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 device and a method for the measurement of the angle of drift of a borehole that extends from the surface of the ground downwardly into the earth. The device comprises a generally hollow protective exterior casing, a microprocessor control; and, a sensor pack. At least the microprocessor control and the sensor pack are received and contained within the exterior casing. The sensor pack includes one or more accelerometers mounted upon a sensor chassis that is positioned within the casing and situated generally parallel to its longitudinal axis. The sensor chassis has one or more mounting surfaces for receiving and securing the one or more accelerometers to the sensor chassis. The mounting surfaces are configured such that the one or more accelerometers when secured to the chassis are held and retained at an inclined angle relative to the longitudinal axis of the exterior casing and the device.

Abstract: A continuous measurement-while-drilling surveying apparatus includes a fiber optic gyroscope sensitive to rotation about the tool spin axis of the bottom hole assembly and a second fiber optic gyroscope sensitive to rotation of the bottom hole assembly about an axis normal to the tool spin axis. The first gyroscope may be shaped as a torus to accommodate flow of drilling mud through the bottom hole assembly. The outputs of the first and second gyroscopes are processed together with acceleration signals from three accelerometers in a microprocessor which determines the orientation, velocity and position of the bottom hole assembly on a continuous basis.

Abstract: Wellbore survey methods and apparatus for wireline and measurement-while-drilling operations are disclosed which include a gyroscope, wherein the gyroscope has a spin axis, aligned with the instrument axis, and further having two sensitive axis orthogonally related to the spin axis and to each other. In addition, the wellbore survey apparatus contains a drive means, functionally connected with the gyroscope, to rotate the gyro about the instrument axis. The wellbore survey apparatus also contains a set of accelerometers, wherein the sensitive axis are aligned orthogonally to each other, and said drive means is functionally connected to the accelerometers to rotate the accelerometers about the instrument axis. Sensors determine the azimuthal direction of inclination of the wellbore at a first location therein and while traversing from said first location.

Abstract: A method of drilling under barriers (rivers, highways and the like) is set out. The horizontal drilling system mounts a guidance tool on the end of the drill string just behind the drill bit. The guidance tool includes a pair of right angle accelero meters and a 3-axis mounted gyro. The gyro furnishes data in a plane at right angles to the z-axis. This defines four data streams to the CPU enabling determination of drill bit location and pathway.

Abstract: Wellbore survey methods and apparatus for wireline and measurement-while-drilling operations are disclosed which include a gyroscope, wherein the gyroscope has a spin axis, aligned with the instrument axis, and further having two sensitive axis orthogonally related to the spin axis and to each other. In addition, the wellbore survey apparatus contains a drive means, functionally connected with the gyroscope, to rotate the gyro about the instrument axis. The wellbore survey apparatus also contains a set of accelerometers, wherein the sensitive axis are aligned orthogonally to each other, and said drive means is functionally connected to the accelerometers to rotate the accelerometers about the instrument axis. Sensors determine the azimuthal direction of inclination of the wellbore at a first location therein and while traversing from said first location.

Abstract: A method for drilling a wellbore to assure the avoidance of a substantially adjacent known magnetic source comprising the steps of obtaining survey data for the path or trajectory of a known magnetic source; preparing a wellplan comprising a path or trajectory for a second wellbore to be drilled, from the survey data and the wellplan, computing expected values of one or more parameters of the magnetic field to be encountered along a second wellbore during drilling operations, in association with drilling a second wellbore, measuring components of a magnetic field encountered and computing one or more magnetic field parameters at a series of survey points along the wellbore path or trajectory; and determining if measured values differ systematically from expected values, thereby indicating a deviation from the planned wellpath.

Abstract: A method and apparatus for use in creating a magnetic declination profile for a borehole. The method includes the steps of making a magnetic directional measurement at a longitudinal location in the borehole and at a magnetic measurement orientation, making a reference directional measurement at a reference measurement orientation, wherein there is a known orientation relationship between the magnetic measurement orientation and the reference measurement orientation, and obtaining a value of magnetic declination at the longitudinal location in the borehole using the magnetic directional measurement, the reference directional measurement and the orientation relationship.

