1. n. 
A calibrated wheel used to drive the depth recording system in wireline logging. The wheel is pressed against the logging cable as the cable is spooled onto the drum and therefore turns as the cable is run in and out of the borehole. After zeroing the depth on surface, the depth wheel provides the depth input to the recording system. Small errors in calibration and slippage can cause the wheel to introduce systematic errors in the depth recorded. For this reason, the depth is checked and corrected using depth marks. The depth wheel is also referred to as a depth encoder. Modern encoders have two wheels so that slippage can be detected by differences between the two measurements.
See: depth mark