1. n. [Formation Evaluation]
The depth of the first reliable reading of a curve on a log. For the typical bottom-to-top survey, the curve readings before the tool is picked up from the bottom of the hole are not reliable--they are straight lines that do not represent the formation at the depth indicated. With several logging tools in a tool string, the first reading of each curve will be at a different depth, depending on the measure point of each tool.
Antonyms: last reading