Abstract: A method and a rock drilling apparatus for drilling holes in rock according to a predetermined drilling plan. The method comprises measuring the location of each hole in the rock by inserting a measuring device into the hole, and calculating the deviation of the end of the measured hole from the location determined in the drilling plan. The rock drilling apparatus comprises a measuring device that can be inserted into a drill hole, and means for transferring the measurement values measured by the measuring device to control means of the rock drilling apparatus.

Abstract: Systems and associated methods for tracking and/or guiding an underground boring tool use one or more detectors to measure the intensity of an electromagnetic field which is transmitted from an underground boring tool. The measured intensities may then be used to determine the location of the boring tool. In a dead reckoning embodiment of the invention, one detector may be employed while, in a position determination embodiment, two or more detectors may be employed. In any embodiment, physically measurable parameters may be used in addition to measured magnetic intensities. A highly advantageous mapping tool instrument for use in the position determination embodiment and a cubic antenna for use in any magnetic field detector are employed herein.

Abstract: A method of drilling under barriers (rivers, highways and the like) is set out. The horizontal drilling system mounts a guidance tool on the end of the drill string just behind the drill bit. The guidance tool includes a pair of right angle accelero meters and a 3-axis mounted gyro. The gyro furnishes data in a plane at right angles to the z-axis. This defines four data streams to the CPU enabling determination of drill bit location and pathway.

Abstract: A method for determining azimuth in well bore hole where the well is being drilled utilizing Measurement-While-Drilling (MWD) equipment, including gravitometer and magnetometer instruments in the presence of an adjacent well. Interference fields introduced by adjacent wells are modeled as a single or series of magnetic dipoles. The interference fields axial interference field IBz is tracked and a position determined where the IBz component is at its maximum, minimum or a zero value. The total magnetic field is then measured at that point and the magnetic inclination and therefrom, the azimuth may be determined. Alternately, the interference field in the x,y plane (IB(x,y)) may be tracked and a position determined where the IB(x,y) component is equal to a minimum, maximum or zero value. The azimuth may then be determined by an alternate numeric or curve fitting methods.

Abstract: A magnetic observatory device is deployed at a fixed location such as the sea bed to provide data measuring variations in the local earth magnetic field. This data is used to correct information from magnetic direction sensors, for example in a measurement-while-drilling system of a drill string. Also described is a reference apparatus for initial calibration of the device or for marine magnetic mapping purposes.

Abstract: Scraper for determining the position of pipelines with at least one scraper body and on the outer circumference of the latter are provided support elements for guiding in the pipeline, the scraper body containing a gyroscopic platform, as well as supply and processing means for measured results, characterized in that the scraper body is guidable by a combination of support elements under a defined, constant angle with respect to the pipe guidance axis, whereof a first part is radially rigid and a second part is radially resilient.

Abstract: The essence of the proposed method resides in that assumed as the datum point of measurement of the well azimuth is an azimuthally stabilized housing of a down-the-hole inclinometer and the magnitude of the azimuth is determined by measuring the angle of rotation of the housing of the down-the-hole instrument (1) round its longitudinal axis, which rotation occurs in response to an azimuthal deviation of the longitudinal axis of a well (5). According to a first embodiment of the proposed device, it comprises a plurality of spring-loaded arcuate elements (3) secured on an external surface (2) of the housing of the down-the-hole instrument (1) so as to form at least three rows, each row consisting of at least three elements (3), each of the latter establishing, together with an internal surface (4) of the well (5), a contact spot (6) whose portion having a greater size (h) is arranged lengthwise the longitudinal axis of the well (5).

Abstract: An extendible surveying probe comprises two collinear position detecting units that allow the probe to be used in an inclined position for accessing vertically inaccessible points with a tip thereof. The two position detecting units may consist of two GPS or GLONASS units used in association with another GPS or GLONASS unit installed on a known point or, alternatively, of two corner cubes used in association with a ground total station. The position detecting units provide spatial coordinates of two spaced-apart collinear points which may be used to extrapolate the coordinates of the point in contact with the tip of the probe.

