Patent ID: 6985814

Claim:
A method for surveying a borehole, the method comprising: (a) providing a downhole tool including first and second magnetic field measurement devices disposed at corresponding first and second positions in the borehole, the first and second positions selected to be within sensory range of magnetic flux from a target subterranean structure; (b) measuring local magnetic fields at the first and second positions using the corresponding first and second magnetic field measurement devices; (c) processing (1) the local magnetic fields at the first and second positions, and (2) a reference magnetic field, to determine a portion of the local magnetic fields attributable to the target subterranean structure; (d) generating interference magnetic field vectors at the first and second positions from the portion of the local magnetic fields attributable to the target subterranean structure; and (e) processing the interference magnetic field vectors to determine a tool face to target angle at each of the first and second positions, the tool face to target angles representing a corresponding direction from each of the first and second positions to the target subterranean structure; and (f) processing the tool face to target angles at the first and second positions to estimate a local change in direction of the borehole relative to the target subterranean structure.