Quantitative evaluation of uranium ore zones

A logging method is described for quantitatively evaluating the abundance of uranium in ore zones in the vicinity of earth formations penetrated by a borehole, comprising: (a) obtaining measurements in the borehole of the intensities of the natural gamma ray activity of the formation in at least two energy bands of a gamma ray energy spectrum. The first band encompassing gamma rays at energies in an energy range from substantially 200 KeV to substantially 500 KeV where photoelectric absorption in uranium is significant and photoelectric absorption from other downhole elements is not significant. The second band encompassing gamma rays at energies in an energy range greater than substantially 800 KeV where photoelectric absorption is not significant for any elements, including uranium, and (b) forming a ratio of the measurements of intensities then comparing the ratio with a predetermined mathematical function to derive a quantitative indication of the uranium concentration present from the ratio of the measurements in the two energy bands and recording the quantitative indication as a function of the borehole depth of the measurement system