July 30, 2024
The cable gate provides an easy way to align cable marks to the top of the casing. With the pulley assembly, depth marks are typically aligned with the cleats at the top of the pulley, which is 0.3m or 1 foot above the top of the casing.
Metric Depths:
- AT system: When the cable mark shows 10 m, the probe is 10 m down hole.
- Classic system: When the cable mark shows 10 m, the probe is only 9.7 m down hole.
English Depths:
- AT system: When the cable mark shows 30 feet, the probe is 30 feet down hole.
- Classic system: When the cable mark shows 30 feet, the probe is only 29 feet down hole.
Summary: If classic surveys are not corrected for the pulley offset and the middle of the probe measurements, plots will show displacements a full interval deeper than they actually are.