Downhole magnetic measurement while rotating and methods of use

Downhole magnetic measurement devices and methods are provided for estimation of magnetic fields during drilling. In certain embodiments, a triaxial vector magnetometer 150 is provided and affixed to a drill string subassembly 125. While the drill string subassembly 125 is rotating, as during drilling operations, an average total magnetic field value (BTwR) is determined as an average of the instantaneous vector magnitudes of the three measured orthogonal component magnetic fields. In this way, near-bit 127 estimates of the downhole magnetic field may be obtained and used for a variety of functions. These downhole magnetic measurement devices may be useful determining magnetic fields, and estimating borehole inclination and azimuth of drill string subassemblies 125, 100 during drilling operations. Additionally, downhole magnetic measurement devices may be used for detecting the presence and distance to external ferromagnetic materials. In this way, these devices may be used for various applications, including, well avoidance, well intercept, and verification of instrument integrity.