A [[well log]] of the natural [[formation]] [[radioactivity]] level.

(1) In sediments the log mainly reflects clay content because clay contains the [[radioisotope|radioisotopes]] of potassium, uranium, and thorium. Potassium [[feldspar|feldspars]], [[volcanic]] ash, granite wash, and some salt deposits containing potassium (potash for example) may also give significant gamma-ray [[reading|readings]]. The log often functions as a substitute for the [[SP]] for [[correlation]] purposes in nonconductive borehole fluids in open holes, for thick carbonate intervals, and to correlate [[cased hole|cased-hole]] logs with [[open hole|open-hole]] logs.

(2) Used in exploration for radioactive [[mineral|minerals]].