Surveying of boreholes using shortened non-magnetic collars

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. 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. .Iaddend.

What is claimed is: .[.1. A system for determining the orientation of a downhole instrument positioned in a drill collar in a borehole comprising: a means for determining inclination angle of the instrument at a location thereof in said borehole; a means for determining the highside angle of said instrument at said location; a means for determining the true horizontal and vertical components of the earth's magnetic field at the location of the borehole; a means for determining components of the local magnetic field perpendicular to the direction of a primary axis of the instrument aligned with the borehole at said location, said drill collar being constructed of non-magnetic material, and having a minimum length, L, determined from the equation: ##EQU13## where P u  is the magnetic pole created by the magnetic material above the sensor, P L  is the magnetic pole created by the magnetic material below the sensor, d is the displacement of the poles P u  and P L  from the axis of the instrument, B n  is the North component of the earth's magnetic field at the tinstrument, and δψ is the error 
     
        in the azimuth angle..]. .[.2.  The orientation system of claim 1 wherein said means for determining the components of local magnetic field comprises a means for sensing measured components of said local magnetic field, said sensing means being located at least one third of said length of said drill collar from an end of said drill collar..]. .[.3. The orientation system of claim 1 wherein said instrument is located in a drill string extending in said borehole, said system being located between the lower drill string end connecting to the drill bit and an upper drill string end connecting to the surface..]. .[.4. The orientation system of claim 3 wherein said drill string is comprised of magnetic material..]. 
     
     
        .Iadd.5.  A method of determining the orientation of a surveying instrument in a borehole comprising the steps of: a. determining the inclination angle of the instrument in the borehole;   b. determining highside angle of the instrument in the borehole;   c. determining two transverse components of the local magnetic field perpendicular to the longitudinal axis of said instrument in the borehole;   d. determining, without directly measuring, a value for the component of the local magnetic field along the longitudinal axis of the instrument in the borehole utilizing the inclination angle; and   e. determining a value of azimuth angle of the instrument utilizing the local magnetic field components, the inclination angle and the highside angle, and without utilizing a directly measured value for the local magnetic field component along the longitudinal axis of said instrument.   
     
     
        .Iaddend. .Iadd.6.  A method as defined in claim 5 further comprising: a. providing data indicative of the earth's magnetic field at said borehole; and   b. using said earth's magnetic field data in the step of determining a value for the component of the local magnetic field along the longitudinal axis of the instrument in the borehole. .Iaddend. .Iadd.7. A method as defined in claim 6 further comprising:   a. utilizing the inclination angle, the highside angle, earth's magnetic field data and said transverse components of the local magnetic field perpendicular to the longitudinal axis of the instrument to obtain an approximate value of azimuth angle;   b. using the approximate value of the azimuth angle also in determining the value for the component of the local magnetic field along the longitudinal axis of the instrument in the borehole; and   c. so determining a more accurate value for azimuth angle. .Iaddend.   
     
     
        .Iadd. .  A method as defined in claim 7 comprising the additional steps of using such more accurate value of azimuth angle as an approximate value of azimuth angle to determine a further value for the component of the local magnetic field along the longitudinal axis of the instrument in the borehole, and determining a new more accurate value of azimuth angle using said further value for the component of the local magnetic field along the longitudinal axis of the instrument as in claim 7. .Iaddend. .Iadd.9. A method as defined in claim 8 comprising the additional steps of repeating the steps of claim 8 until the obtained values of azimuth angle converge to within an acceptable error. .Iaddend. .Iadd.10. A method as defined in claim 6 wherein the earth's magnetic field data is determined by utilizing sensing means included in such a surveying instrument. .Iaddend. .Iadd.11. A method as defined in claim 6 wherein the earth's magnetic field data is determined at the surface of the earth in the vicinity of said borehole. .Iaddend. .Iadd.12. A method as defined in claim 6 wherein the earth's magnetic field data is determined in terms of horizontal and vertical components of said field. .Iaddend. .Iadd.13. A method as defined in claim 5 wherein said instrument is provided located in a drill string in said borehole, said instrument being located between the lower drill string end connecting to a drill bit and an upper drill string end connecting to the 
     
     
        surface. .Iaddend. .Iadd.14.  A method as defined in claim 5 further including providing said surveying instrument positioned in non-magnetic material having a length no shorter than a length L determined by the equation: ##EQU14##.Iaddend. where P U  is the magnetic pole of magnetic material above said non-magnetic material, P L  is the magnetic pole of magnetic material below said non-magnetic material, d is the displacement of the poles P U  and P L  from the axis of the instrument, B n  is the north component of the earth's magnetic field at the borehole, and 
     
     
        δψ is an acceptable error in the azimuth angle.  .Iadd.15.  A method as defined in claim 14 wherein the transverse components of the local magnetic field are determined by utilizing sensing means included in such a surveying instrument and located at least one third of said length of said non-magnetic material from an end of said non-magnetic material. 
     
