Patent ID: 6937023

Claim:
A method for determining the location of a target subterranean structure from within an adjacent borehole, said subterranean structure generating magnetic flux, the method comprising: (a) providing a downhole tool including first and second gravity measurement devices and 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 the subterranean structure, the first and second gravity measurement devices constrained from rotating with respect to one another about a substantially cylindrical borehole axis; (b) measuring first and second gravity vector sets using the first and second gravity measurement devices, respectively; (c) processing the first and second gravity vector sets to determine a local borehole azimuth; (d) measuring a total local magnetic field at each of the first and second positions using the first and second magnetic field measurement devices; (e) processing the total local magnetic field, the local azimuth and a reference magnetic field to determine a portion of the total magnetic field attributable to the target subterranean structure at each of the first and second positions; (f) re-positioning the tool at new locus for first and second positions in the borehole so that the first and second magnetic field measurement devices remains within sensory range of the magnetic flux from the subterranean structure; (g) repeating (b), (c), (d), (e), and (f) for a predetermined number of cycles; (h) generating first and second interference magnetic field vectors for each locus for first and second positions, each of the interference magnetic field vectors corresponding to the portion of the total magnetic field determined in (e) at the corresponding locus; and (j) extending the interference magnetic field vectors to determine a location of the target subterranean structure.