A [[well log]] of scattered [[gamma ray|gamma rays]], differing from the [[density log]] in that the [[gamma ray source|gamma-ray source]] and [[gamma ray detectors|detector]] are so spaced as to be sensitive to the [[density]] of material in the [[annulus]] between casing and formation. Used for distinguishing between cement and fluids behind casing. Can be run in liquid-filled or [[empty hole|empty holes]].