Electromagnetic orientation system for deep wells

1. Apparatus for target detection from a borehole being drilled, comprising:

a drill string having multiple drill pipe sections connected end to end and carrying a drill bit;

at least one of said drill pipe sections being electrically conductive to provide a drill pipe electrode section;

at least one electrically insulating drill pipe sub electrically isolating said electrode section from adjacent drill pipe sections;

a power supply in electrical communication with said at least one drill pipe electrode section and energizable to inject a time varying current into Earth formations surrounding said borehole to cause current flow in a target;

an alternating electromagnetic dipole source in said drill string whose magnetic axis is substantially perpendicular to said drill string, for producing an auxiliary magnetic field;

a sensor instrument at said drill bit for detecting magnetic fields produced by said current flow in a target and by said dipole source to determine a rotational orientation of the direction of drilling with respect to the direction to said target; and

communication electronics located in said drill string for establishing communication between said sensor instrument and surface instrumentation for sending data to said surface instrumentation.

2. The apparatus of claim 1 , wherein said dipole source is a solenoid mounted in said drill string.

3. The apparatus of claim 1 , wherein said dipole source comprises electrical coils mounted in said drill string.

4. The apparatus of claim 1 , wherein said magnetic fields produced by said injected current and by said dipole source have different frequencies.

5. A method for target detection from a borehole being drilled, comprising:

connecting multiple drill pipe sections end to end to form a drill string carrying a drill bit, wherein at least one of said drill pipe sections is electrically conductive to provide a drill pipe electrode section;

electrically isolating an electrode section of said drill string from adjacent drill pipe sections;

supplying power to said electrode section to inject a time varying current into Earth formations surrounding said borehole to produce a target magnetic field;

locating a dipole source in said drill string to produce an auxiliary alternating electromagnetic field whose magnetic axis is perpendicular to said drill string;

detecting the magnetic fields produced by said injected current and by said dipole at a sensor instrument at said drill bit to determine a rotational orientation of said sensor instrument with respect to said target; and

establishing communication between said sensor instrument and surface instrumentation for sending detected magnetic field data to the surface instrumentation.

6. The method of claim 5 , including mounting a solenoid in said drill string to produce said dipole source.

7. The method of claim 5 , including mounting electrical coils in said drill string to produce said dipole source.

8. The method of claim 5 , wherein producing said injected current and said dipole magnetic fields includes supplying said injected current and supplying energizing current to said dipole at different frequencies.

9. A method for determining the direction of drilling a borehole with respect to the direction to a target location, comprising:

positioning a dipole source on a drilling assembly to produce a first alternating magnetic field having an axis substantially perpendicular to the borehole;

producing a second alternating magnetic field at the target location;

measuring the first and second alternating magnetic fields at a drill bit instrumentation sub;

measuring an angular orientation of the dipole source with respect to the angular orientation of the drill bit instrumentation sub; and

determining, from said first and second fields and the angular orientation of the dipole source with respect to the angular orientation of the drill bit instrumentation sub, the direction of drilling with respect to the direction to the target.

10. The method of claim 9 , further including:

energizing the dipole source to produce a magnetic field having a first frequency; and

producing said second magnetic field at a second frequency.

11. Apparatus for determining the azimuthal direction to a target location, comprising:

a drilling assembly including a drill bit instrumentation sub in a borehole;

a dipole source on the drilling assembly to produce a first alternating magnetic field having an axis substantially perpendicular to the borehole, said dipole source having a known azimuthal orientation with respect to said drilling assembly;

a second alternating magnetic field produced at the target location; and

sensors in the drill bit instrumentation sub for detecting vector components of the first and second alternating magnetic fields, whereby the azimuthal direction from the drill bit instrumentation sub to the target is determined from said first and second fields and the azimuthal orientation of the dipole source with respect to the drill bit instrumentation sub.

12. The apparatus of claim 11 , wherein said first and second magnetic fields have different frequencies.

13. The apparatus of claim 11 , wherein said dipole source is mounted on a drilling motor sub above and near said drill bit instrumentation sub.

14. The apparatus of claim 11 , wherein said drilling assembly includes a drilling motor sub, and wherein said dipole source is mounted on said drilling assembly above the drilling motor sub.

15. The apparatus of claim 14 wherein said dipole source is incorporated in a measurement while drilling package.