Abstract: A method for well borehole survey is set out. A sonde supports X and Y accelerometers and X and & sensors on a rate gyro having a Z axis aligned with the sonde. On a slickline, or within a drill string, the sonde is used to measure four variables, these being G.sub.x and G.sub.y, A.sub.x and A.sub.z. This enables well azimuth and inclination to determined. Measuring depth enables a survey to be made.

Abstract: A method is disclosed for correcting for the distorting effect of cross-axial magnetic interference on the readings of a well survey tool using a reduced number of measurements and measuring locations. A well survey tool connected to a drill string assembly measures the gravitational and distorted terrestrial magnetic fields at two or more axially displaced locations in an axially extending well bore. The measurements are made from two or more different orientations (tool faces) of the tool about the well axis. Initial values for the tool inclination, azimuth and tool face angle are first determined without correction for the cross-axial magnetic interference introduced by the drill string assembly and the survey tool. Measurements made at each location and tool face are referred to the toots coordinate system and variations introduced by the different tool face angles are used to estimate the cross-axial interference. The estimated interference is used to improve the estimate of the azimuth.

Abstract: A method for well borehole survey is set out. A sonde supports X and Y accelerometers and X and & sensors on a rate gyro having a Z axis aligned with the sonde. On a slickline, or within a drill string, the sonde is used to measure four variables, these being G.sub.x and G.sub.y, A.sub.x and A.sub.z. This enables well azimuth and inclination to determined. Measuring depth enables a survey to be made.

Abstract: The present invention discloses an MWD system which permits an indirect measurement of the wellbore inclination angle at the drill bit, in situations where it is difficult or expensive to measure that angle directly. The system includes a first inclinometer included as part of a conventional directional sensor, which is located in the bottomhole assembly a relatively large distance above the bit. The first inclinometer provides a measure of the inclination angle .alpha. at point A. The system also includes a second inclinometer, which is provided closer to the drill bit to provide a measure of the inclination angle .beta. at point B. The inclination angles .alpha. and .beta. are used, together with bottomhole assembly configuration information, and other information relating to drilling tendencies, to determine with a reasonable degree of accuracy the inclination angle .theta. of the wellbore at the bit.

Abstract: A method is disclosed for determining wellbore azimuth accurately, notwithstanding the corrupting influence of axial and transverse magnetic field biasing error components.A reliable value of the axial magnetic field component (B.sub.x) is determined by minimizing the vector distance between B (as measured) and its expected value B. Preferably, this is accomplished utilizing a method of radicals to solve a quadratic equation.Given a reliable B.sub.x, the transverse magnetic field components (B.sub.x and B.sub.x) can be determined from taking a number of surveys as the drillstring is rotated about. B.sub.x and B.sub.x can be determined by utilizing the assumption that the transverse magnetic field components should remain invariant as the drillstring is rotated.

Abstract: A method is provided for deriving wellbore azimuth in a manner which reduces the impact of axial magnetic field errors, which utilizes a plurality of estimates of the axial component of the magnetic field which are combined in a manner which emphasizes the better estimates and deemphasizes the poorer estimates.

Abstract: A method for determining the direction of a borehole during drilling comprises determination of inclination angle .theta. and highside angle .phi. from gravity acceleration (g) measurements and determination of azimuth angle .psi. from magnetic field (B) measurements along with the calculated inclination angle .theta. and highside angle .phi., the determinations being carried out in conventional XYZ-and-NEV coordinate systems coupled by Euler-angle coordinate transformations. In particular g and B are measured at least at two borehole depths such that .phi..sub.i does not equal .phi..sub.i+1, and .psi..sub.i and .psi..sub.i+1 are calculated from B.sub.i =[.phi..sub.i ].sup.T [.theta..sub.i ].sup.T {[.psi..sub.l ].sup.T B.sub.e }+B.sub.p with i as number of measurement, B.sub.e as local earth magnetic field, and B.sub.p as perturbating magnetic field.

