Surveying of subterranean magnetic bodies from an adjacent off-vertical borehole

A method of directional subsurface drilling of an off-vertical relief well borehole to intersect an adjacent well having remanent magnetization, comprising the steps of:

• running magnetic field sensing apparatus into the relief well borehole, stopping at a location therein sufficiently far from the adjacent well that the magnetic field existing about the well is not substantially detected and measuring components of the earth's magnetic field along three mutually perpendicular axes;
• determining relief well borehole azimuth and slope angles with referenct to the earth's magnetic field;
• lowering said magnetic field sensing apparatus and simultaneously measuring components of the total magnetic field along three mutually perpendicular axes at a plurality of spaced locations of defined separation along the relief well borehole, with one of the measured components being that which is in the direction of the relief well borehole;
• determining the direction to the adjacent well by measuring three components of the total magnetic field, subtracting the measured component values of the earth's field and resolving the remaining quantities of the components into a resultant vector;
• determining the gradient of the remanent magnetic field intensity in the direction of the relief well borehole over defined portions of the relief well;
• determining the range from a location along the relief well borehole to the adjacent well using the measurements of the component total magnetic field intensity in the direction of the relief well borehole and the gradient of the remanent magnetic field intensity in the direction of the relief well borehole;
• and
• orienting the course of drilling of the off-vertical relief well borehole based upon the range and direction of the adjacent well from the relief well borehole.

A method of surveying the range to a subterranean body having remanent magnetization from an adjacent borehole comprising the steps of:

• measuring the component of the total existing magnetic field in the direction of the borehole at a plurality of locations along the borehole;
• determining the gradient of the magnetic field emanating from the subterranean body in the direction of the borehole;
• and
• determining the range of the subterranean body from one of said plurality of locations along the borehole using the gradient determination and the measured component of the magnetic field.

A method of directional subsurface surveying from a borehole to locate a subterranean ferromagnetic target, comprising the steps of:

• measuring the components of the earth's magnetic field along orthogonal axes at a first location in the borehole sufficiently remote from the target to be unaffected by any magnetic field of the target;
• measuring components of the total magnetic field along orthogonal axes at a second location in the borehole sufficiently proximate the target to detect the magnetic field of the target superimposed upon the earth's field;
• determining the direction of the superimposed magnetic field of the target from the second location using the measurements of the components of the total magnetic field and the measurements of the components of the earth's magnetic field;
• measuring the component of the total field in the direction of the axis of the borehole at a plurality of locations in the borehole to determine the gradient of the target field in the direction of the axis of the borehole;
• and
• determining the range of the target from one of said plurality of locations using the measurements of the component of the total field in the direction of the axis of the first borehole.

A directional subsurface surveying from a borehole to locate a subterranean ferromagnetic target, comprising the steps of:

• measuring the components of the earth's magnetic field along orthogonal axes at a first location in the borehole sufficiently remote from the target to be unaffected by any magnetic field of the target;
• measuring components of the total magnetic field along orthogonal axes at a second location in the borehole sufficiently proximate the target to detect the magnetic field of the target superimposed upon the earth's field;
• determining the direction of the superimposed magnetic field of the target from the second location using the measurements of the components of the total magnetic field and the measurements of the earth's magnetic field;
• measuring one component of the total field at a plurality of locations along the borehole to determine the rate of change of the target field with distance down the borehole;
• and
• determining the range of the target from one of said plurality of locations using the measurements of the component of the total field in the direction of the axis of the first borehole.

A magnetic field sensing apparatus for disposition in a borehole to locate a subterranean magnetic body exhibiting a static magnetic field, comprising:

• an outer housing or non-magnetic material;
• a pair of mutually perpendicular radially oriented magnetic field sensors disposed within said housing;
• a pair of aligned axial magnetic field sensors spaced apart a predetermined separation within said housing and oriented perpendicular to said radial magnetic sensors;
• said axial and radial magnetic sensors measuring components of the magnetic field along orthogonal axes.

A survey system for determining the range and direction to a subterranean ferromagnetic body exhibiting a magnetic field from a location within an adjacent subterranean borehole;

• which comprises:
• subsurface magnetic field sensing apparatus for disposition in the adjacent borehole, said apparatus having magnetic sensors for measuring components of a magnetic field along orthogonal axes including a pair of aligned magnetic sensors oriented to make measurements of a component of the magnetic field in the direction of the borehole;
• and
• surface instrumentation operably coupled to said subsurface magnetic field sensing apparatus for determining range and direction from said apparatus to the subterranean body.

An apparatus for disposition in a borehole to locate a subterranean electro-conductive body exhibiting a time-varying electric field comprising:

• an electric field potential probe for sensing the potential gradient of a time-varying electric field of a predetermined frequency;
• a frequency selective amplifier coupled to said electric field potential probe;
• and
• first and second magnetic field sensors having axes of maximum sensitivity perpendicular to one another and perpendicular to the axis of the borehole, said sensors being responsive to the earth's field.

A method of directional subsurface surveying from a borehole to locate a subterranean ferromagnetic target, comprising the steps of:

• a. establishing a time-varying magnetic field about said ferromagnetic target;
• b. detecting said time-varying magnetic field at a plurality of locations in the borehole to measure the intensity of said field at the locations;
• c. determining the intensity gradient of said time-varying field from the measurements of magnetic field intensity;
• d. determining the range of the target from one of said plurality of locations using the field intensity measurements obtained in step b and the intensity gradient determined in step c;
• and
• e. determining the direction to the target from the detection of said time-varying magnetic field made in step b.