Abstract: The invention relates to a method and an apparatus for use in surveying boreholes. The method of the invention comprises the following steps: providing an instrument package in a leading end of a drillstring, the instrument package comprising first and second single-axis sensors mounted for rotation with the drillstring about the rotational axis of the drillstring, the first sensor being an accelerometer and the second sensor being a magnetic fluxgate or a rate gyro; rotating the drillstring; deriving from the first sensor the inclination angle of the drillstring at the instrument package; and deriving from the second sensor the azimuth angle of the drillstring at the instrument package.

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: The invention relates to a method and an apparatus for use in surveying boreholes. The method of the invention comprises the following steps: providing an instrument package in a leading end of a drillstring, the instrument package comprising first and second single-axis sensors mounted for rotation with the drillstring about the rotational axis of the drillstring, the first sensor being an accelerometer and the second sensor being a magnetic fluxgate or a rate gyro; rotating the drillstring; deriving from the first sensor the inclination angle of the drillstring at the instrument package; and deriving from the second sensor the azimuth angle of the drillstring at the instrument package.

Abstract: Borehole surveying methods and apparatus for surveying the true longitudinal magnetic field within a substantially non-magnetic drill collar. A plurality of longitudinal magnetic field measurements are made to provide a longitudinal-position-dependent series of magnetic field measurements BZ(z) which enable the true magnitude of the terrestial magnetic field BZe in the direction of the longitudinal axis of the borehole to be calculated on the basis that BZ(z)=BZe+E(z), where E(z) is the longitudinal-position-dependent longitudinal magnetic field error induced by the magnetism of the drill string and the bottom-hole assembly. Several different methods of calculation are described, including polar and non-polar magnetic error function models.

Abstract: A method of deriving the true magnitude of the terrestial magnetic field in the direction of the longitudinal axis of a borehole at a region of the borehold occupied by an instrumentation package equipped with magnetic vector measuring means for measuring the lateral magnetic vectors Bx and By and further equipped with gravitational vector measuring means for measuring each of the three gravitational vectors Gx, Gy, and Gz.The method comprises the steps of performing said measurements of the magnetic vectors and the gravitational vectors and from said vector measurements solving the equation:Bze=(Bve.Ge-Bx.Gx-By.Gy)/Gzwhere BVe is the vertical component of the terrestial magnetic field at the location of the borehole, and Ge is the terrestial gravity field. The terrestial gravity field Ge may be derived as the square root of the sum of the individual squares of the individual gravity vectors Gx, Gy and Gz, according to the equation:Ge=(Gx.sup.2 +Gy.sup.2 +Gz.sup.2).sup.1/2.

Abstract: A transmitter comprises an impeller assembly rotatable by a liquid flow, and an electrical generator having a stator and a rotor arranged to be driven by the impeller assembly. The impeller assembly comprises a main impeller portion and a secondary impeller portion angularly displaceable relative to one another about the axis of rotation of the impeller assembly in response to a change in load of the generator so as to vary the pressure drop across the rotating impeller assembly. Thus appropriate variation of the load of the generator may be used to control the impeller assembly in such a manner as to transmit pressure signals within the liquid flow. Such a transmitter is particularly suitable for transmitting measurement data to the surface from a measuring instrument at the end of a drill string within a borehole. Furthermore such a transmitter can be constructed so as to be readily retrievable without requiring withdrawal of the drill string from the borehole.

Abstract: An accelerometer has a moving magnet within a housing. The magnet is mounted within a support member connected to the center of a diaphragm so as to permit movement of the magnet along a measurement axis extending in the axial direction of the support member by deformation of the diaphragm and so as to permit movement of the magnet along two mutually orthogonal measurement axes parallel to the diaphragm by flexing of the diaphragm to allow for pivoting of the support member. Furthermore displacement of the magnet with respect to the three mutually transverse measurement axes in response to an applied force is sensed by a pickoff capacitor, and a respective output signal proportional to the component of the applied force along each of the measurement axes is supplied by associated sensing circuitry. Simultaneously the magnet is restored to its null position by associated force coil pairs.

Abstract: For the purposes of determining the orientation of a surveying instrument in a borehole, while compensating for the effects of perturbing magnetic fields associated with magnetized sections of the drill string both above and below the instrument, the inclination angle .theta. and the highside angle .phi. of the instrument are determined, together with two transverse components Bx and By of the local magnetic field as measured at the instrument in the borehole. Additionally either a single component of, or the magnetide of, the true earth's magnetic field at the location of the borehole is ascertained from a look-up table or directly by measurement away from the influence of the drill string. A value .PSI.a for the azimuth angle of the instrument is then determined from the inclination and highside angles, the measured component(s) of the local magnetic field, and only the single component of, or the magnitude of, the true earth's magnetic field.