Abstract: A measuring tool including a device (4) for measuring the terrestrial magnetic field is inserted in a drill string (2) linking a drilling tool (3) to a surface installation. The method allows to be free from the disturbance affecting the measured terrestrial magnetic field and which is due for example to the drill string. It comprises stopping the tool during the progression thereof in the well in successive stop positions longitudinally spaced out in relation to one another, the angular position of measuring device (4) in these positions being random, and using a statistic calculation method for combining the different measurements and determining the disturbing field.

Abstract: The calibration of time-depending measurement uncertainties of gyros and inertial systems by the differences of angular and/or velocity and/or positional measurements which are carried out and repetitiously executed with a view to undetermined reference directions and/or fixed locations varying with time. The differentiation allows to forego precise reference data. Measurement uncertainties can be corrected through calibration subsequently to the measurement process. For the measurement of spatial angles a three-axis gyro measurement package comprising a computer and the possibility of feeding or automaticly reading-in of reference data characterizing the reference directions. The reference data can be a number, a marker of the measurement point or in the case of positional measurements a terrestrial aiming point. When measuring angular characteristics, i.e. the interdependence of angles and external forces or moments, the latter are considered as references.

Abstract: A method for determining the orientation of the axis of a borehole with respect to an Earth-fixed reference coordinate system at a selected series of locations in the borehole, the borehole having a trajectory, and adapted to receive a drill string, comprising defining a model for the influence of magnetic interference from elements of the drill string on the measurement of components of the Earth's magnetic field in the borehole in terms of an unknown vector, a measurement vector, and a measurement matrix relating the unknown vector and the measurement vector; measuring at two or more selected locations along the borehole trajectory at least one of two cross-borehole components of the Earth's gravity field at the selected locations in the borehole, and two cross-borehole components and an along-borehole component of the Earth's gravity field at the selected locations in the borehole; two cross-borehole components and an along-borehole component of the Earth's magnetic field at the selected locations; computi

Abstract: There is disclosed a system and method for providing a portable visual display device. The device, in one embodiment, can be worn over the head by a viewer in the manner of a helmet. The viewer then looks at a screen within the helmet and "sees" created images. The helmet is equipped with sensors which determine the geographical direction as well as the relative movement of the viewer. The images are stored in a processor as a series of objects, each having associated therewith physical characteristics, including both audio and visual.

Abstract: Apparatus and method for positioning a tool, such as a surveying instrument, in the deviated section of a borehole includes a flexible rod and a positioning assembly. The flexible rod extends from the vertical section into the deviated section of the borehole with the tool being connected to the downhole end of the rod. The positioning assembly is connected to the uphole end of the rod for positioning the downhole end of the rod in the deviated section of the borehole when the positioning assembly and the uphole end of the rod are in the vertical section of the borehole. In one embodiment, the positioning assembly is a biasing assembly, such as a weight, for urging the downhole end of the rod and tool into the deviated section. In another embodiment, the positioning assembly is an orienting assembly for rotationally orienting the rod and tool in the borehole as the rod is lowered into the borehole.

Abstract: A roll sensor is provided for determining the roll angle of a rotatable member. The roll sensor includes a plurality of elements arranged in a predetermined manner, each element having a first state and a second state. A controller controls the respective state of the plurality of elements. A detector detects an output of the elements which are in the first state. A liquid responsive to the rotation of the rotatable member causes the output of at least one of the elements in the first state detected by the detector to be different than the output of the remaining elements in the first state detected by the detector to determine the roll angle of the rotatable member.

Abstract: A borehole deviation monitor comprises a probe body adapted to be lowered in a borehole and including two inclinometers mounted in an orthogonal configuration to provide simultaneous X and Y axis inclination signals indicating the dip angle of the borehole with respect to the vertical and two triaxial magnetometers to provide reference signals indicating the azimuth angle of the borehole. The magnetometers are separated by a predetermined distance along the axis of the probe. A cable takeup winch mechanism is provided to lower the probe body into the borehole by predetermined increments equal to the distance between the two magnetometers. A control and data storage station is connected to the two inclinometers and magnetometers for reading and storing the data collected at each incremental distance and for correcting the azimuth reading given by the top magnetometer by substracting any difference in readings between the two magnetometers at the previous position of the probe.

