to be derived.

Abstract: Inclination and high side angle of a borehole is measured by a tool that may be positioned inside a drill string and lowered into the borehole. The tool includes three gravity transducers positioned in a novel array. The transducers are positioned in a case whose axis coincides with that of the borehole and are arranged with their axes angularly displaced from each other around the case axis by approximately 120 degrees and from the case axis by approximately 45 degrees. An array of fluxgate magnetic transducers may be added to the apparatus to provide combination surveying and steering capability.

Abstract: A linear accelerometer comprises a proof mass in the form of a permanent bar magnet located in a tube containing a magnetic fluid such as a liquid with particles of magnetic material suspended therein. A servo circuit responsive to A.C. energized sensing coils is used to energize a D.C. electromagnet to maintain the proof mass in a central position in the tube.

Abstract: The electrical power required to drive downhole drilling tools and/or measuring instruments is obtained using means responsive to turbulence in the mud flow past a unit forming part of the drill string. The responsive means may comprise a piezo-electric disc connected to rectifying and smoothing circuits whereby a D.C. output is obtained.

Abstract: Apparatus is provided for measuring both the angle between the vertical and a particular axis of a body and the azimuth angle of the displacement of such axis from the vertical. The body houses a trasnducer having a bar magnet mounted on the free end of a pendulum beam and confronting four solenoids forming two differential fluxgate pairs disposed symmetrically with respect to the neutral position of the pendulum beam. The two differential fluxgate pairs are preferably so interconnected that the magnetic flux in each fluxgate pair is equal to zero.

Abstract: For the directional drilling of boreholes, the orientation or roll angle of the drill head or mud-motor in the borehole has to be ascertained for steering purposes. The roll angle is determined, during drilling, by sensing at the location of the drilling tool in the borehole the values of the earth's magnetic field strength in two axes transverse to the main tool axis, signalling these values to the drilling station and translating them according to the direction and inclination of the borehole axis, which are known and normally updated at intervals between drilling runs. The down-hole assembly, located in a fixed attitude to the mud-motor, requires only static magnetic sensors. A resolver in a surface indicating unit provides for the indication of the roll angle as the angle determined by the sensors, translated according to the borehole information by the addition of an angle precalculated and selected from tables, calculated in a computation center, or calculated by computation in the indicating unit.