     
        .Iaddend. .Iadd.16.  A method of determining the orientation of a surveying instrument in a borehole, comprising the steps of: a. determining the inclination angle of the instrument in the borehole;   b. determining the highside angle of the instrument in the borehole;   c. providing data indicative of the earth's magnetic field at the borehole;   d. determining two transverse components of the local magnetic field perpendicular to the longitudinal axis of the instrument in the borehole;   e. calculating, without directly measuring, a value for the component of the local magnetic field along the longitudinal axis of the instrument in the borehole utilizing the earth's magnetic field data; and   f. determining a value for the azimuth angle of the instrument utilizing said local magnetic field components, the inclination angle and the highside angle, and without utilizing a directly measured value for the local magnetic field component along the longitudinal axis of the   
     
     
        instrument in the borehole. .Iaddend. .Iadd.17.  A method according to claim 16 wherein said value for the component of the local magnetic field along the longitudinal axis of the instrument is determined also utilizing the inclination angle and an approximation of the azimuth angle of the instrument. .Iaddend. .Iadd.18. A method according to claim 17 wherein the approximation of the azimuth angle of the instrument is determined from the inclination angle, the highside angle, the transverse components of the local magnetic field perpendicular to the longitudinal axis of the instrument, and earth's magnetic field data. .Iaddend. .Iadd.19. A method according to claim 17 comprising the additional steps of determining a further value for the component of the local magnetic field along the longitudinal axis of the instrument utilizing the previously determined value for the azimuth angle, and determining a new, more accurate value for the azimuth angle utilizing said further value for the component of the local magnetic field along the longitudinal axis of the instrument. .Iaddend. .Iadd.20. A method according to claim 19 comprising the additional steps of repeating the steps of claim 19 until the obtained values of azimuth angle converge to within an acceptable error. .Iaddend. .Iadd.21. A method according to claim 16 which comprises: a. utilizing the inclination angle, the highside angle, earth's magnetic field data and the transverse components of the local magnetic field perpendicular to the longitudinal axis of the instrument to obtain an approximate value of the azimuth angle for the instrument;   b. utilizing the inclination angle and said approximate value for the azimuth angle also in determining said value for the component of the local magnetic field along the longitudinal axis of the instrument; and   c. so determining a more accurate value for the azimuth angle. .Iaddend. .Iadd.22. A method according to claim 21 comprising the additional steps of utilizing such more accurate value for the azimuth angle as an approximate value for the azimuth angle to determine a further value for the component of the local magnetic field along the longitudinal axis of the instrument, and determining a new, more accurate value for the azimuth angle using said further value for the component of the local magnetic field along the longitudinal axis of the instrument as in claim 21.   
     
     
        .Iaddend. .Iadd.23.  A method according to claim 22 comprising the additional steps of repeating the steps of claim 22 until the obtained values of azimuth angle coverage to within an acceptable error. .Iaddend. .Iadd.24. A method according to claim 16 wherein the local magnetic field components are determined by utilizing sensing means included in such a surveying instrument. .Iaddend. .Iadd.25. A method according to claim 16 wherein the earth's magnetic field data is determined by utilizing sensing means included in such a survey instrument. .Iaddend. .Iadd.26. A method according to claim 16 wherein the earth's magnetic field data is determined at the surface of the earth in the vicinity of said borehole. .Iaddend. .Iadd.27. A method according to claim 16 wherein said instrument is provided located in a drill string in said borehole, said instrument being located between a lower drill string end connecting to a drill bit and an upper drill string end connecting to the surface. .Iaddend. .Iadd.28. Apparatus for determining the orientation of a surveying instrument in a borehole, comprising: a. means for determining the inclination angle for the instrument in the borehole;   b. means for determining the highside angle of the instrument in the borehole;   c. means for storing data indicative of the earth's magnetic field at the borehole;   d. means for determining two transverse components of the local magnetic field perpendicular to the longitudinal axis of the instrument in the borehole;   e. first calculating means for calculating, without directly measuring, a value for the component of the local magnetic field along the longitudinal axis of the instrument in the borehole utilizing the earth's magnetic field data; and   f. second calculating means for calculating a value for the azimuth angle of the instrument utilizing said local magnetic field components, the inclination angle and the highside angle, and without utilizing a directly measured value for the local magnetic field component along the longitudinal axis for the instrument in the borehole. .Iaddend.