1. To allow current to pass in one direction only. 2. To adjust a borehole log for true vertical depth. Logs in slant holes are usually recorded in distance measured along the hole, which makes them difficult to correlate with other logs. Also sometimes used for adjusting a log to what would be seen without formation dip. 3. To replace an inverted image with an erect one. 4. To eliminate time differences caused by weathering, elevation, or normal-moveout differences.