A method for surveying a borehole is provided. The method includes providing a
tool having a magnetic field measurement device disposed thereon and positioning
the tool in a borehole. Magnetic interference vectors are determined at at least
two positions in the borehole by comparing the measured magnetic fields at those
positions with a known magnetic field of the earth. The magnetic interference vectors
indicate a direction to a target subterranean structure. Various embodiments of
the invention compare the directions to the target subterranean structure with
a historical survey thereof, so as to determine a distance between the borehole
and the subterranean structure and an azimuth of the borehole. The surveying methodology
of this invention may advantageously improve borehole surveying data obtained,
for example, in relief well and/or well twinning drilling applications.