Thermal neutron capture gamma rays have been observed in boreholes drilled in shales, sandstones, and limestones. A capsuled source of neutrons and a scintillation crystal detector, connected through 5000 ft of logging cable to a transistorized, multi-channel, pulse-height analyzer, were used. Resolved peaks were identified on the basis of the known energies of expected gamma rays and results obtained in models where conditions of porosity, casing, and fluid were controlled. To properly interpret borehole spectral data a system with good energy resolution and an accurate means of energy calibration are necessary. This is accomplished by using H and Fe to give prominent gamma-ray peaks at opposite ends of the energy range of interest. On field spectra, identification was made of gamma rays from Cl, Si, Ca, H, and Fe. On the basis of Cl gamma rays, salt water can be differentiated from oil or fresh water. Gamma rays from Fe casing are an undesirable background and reduce the sensitivity of the method compared to that possible in an uncased hole. Two examples of natural gamma-ray spectra show well resolved lines from U-Ra and Th.

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