Radioactivity logging is the only known method of making accurate lithologic records through the casing and cement. The radioactivity of the strata is determined by measuring the variations in conductivity produced by gamma rays in the gas in an ionization chamber contained in the subsurface instrument. It has been found that each formation tends to produce a curve of characteristic shape on the radioactivity logs, and that some of these characteristic curves are highly persistent laterally.

At present, the radioactivity logs are used chiefly to determine the position of the producing sands behind the casing, in order that the pipe may be perforated to drain them most effectively, and the process has been highly successful in this use. Other applications consist in determining the amount of sample lag, making correlations and cross sections, mapping subsurface structure for deeper drilling, and surveying potash deposits. A similar process may be used in making radioactivity surveys at the surface and in mines.

A method has also been developed for determining the radioactivities of cores and samples which has proved its value in interpreting the logs and in solving problems of sedimentation.