Elongated coil assembly for electromagnetic borehole surveying

Abstract

An apparatus and method for surveying the path of a borehole includes three single-axis electromagnetic field sensors and three single axis gravity direction sensors in the borehole being surveyed. A pair of closely spaced, parallel guide wires carrying oppositely directed electric current in a reference borehole generate a known electromagnetic field, which is detected by the sensors. Mathematical analysis of electromagnetic and gravity measurements determines the lateral location and the direction of drilling in the neighboring borehole.

1 . Apparatus for guiding the drilling of a borehole in the earth in spaced relationship to a guide borehole in the earth, comprising: 
 a coil assembly incorporating an elongated core carrying a longitudinal coil winding located in the guide borehole;    a source of current connected to said coil to produce a magnetic field in the earth surrounding the guide borehole and in the region of the borehole being drilled;    sensors in said borehole being drilled for detecting vectors of gravity and of the generated magnetic field; and    a controller responsive to the generated magnetic field and the measured magnetic field vectors to control the direction of further drilling.    
   
   
       2 . The apparatus of  claim 1 , where the coil assembly core is generally tubular with a longitudinal axis and incorporates diametrically opposite external ridges parallel to said axis for receiving and positioning said coil winding.  
   
   
       3 . The apparatus of  claim 2 , wherein said coil includes multiple windings positioned by said ridges to lie in a plane that includes said axis.  
   
   
       4 . The apparatus of  claim 3 , wherein said coil passes around a first, distal end of said core, and passes over the exterior of said core to cause the second, near end of the core to extend out of the coil.  
   
   
       5 . The apparatus of  claim 4 , wherein said near end of said core is extendable to permit said coil to be positioned at any location in said guide borehole.  
   
   
       6 . The apparatus of  claim 5 , wherein said core includes internal grooves for receiving and guiding instrumentation along the length of the core.  
   
   
       7 . The apparatus of  claim 6 , wherein said instrumentation includes an inclinometer for measuring the roll angle of said coil.  
   
   
       8 . The apparatus of  claim 7 , wherein said instrumentation is a survey instrument for surveying said guide borehole.  
   
   
       9 . The apparatus of  claim 5 , further including spaced straps for securing said coil winding on said core.  
   
   
       10 . The apparatus of  claim 4 , further including a cable connected to said winding for connecting said winding to said source of alternating current.  
   
   
       11 . The apparatus of  claim 10 , wherein said controller includes a computer for iteratively calculating theoretical magnetic fields produced by said coil and comparing the theoretical fields with magnetic fields measured by said sensors for determining the location of said sensors with respect to said coil.  
   
   
       12 . A method for controlling the drilling of a borehole with respect to a guide borehole, comprising: 
 drilling a guide borehole;    obtaining a survey of said guide borehole to determine its path;    inserting in said guide borehole a coil assembly including a tubular, nonmagnetic core and at least one winding wound externally on said core in a plane parallel to the longitudinal axis of the core;    energizing said core with an excitation current to produce a corresponding magnetic field;    partially drilling with a drilling tool a second borehole near said guide borehole;    detecting the produced magnetic field at the drill head of the drilling tool;    iteratively comparing the detected magnetic field with theoretical magnetic fields calculated from said current and the location of said coil assembly to determine the location of the drill head with respect to said coil assembly; and    guiding the further drilling of said second borehole in accordance with the calculated location of the drill head.