Subterranean target location by measurement of time-varying magnetic field vector in borehole

Claim 1

A method of locating, from a homing tool location, the direction to a target underground location, comprising: drilling a borehole at a location spaced from a target underground location; lowering into said borehole a homing tool having a longitudinal axis which is aligned with the axis of said borehole and incorporating a magnetic field sensor having an axis of maximum sensitivity aligned with said homing tool axis; producing a time-varying current flow on the target which provides a corresponding time-varying magnetic field at the homing tool location in said partially drilled borehole; measuring, by means of said magnetic field sensor, the component of said time-varying magnetic field which is parallel to said homing tool axis; and determining from said measured time-varying magnetic field component the direction to the target location from said borehole.

Claim 2

A method of locating, from a homing tool location, the direction to a target underground location, comprising: drilling a borehole at a location spaced from a target underground location; lowering into said borehole a homing tool having a longitudinal axis which is aligned with the axis of said borehole and incorporating a magnetic field sensor having an axis of maximum sensitivity aligned with said homing tool axis; producing a time-varying current flow on the target which provides a corresponding time-varying magnetic field at the homing tool location in said partially drilled borehole; measuring, by means of said magnetic field sensor, the component of said time-varying magnetic field which is parallel to said homing tool axis; and determining from said measured time-varying magnetic field component the direction to the target location from said borehole.

Claim 8

A method of locating from a relief borehole the direction to a target well which is to be intersected by the relief well, comprising: drilling a relief borehole at a location spaced from a generally vertical target well, said relief borehole being directed generally horizontally toward the target well to intersect therewith at a large angle of intersection; lowering into said relief borehole a homing tool having a longitudinal axis which is aligned with the longitudinal axis of said relief borehole, said homing tool incorporating a single magnetic field sensor having an axis of maximum sensitivity aligned with said homing tool axis; injecting a time-varying current into the earth surrounding the target well to produce in the target well a time-varying current I which provides a corresponding time-varying magnetic field H which is generally horizontal at the homing tool location in the partially drilled relief borehole; measuring by said magnetic field sensor, the component Ha of said time-varying magnetic field which is parallel to said homing tool axis; determining the phase relationship between said measured component H a and said time-varying current I; determining from the magnitude of said meaured component Ha of said time-varying magnetic field the angle D between the longitudinal axis of said homing tool and the target well in accordance with the relationship Ha=I·sin D/2πR, where r is the vectorial distance betwenen the target well and the homing tool; determining from said phase relationship whether the angle D is to one side or the other of said relief borehole; continuing the drilling of said relief well in accordance with the determination of said angle D and said phase relationship.

Claim 9

A method of locating from a relief borehole the direction to a target well which is to be intersected by the relief well, comprising: drilling a relief borehole at a location spaced from a generally vertical target well, said relief borehole being directed generally horizontally toward the target well to intersect therewith at a large angle of intersection; lowering into said relief borehole a homing tool having a longitudinal axis which is aligned with the longitudinal axis of said relief borehole, said homing tool incorporating a single magnetic field sensor having an axis of maximum sensitivity aligned with said homing tool axis; injecting a time-varying current into the earth surrounding the target well to produce in the target well a time-varying current I which provides a corresponding time-varying magnetic field H which is generally horizontal at the homing tool location in the partially drilled relief borehole; measuring by said magnetic field sensor, the component Ha of said time-varying magnetic field which is parallel to said homing tool axis; determining the phase relationship between said measured component H a and said time-varying current I; determining from the magnitude of said meaured component Ha of said time-varying magnetic field the angle D between the longitudinal axis of said homing tool and the target well in accordance with the relationship Ha=I·sin D/2πR, where r is the vectorial distance betwenen the target well and the homing tool; determining from said phase relationship whether the angle D is to one side or the other of said relief borehole; continuing the drilling of said relief well in accordance with the determination of said angle D and said phase relationship.