Definition - What does Non-Mag (NMDC) mean?
Non-mag is a construction industry term commonly used instead of the full term "non-magnetic drill collars" (NMDC). Non-magnetic collars are used in every bottom hole assembly (BHA) involving magnetic surveys. The non-mag prevents the Earth's magnetic field from interfering with BHA parts that cannot degmagnetized. As the bottom hole assembly is an important aspect of a drill string in a drilling operation, non-magnetic drill collars are functionally essential.
Trenchlesspedia explains Non-Mag (NMDC)
A non-mag is another name commonly used for a non-magnetic drill collar (NMDC). To ensure the magnetic field is reduced to a minimum, the correct length of non-mag must be used. The bottom hole assembly (BHA) is the drill bit which is located at the bottom of the drill string. Moving up the drill string, the bottom hole assembly fits into the bit sub. Drill collars are found between the bit sub and the drill string.