1. n. [Formation Evaluation]

The measurement versus depth or time, or both, of one or more physical quantities in or around a well. The term comes from the word "log" used in the sense of a record or a note. Wireline logs are taken downhole, transmitted through a wireline to surface and recorded there. Measurements-while-drilling (MWD) and logging while drilling (LWD) logs are also taken downhole. They are either transmitted to surface by mud pulses, or else recorded downhole and retrieved later when the instrument is brought to surface. Mud logs that describe samples of drilled cuttings are taken and recorded on surface.

See related terms: real-time data,  recorded data

2. n. [Formation Evaluation]

The display of one or more log measurements on a strip of paper or film (a hard copy) with depth in one axis. In this sense, the term refers to the display not only of the measurement but of other relevant information. A typical log is presented on folded paper of indeterminate length, but about 8.5-in. [21.5-cm] wide. It consists of a heading, well sketch, logging tool sketch, insert, main log, repeat section and tail. When the term is used in this sense, each log measurement is usually referred to as a curve.