Gamma-ray measurements detect variations in the natural radioactivity originating from changes in concentrations of the trace elements uranium (U) and thorium (Th) as well as changes in concentration of the major rock forming element potassium (K). Since the concentrations of these naturally occurring radioelements vary between different rock types, natural gamma-ray logging provides an important tool for lithologic mapping and stratigraphic correlation. Gamma-ray logs are important for detecting alteration zones, and for providing information on rock types. For example, in sedimentary rocks, sandstones can be easily distinguished from shales due to the low potassium content of the sandstones compared to the shales.

In sedimentary rocks, potassium is, in general, the principal source of natural gamma radiation, primarily originating from clay minerals such as illite and montmorillonite. In igneous and metamorphic geologic environments, the three sources of natural radiation may contribute equally to the total gamma radiation detected by the gamma probe. Often in base metal and gold exploration areas, the principal source of the natural gamma radiation is potassium, because alteration, characterized by the development of sericite (sericitization), is prevalent in some of the lithologic units and results in an increase in the element potassium in these units. The presence of feldspar porphyry sills, which contain increased concentrations of K-feldspar minerals, would also show higher than normal radioactivity on the gamma-ray logs. During metamorphism and hydrothermal alteration processes, uranium and thorium may be preferentially concentrated in certain lithologic units.

For details on natural gamma ray logging in volcanic rocks, see Mwenifumbo and Killeen, 1987.

A gamma-ray probe's sensor is usually a sodium iodide or cesium iodide scintillation detector. Unlike a total count gamma-ray probe, the spectral gamma-ray probe measures the energy of each gamma ray detected. K, U and Th emit gamma rays with characteristic energies, so estimates of the concentrations of the three radioelements can be made.

Potassium decays into two stable isotopes (argon and calcium) which are no longer radioactive, and emits gamma rays with energies of 1.46 MeV. Uranium and thorium, however, decay into daughter- products which are unstable (i.e. radioactive). The decay of uranium forms a series of about a dozen radioactive elements in nature which finally decay to a stable isotope of lead. The decay of thorium forms a similar series of radioelements. As each radioelement in the series decays, it is accompanied by emissions of alpha or beta particles or gamma rays. The gamma rays have specific energies associated with the decaying radionuclide. The most prominent of the gamma rays in the uranium series originate from decay of ²¹⁴Bi (bismuth), and in the thorium series from decay of ²⁰⁸Tl (thallium).

Because there should be an equilibrium relationship between the daughter product and parent, it is possible to compute the quantity (concentration) of parent uranium (²³⁸U) and thorium (²³²Th) in the decay series by counting gamma rays from ²¹⁴Bi and ²⁰⁸Tl respectively, if the probe has been properly calibrated (Killeen, 1982).

While the probe is moving along the hole, the gamma rays are sorted into an energy spectrum and the number of gamma rays in three pre-selected energy windows centred over ⁴⁰K, ²¹⁴Bi and ²⁰⁸Tl peaks in the spectrum are computed each second, as is the total gamma-ray count. These four numbers represent gamma rays originating from potassium, uranium, thorium and Total Count (TC) detected during that one second of counting time.

These data are recorded along with the depth and are displayed on the chart recorder to produce gamma-ray spectral logs. The raw gamma-ray spectral logs (Total Count log, K log, U log and Th log) provide more information than a non-spectral (gross count) log, and it is possible to convert them to quantitative logs of K, U and Th concentrations. This requires that the probe be calibrated in model boreholes with known concentrations of K, U and Th such as the models constructed by the GSC at Bells Corners near Ottawa (Killeen, 1986).

Because gamma rays can be detected through steel, logging can be done inside drill rod or casing with a slight decrease in sensitivity.

The GSC R&D logging system utilizes gamma-ray spectral data acquisition equipment similar to that found in modern airborne gamma-ray spectrometers. Full 256-channel gamma-ray spectra over an energy range of approximately 0.07 to 3.0 MeV are recorded from a scintillation detector in the probe. Scintillation detectors of different materials, and of different sizes are used by the GSC. These include:

The probe (and detector) selection is determined by the hole diameter. The largest diameter probe that will safely fit in the borehole will maximize the count rate and provide the best counting statistics.

For smaller probes, the higher density (higher efficiency) materials are chosen. (These are also higher cost). If the count rate is too low due to the extremely low concentrations of K, U and Th, as is often the case in limestones for example, it is not possible to produce K, U and Th logs. In that case only the Total Count log, which is the count rate of all gamma rays above a preselected threshold energy (usually 100 KeV or 400 KeV), is produced. A number of factors determine the logging speeds and sample times during the acquisition of gamma-ray data. The critical factors are the anticipated levels of radioactivity and the size of detector in the probe. Gamma-ray spectral logging is usually done at 3 m/minute but can be done as fast as 6 m/minute or as slow as 0.5 m/minute for more detailed information. The volume sampled is about 0.5 cubic metres of rock surrounding the detector, at each measurement (i.e. 10 to 30 cm radius depending on the rock density).