Two coil guidance system for tracking boreholes

Abstract

A method and apparatus for tracking the drilling progress of a borehole such as a horizontal borehole that is drilled under an obstacle along a prescribed path is based upon triangulation from two laterally separated loops of wire lying on the ground, which generate corresponding alternating magnetic fields.

1 . A method for drill guidance, comprising:
 determining a proposed path for a borehole;   positioning first and second current-conducting loops in known, laterally separated locations with respect to said proposed path;   causing said first and second loops to generate corresponding first and second magnetic fields, each of said fields having an alternating polarity;   positioning magnetic field sensors at a measurement location in said borehole;   measuring, at said measurement location, the vector components of said first and second magnetic fields and   determining, from said measured vector components of said first and second fields, the location of the sensors with respect to the locations of the magnetic loop sources.   
     
     
         2 . The method of  claim 1 , further including:
 determining the direction of the gravity vector at said measurement location; and determining from said gravity vector and said measured magnetic field vectors the direction of the borehole at said sensors with respect to a surface direction.   
     
     
         3 . The method of  claim 1 , wherein generating electromagnetic fields includes directing alternating currents of known amplitude and frequency through said loops. 
     
     
         4 . The method of  claim 1 , wherein generating electromagnetic fields includes simultaneously directing alternating currents of known amplitudes and different frequencies through each of said loops. 
     
     
         5 . The method of  claim 1 , wherein positioning said first and second loops includes placing said loops on opposite sides of said proposed path. 
     
     
         6 . The method of  claim 1 , further including determining the lateral distance of said measuring location from the proposed path and the direction of said borehole with respect to the direction of the proposed path. 
     
     
         7 . Apparatus for borehole surveys comprising:
 a downhole tool located in a borehole in the Earth, said tool having an axis and including a magnetometer for measuring x, y and z vector components of magnetic fields at the tool;   an electromagnetic excitation system including two electrical loop conductors at known locations, laterally spaced from each other and from a proposed path of said borehole, and carrying known alternating electrical currents producing corresponding alternating electromagnetic fields in the region of said magnetometer;   computer circuitry;   a telemetry link for transmitting measurement data signals corresponding to the respective measured vectors of magnetic fields generated by each of said loops from said magnetometer to said computer circuitry; and   circuits in said computer circuitry responsive to said measurement signals for determining the location of the borehole sensors.   
     
     
         8 . The apparatus of  claim 7 , wherein said downhole tool further includes sensors for measuring the Earth's gravity vector and for determining the orientation of the downhole tool, said telemetry link transmitting measurement data signals corresponding to said gravity vector and orientation measurement to said computer for determination of the direction of the borehole with respect to said proposed path. 
     
     
         9 . The apparatus of  claim 8 , wherein said downhole tool is a measurement while drilling instrument. 
     
     
         10 . The apparatus of  claim 8 , wherein the electrical current supplied to each of said loops is an alternating current of a different frequency. 
     
     
         11 . The apparatus of  claim 8 , wherein:
 said two electrical loop conductors carry first and second alternating currents, respectively, said currents producing corresponding first and second electromagnetic fields in the region of said downhole tool in said borehole; and   said electrical loop conductors are spaced on opposite sides of said proposed path of said borehole.   
     
     
         12 . The apparatus of  claim 11 , wherein each of said loop conductors comprises multiple turns of wire. 
     
     
         13 . A method for location determination, comprising:
 positioning first and second current-conducting loops at known spaced locations;   energizing said loops with known electrical currents to produce corresponding first and second electromagnetic fields;   positioning a magnetic field vector sensor to detect x, y and z vector components of said first and second fields; and   determining from said vector components the location of said sensor with respect to said loops.   
     
     
         14 . The method of  claim 13 , wherein:
 positioning said loops includes positioning the loops on opposite sides of a proposed path to be followed by said sensor; and   determining the location of said sensor includes locating said sensor with respect to said path.   
     
     
         15 . The method of  claim 13 , wherein energizing said loops includes supplying alternating current of a known amplitude and frequency to one loop at a time. 
     
     
         16 . The method of  claim 13 , wherein energizing said loops includes supplying alternating current of known amplitude and different known frequencies to each loop. 
     
     
         17 . The method of  claim 13 , further including:
 detecting, at the location of said sensor, the Earth's gravity vectors and the orientation vectors of the sensor; and   determining from said vectors the direction of the sensor with respect to the locations of said loops.   
     
     
         18 . The method of  claim 13 , wherein determining the location of the sensor includes:
 time averaging magnetic field vector data obtained for each of said loops by the sensor;   calculating theoretical location vectors for the sensor; and   optimizing the match between theoretical location values and measured magnetic field vector data to determine the location of the sensor.   
     
     
         19 . The method of  claim 18 , wherein;
 positioning said loops includes positioning the loops on opposite sides of a proposed path to be followed by said sensor; and   determining the location of said sensor includes locating said sensor with respect to said path.   
     
     
         20 . The method of  claim 19 , wherein energizing said loops includes supplying alternating current of a known amplitude and frequency to one loop at a time. 
     
     
         21 . The method of  claim 19 , wherein energizing said loops includes supplying alternating current of known amplitude and different known frequencies to each loop.