Abstract: A method for determining the orientation of the axis of a borehole with respect to an earth-fixed reference coordinate system at a location in the borehole comprising the steps of: measuring two cross-borehole components or two-cross-borehole components and an axial component of the earth's gravity field at a location in the borehole; measuring two cross-borehole components of the earth's magnetic field; determining the inclination angle of the borehole axis from the gravity component measurements; determining the highside angle reference of the cross-borehole measured components of the earth's gravity and magnetic fields from the gravity component measurements; determining more than one individual estimate of the azimuthal orientation of the borehole axis from the inclination angle, the highside angle reference and the two measured cross-borehole components of the earth's magnetic field; determining an error indicative parameter for each individual estimate of the azimuthal orientation of the borehole axis;

Abstract: A method and apparatus is presented for determining the inclination angle of a borehole being drilled, the data for determining the inclination angle being obtained while the drillstring is rotating.

Abstract: A method and apparatus is presented for measuring the azimuth angle of a borehole being drilled, the data for determining the azimuth angle being obtained while the drillstring is rotating.

Abstract: A wellbore inertial directional surveying system for providing a complete directional survey of an oil or gas well borehole to determine the displacement in all three directions of the borehole path relative to the well head at the surface. The information generated by the present invention is especially useful when numerous wells are drilled to different geographical targets from a single off-shore platform. Accurate knowledge of the path of the borehole allows proper well spacing and provides assurance that target formations are reached. The tool is lowered down into a borehole on the electrical cable. A computer positioned on the surface communicates with the tool via the cable. The tool contains a sensor block which is supported on a single gimbal, the rotation axis of which is aligned with the cylinder axis of the tool and, correspondingly, the borehole.

Abstract: Land vehicle navigation apparatus combining an inertial angular rate sensor having a sensitive axis, with an inertial acceleration or tilt sensor having at least two sensitive axes, these sensitive axes having a known relationship to the inertial angular rate sensor's sensitive axis. Also provided is circuitry to accept from an external source an initial input valve (or self initialization, by rotary multiple positioning) of the vehicle's azimuthal orientation angles, roll, pitch, and azimuth, with respect to an Earth-fixed coordinate set as the land vehicle travels over the Earth surface. Changes to the azimuth angle during travel are determined by integration of the output of the inertial angular rate sensor.

Abstract: A surveying and mapping implement indicates the level differences of the ground over which it travels, in relation to the distance travelled. It comprises a carriage consisting of a swivelling front wheel, two center wheels attached to the outside of a U-shaped vertical frame, and two rear wheels connected to the U-shaped frame by means of a thill and a universal joint positioned co-extensive with the axis of the center wheels. The universal joint permits the rear wheels to move up and down and sideways, as well as to twist in relation to the center wheels. The wheel base between the center wheels and the front wheel is identical with that between the center wheels and the rear wheels, in order to facilitate calculations.

Abstract: A pipeline survey pig includes an elongated hollow pig body having an enclosure for supporting a generally cylindrical tubular instrument chassis which, in turn, supports an inertial reference unit and associated signal processing and recording devices. A plurality of resilient cup members support the pig body for traversal through a pipeline section under the urging of pressure fluid. The cup members are spaced apart and secured to the pig body by spacer members which may be selectively provided to predetermine the axial spacing of the cup members so that the center of stiffness of the pig is in a predetermined position. The instrument chassis is secured in the pig body by a cam mechanism which is actuatable from one end of the chassis after insertion into the pig body to wedge opposed chassis door members into engagement with the pig body to enhance heat transfer between the chassis and the pig body.

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 method is presented for measuring the azimuth angle of a borehole being drilled, the data for determining the azimuth angle being obtained while the drillstring is rotating.

Abstract: For seismic exploration, a method and apparatus are set forth wherein geophones, preferably arranged in XYZ relationship to one another fixedly within a hermetically sealed housing, are located in a deviated well. The position of the housing with respect to a reference such as the earth's north axis is determined. This reference enables determination of the geophone location and the direction of propagation of seismic impulse.

