A magnetic mark placed on a logging cable as a reference for depth measurements. The marks are placed on the cable at regular intervals, usually 100 ft (30 m) or 50 m (164 ft), under a certain tension in a workshop. The intervals may change slightly as a function of tension downhole, but this change can be corrected for. During logging operations, the marks are detected by a magnetic mark detector, and then used to check and correct the depth read by the depth wheel.