A well log of the natural formation radioactivity level.

(1) In sediments the log mainly reflects clay content because clay contains the radioisotopes of potassium, uranium, and thorium. Potassium feldspars, volcanic ash, granite wash, and some salt deposits containing potassium (potash for example) may also give significant gamma-ray 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 logs with open-hole logs.

(2) Used in exploration for radioactive minerals.