Magnetic surveying techniques
The azimuth of the borehole is determined from the Earth's magnetic field using a magnetic survey tool.
Two basic types of tools exist, i.e. conventional photo-mechanical and solid state tools. For conventional tools the length of NMDC should be such that the interference from the steel drillstring is acceptable. For solid state survey instruments, a shorter length of NMDC can be used and the magnetic interference from the drillstring measured and corrected for.
Magnetic interference from adjacent wells or objects cannot be corrected for and will produce erroneous azimuth . . .