1. n. [Perforating]

The point on a gun assembly or perforating string that is used when correlating depth. With use of a surface depth reference or datum point, any convenient point may be used, providing it is applied accurately and consistently. For downhole correlation on wireline applications, the gun zero point will relate to the correlating device, such as the casing collar locator (CCL) or gamma ray log tool. For nonwireline applications, the gun zero point will relate to the locating device, such as the tubing-end locator. The correlation process is crucial to a successful perforating operation; and therefore the importance of ensuring correct placement of the gun assembly prior to firing cannot be overstated.

See: casing collar locator (CCL),  gamma ray log,  tubing-end locator