Abstract: A down-hole signal generator for a mud-pulse telemetry system comprises a flow constrictor defining a throttle orifice for the mud passing along a drill string, a throttling member displaceable with respect to the throttle orifice to modulate the mud pressure for the purpose of transmitting measurement data up the drill string, and a turbogenerator. The turbogenerator incorporates an annular impeller surrounding a casing and arranged to be driven by the mud passing along the drill string, and a rotatable magnet assembly disposed in a mud-free environment within the casing.

Abstract: An instrument for monitoring the direction of a borehole comprises an elongate casing having its longitudinal axis coincident, in use, with the axis of the borehole, an outer gimbal pivotally mounted within the casing with its pivot axis coincident with the longitudinal axis of the casing, a gyroscope unit mounted in the outer gimbal, a torque motor and a gravity sensor unit. At the beginning of a run the instrument is positioned at the mouth of the borehole. The rate of rotation about an axis transverse to the outer gimbal axis is then sensed by the gyroscope unit, and the outer gimbal is slewed by the torque motor in dependence on the sensed rate so as to align the transverse axis along an East/West direction. The instrument is then moved along the borehole and the rate of rotation about the outer gimbal axis is continually sensed by the gyroscope unit, the outer gimbal being torqued by the motor in dependence on the sensed rate to stabilize the outer gimbal about its axis.

Abstract: A down-hole signal generator for a mud-pulse telemetry system comprises an annular impeller surrounding a casing and arranged to be driven by the mud passing along the drill string. The impeller serves to drive a torque control arrangement, and preferably also an electrical generator, within the casing. The torque control arrangement is switchable between two states by a signalling actuator in response to an electrical input signal. In a first state the impeller may be driven relatively easily so that it is rotated at a relatively fast speed by the mud flow, whereas, in a second state, a greater torque is required to drive the impeller so that it is rotated at a relatively slow speed. Thus suitable variation of the input signal may be used to vary the impeller speed to transmit a modulated pressure signal in the mud flow which may be sensed at the surface.

Abstract: A down-hole signal generator for a mud-pulse telemetry system comprises a flow constrictor defining a throttle orifice for the mud passing along a drill string, a throttling member displaceable with respect to a casing to modulate the mud pressure for the purpose of transmitting measurement data up the drill string, and a turbogenerator incorporating an electrical generator within the casing. The flow constrictor and casing form an integrated unit which is installed within a drill collar and is retrievable by drawing it up the inside of the drill string. The throttling member is displaced by a pump according to the torque required to drive the rotor of the generator which is dependent on the electrical load of the generator. The pump incorporates a rotary valve member which supplies the output of the pump to one or other side of a double-acting ram according as to whether the valve member is rotating with a first phase of rotation or a second phase of rotation.

Abstract: A down-hole signal transmitter for a mud pulse telemetry system comprises a flow constrictor defining a throttle orifice for the mud flow, a throttling member displaceable to vary the throughflow cross-section of the throttle orifice, and a pump for displacing the throttling member against the mud flow in order to modulate the mud flow. The displacement of the throttling member is controlled by a hydraulic amplifier, comprising a main pressure relief valve and a subsidiary control valve, and a solenoid to which the output signal of a measuring instrument is supplied. When the main valve is close, the pump displaces a ram, coupled to the throttling member, upwardly.

Abstract: A borehole is surveyed by positioning at the mouth of the borehole a survey instrument having a casing and a three-axis rate gyroscope unit mounted within the casing, and sensing at least two components of gravity in at least two mutually transverse directions with respect to the survey instrument by means of a gravity sensor unit. The survey instrument is then moved along the borehole with the start and finish of the run being at the mouth of the borehole or at some known reference along the path of the borehole. During the run the rates of rotation about three non-coplanar axes are sensed at a series of locations along the length of the borehole by means of the rate gyroscope unit.

Abstract: When surveying a borehole using an instrument responsive to the earth's magnetic field, the assumption is made that the error vector which comprises the difference between the true earth's magnetic field and the measured magnetic field in the borehole is in the direction of the borehole axis. The magnitude of this error vector is determined and used to correct the magnetically measured azimuth.

Abstract: An instrument for measuring the direction of a borehole in order to carry out a spatial survey thereof comprises a gyro-stabilized single-axis platform having its axis coincident with the axis of the borehole and three gravity sensors for measuring three components of gravity in the direction of the borehole axis and in two mutually perpendicular directions in a plane perpendicular to said axis.