Downhole Magnetic Measurement While Rotating and Methods of Use

A method for detecting a presence of external ferromagnetic material comprising:

• providing a triaxial vector magnetometer affixed to a drill string subassembly wherein the triaxial magnetometer is adapted to measure three orthogonal component magnetic fields;
• allowing the drill string subassembly to rotate;
• drilling a section of borehole;
• determining a plurality of average total magnetic fields along the section of borehole wherein the average total magnetic fields is as an average of the instantaneous vector magnitudes of the three orthogonal component magnetic fields during the step of allowing the drill string subassembly to rotate;
• and
• identifying the presence of external ferromagnetic material as a portion of the section where the average total magnetic fields vary from an expected Earth's magnetic field.

A method for assessing the operational quality of downhole instrumentation comprising:

• providing a triaxial vector magnetometer affixed to a drill string subassembly wherein the triaxial vector magnetometer is adapted to measure three orthogonal component magnetic fields;
• allowing the drill string subassembly to rotate;
• determining an average total magnetic field (BTwR) as an average of the instantaneous vector magnitudes of the three orthogonal component magnetic fields during the step of allowing the drill string subassembly to rotate;
• ceasing temporarily rotation of the drill string subassembly;
• measuring a stationary total magnetic field while the drill string subassembly is stationary so as to produce a stationary total magnetic field measurement;
• and
• comparing the average total magnetic field (BTwR) to the stationary total magnetic field measurement so as to determine a magnitude of error between the total magnetic field (BTwR) and the stationary total magnetic field measurement.

A device for determining an average total magnetic field value during drilling comprising:

• a drill string subassembly;
• a processor;
• a first vector magnetometer wherein the first vector magnetometer corresponds to an along-borehole axis wherein the first vector magnetometer is adapted to measure a first vector magnetic field component, the first vector magnetometer having an output communicatively coupled to the processor;
• a second vector magnetometer wherein the second vector magnetometer corresponds to an cross-borehole axis wherein the second vector magnetometer is adapted to measure a second vector magnetic field component, the second vector magnetometer having an output communicatively coupled to the processor;
• a third vector magnetometer wherein the third vector magnetometer corresponds to an cross-borehole axis wherein the third vector magnetometer is adapted to measure a third vector magnetic field component, the third vector magnetometer having an output communicatively coupled to the processor;
• wherein the first vector magnetometer, the second vector magnetometer, and the third vector magnetometer are affixed to the drill string subassembly;
• wherein the along-borehole axis is orthogonal to the cross-borehole axis of the second vector magnetometer and the third vector magnetometer, and wherein the cross-borehole axis of the second vector magnetometer is orthogonal to the cross-borehole axis of the third vector magnetometer;
• wherein the processor is adapted to determine the average total magnetic field value (BTwR) as an average of the instantaneous vector magnitudes of the first vector magnetic field component, the second vector magnetic field component, and the third vector magnetic field component.

A method for determining an average cross-axis magnetic field value during drilling comprising:

• providing a biaxial vector magnetometer affixed to a drill string subassembly wherein the biaxial vector magnetometer is adapted to measure two orthogonal component magnetic fields;
• allowing the drill string subassembly to rotate;
• and
• determining the average cross-axis magnetic field value (BCwR) as an average magnitude of the instantaneous vector sum of the two orthogonal component magnetic fields during the step of allowing the drill string subassembly to rotate wherein the total magnetic field value (BCwR) is computed according to the formula:
• B
• CwR
• =
• ∑
• i
• =
• 1
• n
• 
• (
• B
• x
• i
• 2
• +
• B
• y
• i
• 2
• )
• n
• ,
• wherein Bxi and Byi each correspond to a cross-borehole axis.