Method and apparatus for calibrating and correcting for coherent noise in casing detection

A system comprising:

• a borehole assembly, comprising:
• an electric current driving tool configured to cause a current to flow across an insulated gap in a drill collar of the borehole assembly;
• and
• an internal magnetometer disposed within the drill collar;
• and
• data processing circuitry configured to remove all or part of a noise component of the magnetic field signals.

A method comprising:

• applying a current across a drill collar that surrounds an internal magnetometer, wherein the current travels across the drill collar and along one or more other current paths to return to the drill collar;
• measuring magnetic fields due to the current across the drill collar and the current across the one or more other current paths using the internal magnetometer, while the drill collar is disposed at a first angle of rotation;
• rotating the drill collar to a second angle of rotation;
• measuring the magnetic fields using the internal magnetometer while the drill collar is disposed at the second angle of rotation;
• and
• determining a relationship between the current across the drill collar and a portion measurable to the internal magnetometer of the magnetic field due to the current across the drill collar.

A system for calibrating a measurement sub comprising:

• an electric current driving tool configured to couple to the measurement sub and to apply a current across a drill collar of the measurement sub, wherein the measurement sub comprises an internal magnetometer;
• one or more conductive paths configured to enable the current to exit the drill collar of the measurement sub and to return to the electric current driving tool;
• and
• one or more resistive elements coupled to the one or more conductive paths and configured to adjust the amount of current flowing through each of the one or more conductive paths;
• wherein the magnetic field measurable to the internal magnetometer of the measurement sub is substantially entirely due to the current across the drill collar of the measurement sub.

A method of drilling a new well proximate to an existing cased well comprising:

• drilling into a formation having an existing cased well using a borehole assembly, wherein the borehole assembly includes a measurement sub housing an internal magnetometer;
• applying a current across an insulated gap of a drill collar on the borehole assembly using an electric current driving tool, such that a portion of the current enters the formation and travels along a casing of the existing cased well;
• measuring, while the borehole assembly is rotating, magnetic field signals capable of penetrating a drill collar of the measurement sub;
• and
• processing the measured magnetic field signals to determine a portion of the magnetic field signals arising due to the current traveling along the casing of the existing cased well.