Abstract: Apparatus and method for the surveying of bore holes, for example, oil wells and the like, to permit accurate three-dimensional mapping thereof, using a single rate gyroscope and an accelerometer package in an instrumentation pod which is lowered into the bore hole. Signals from the accelerometers and the rate gyroscope plus the increments by which the pod is lowered into the well permit continual calculation of pod attitude and updated of pod position at all depths in the hole.

Abstract: A method is presented, for use in borehole drilling, for correcting errors in azimuth determination resulting from variations in the earth's magnetic field to which a measuring instrument is exposed. The method distinguishes between variations in the earth's magnetic field caused by the drillstring and variation caused by external sources, and the method corrects only for errors caused by the drillstring. If the error is caused by the drillstring, an azimuth correction is made based on dip angle data, thereby reducing errors caused by scale factor variations since dip angle is a ratio quantity of magnetic field components.

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 pipeline survey pig including an onboard inertial reference unit and signal processing units for receiving acceleration and angular velocity signals generated by the inertial reference unit, calculating resultant values of angular velocity and accelerations and averaging the calculated values to provide recordable signals related to the position of the pig and changes in curvature of the pipe. The pig is supported by a plurality of resilient cup shaped support members which have a stiffness characteristic whereby the natural frequency of vibration which may cause lateral excursions of the pig is less than the signal generating rate yet greater than the frequency of the signal to be measured. The center of stiffness and the center of gravity of the pig are disposed along the central axis of the pig and the pipeline section being measured and are preferably coincident with each other.

Abstract: Apparatus and method for the surveying of bore holes, for example, oil wells and the like, to permit accurate three-dimensional mapping thereof, using an accelerometer package in an instrumentation pod which is lowered into the bore hole. In one embodiment the pod is lowered on the end of a rod string where adjacent rods connected together with Hookes-joint pivots which permit two degrees of freedom of movement between adjacent rods. In another embodiment, rigid interconnections of adjacent rods are used, and the rods are isotropic for twistless flexure as the rods encounter curves in the path of the bore hole. The instrumentation pod is lowered one rod-length at a time, and accelerometer readings are taken which, together with mathematical modeling of rod motion, permits calculation of updated pod position at all depths in the hole.

Abstract: A change in curvature or displacement of a section of submarine or subterranean fluid transmission pipeline is measured by traversing the pipeline with a pig having an onboard instrument package, including accelerometers and a longitudinal position measuring device comprising a magnetometer for counting the girth welds or other known magnetic anomalies along the section of pipeline to be measured. The accelerometers are mounted at the central longitudinal axis of the pig which is substantially coincident with the central longitudinal axis of the section of pipeline being measured so that the roll attitude of the pig does not influence the signals generated by the accelerometers. This arrangement precludes the requirement of mounting the accelerometers on a gyroscopic stabilized platform since the absolute position of the pig in space is not required, but primarily the change in curvature or displacement of the pipeline from a predetermined course is what is desired to be known.

Abstract: A method for determining the true dip and azimuth of bedding planes in a formation penetrated by a borehole using borehole televiewer measurements. The method corrects for borehole deviation and for inclination of the earth's magnetic field.

Abstract: A method for eliminating the influence of drill string magnetization on an azimuth measurement made by a magnetometer package disposed in the drill string. The method comprises first eliminating the influence of the cross-axial components of the drill string magnetization by taking magnetometer readings at various angular orientations of the drill string and then eliminating the influence of the axial component of the drill string magnetization.

Abstract: A method of, and apparatus for, determining the position of an unknown fixed location, e.g. the end of an underground passage inaccessible to human beings, involves establishing a chain of measuring station of known station spacing between a base and a position at the unknown fixed location. Each station (except, optionally, the foremost one) of the chain is provided with light sources and photodetectors as well as a goniometer for measuring in at least one plane the angle of inclination of the station immediately ahead of it. The stations of the chain are spaced apart by a known distance and arranged so that each station is within the operative range of the immediately preceding station. By transmitting the measurement results from each station to the base, and processing the data, the position of the fixed location from the measurement results.

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..]. .Iadd.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.