1. n. [Geophysics]

Magnetic interference caused by nearby structures such as metallic rigs and wells. The magnetic permeability of drillstrings and the remanent magnetization in drillstrings contribute to perturbations of the measured magnetic field. Operators may use nonmagnetic drill collars to reduce these effects along with software techniques to compensate for them.

See: main magnetic field,  crustal magnetic field,  external disturbance field