1. n. [Drilling]

A common and inexpensive measurement of the natural emission of gamma rays by a formation. Gamma ray logs are particularly helpful because shales and sandstones typically have different gamma ray signatures that can be correlated readily between wells.

See: signature

2. n. [Formation Evaluation]

A log of the total natural radioactivity, measured in API units. The measurement can be made in both openhole and through casing. The depth of investigation is a few inches, so that the log normally measures the flushed zone. Shales and clays are responsible for most natural radioactivity, so the gamma ray log often is a good indicator of such rocks. However, other rocks are also radioactive, notably some carbonates and feldspar-rich rocks. The log is also used for correlation between wells, for depth correlation between open and cased hole, and for depth correlation between logging runs. The gamma ray log was the first nuclear well log and was introduced in the late 1930s.

See: cased hole,  corrected gamma ray,  depth correlation,  depth of investigation,  flushed zone,  logging run,  natural gamma ray spectroscopy,  openhole,  potassium,  scintillation detector,  thorium,  uranium