Non-magnetic Drill Collar
As the name suggests, "Non-Magnetic Drill Collar" is a drill collar made of a non-magnetic material. If magnetic survey instruments are run in the hole, they encounter magnetic interference from regular drill collars which causes errors in measurements of the earth's magnetic field. Hence "Non Magnetic Drill Collars" of sufficient length need to be run as part of the Bottom Hole Assembly for accuracy of survey readings. Non Magnetic Drill Collars are made of austenitic stainless steel and are usually non-spiral.