Used to locate casing collars and other features of downhole hardware (e.g., packers, etc.) which often serve as reference depths in subsequent completion operations.

(1) a magnetic casing collar locator. Involves a system of two opposed permanent magnets in two similar magnetic circuits which produce characteristic magnetic fields in which flux lines pass through casing or tubing. A deformation of either of the magnetic fields, caused by the gap between casing joints, packers, sometimes holes, etc. is detected by a winding having a core of high permeability The resulting electromagnetic imbalance, first in one direction and then in another, is telemetered to the surface where it is recorded, depth correlated, as a feature of the downhole hardware arrangement.

(2) a mechanical collar locator. May involve feelers or fingers which produce [[signal|signals]] sent to the surface when the feelers cross pipe connections or other irregular features inside casing or tubing.