Method of determining the location of a deep-well casing by magnetic field sensing

A method of locating a target borehole containing an electrically conductive material or object such as a casing, drill stem, or the like and for directing the drilling of a relief borehole to intersect the target borehole, comprising:

• partially drilling a relief borehole at a location spaced from said target borehole;
• lowering into said relief borehole at least one electrode adapted to inject current into the earth surrounding said relief borehole;
• connecting a source of low frequency current to said electrode, thereby producing a low frequency alternating electric current flow in the conductive material located in said target borehole, the current flow in said target borehole conductive material producing a target magnetic field;
• measuring at selected depth intervals the magnitude, phase and direction of the target magnetic field produced within said relief borehole by the electric current flow in the conductive material in said target borehole;
• measuring at said selected depth intervals the magnitude and direction of the earth's magnetic field;
• determining from said target borehole magnetic field measurements and said earth's magnetic field measurements at each selected depth interval the compass direction and distance of said target borehole;
• and
• further drilling said relief borehole so that it intersects said target borehole at a selected depth below the earth's surface.