1. n. [Formation Evaluation]
The log used as the reference for depths in the well. Each log may record formation features at slightly different depths because of their different response and the difficulty of aligning depths. It is therefore important to select one log to which the other logs are depth matched and that is used as the reference for well-to-well correlation and mechanical operations such as perforating. The gamma ray is most often used as the base log because it can be recorded in both open and cased holes.