The density/SGG logging tool measures rock density and SGG ratio. The SGG ratio (defined below) is related to the effective atomic number of the rock, which depends on the chemical composition of the rock. The SGG ratio log is particularly useful for detecting base metals; these elements have high atomic numbers compared to major rock-forming minerals, and they can occur in high enough concentrations to increase the effective atomic number of the rock significantly . The SGG ratio log may also be useful for lithologic mapping in areas where the iron content differs significantly among different rock types.

The density of rock is affected by porosity, water content and composition. Most of the density variations within igneous and metamorphic rocks are due to variations in mineralogical composition. Rocks with higher percentages of mafic minerals (Fe, Mg silicates) have higher densities than those with higher percentages of felsic minerals (Ca, Na, K, Al silicates). The presence of minerals containing heavy elements such as base metals increases the overall density of the host rock. In sedimentary rocks, density variations may be a result of differing degrees of compaction (induration) rather than changes in elemental composition.

In ore tonnage and reserve computations, one of the factors used is the specific gravity, and hence a knowledge of in-situ densities of the rocks may provide valuable information. The density log is also useful for locating fractures since open fractures intersected by the borehole often appear as low density zones on the density log (Wilson et al, 1989).

The density and SGG ratio (or heavy element indicator) logs are derived from the spectral gamma-gamma probe (Killeen and Mwenifumbo, 1988). The density/SGG tool is essentially a spectral gamma-ray logging tool with the addition of a weak (10 millicurie = 370 MBq) gamma- ray source (e.g. 60Co) on the nose of the probe. The tool has a 23 mm by 76 mm (0.9" x 3") cesium iodide detector which measures gamma rays from the source that are backscattered by the rock around the borehole.

Complete backscattered gamma-ray spectra are recorded in 1024 channels over an energy range of approximately 0.03 to 1.0 MeV. Density information is determined from the count rate in an energy window above 200 keV, while information about the elemental composition or heavy element content is derived from the ratio of the count rates in two energy windows (spectral gamma-gamma ratio, SGG): one at high energy (above 200 keV) and one at low energy (below 200 keV). When the density of the rock increases, the count rate in both windows will decrease due to the change in compton-scattered gamma rays reaching the detector. However, if there is an increase in the content of high-Z (atomic number) elements in the rock, the associated increase in photoelectric absorption (which is roughly proportional to Z⁵) will cause a significant decrease in count rate in the low energy window with a small change in the high energy window. Since the low energy window is affected by both density and Z while the high energy window is mainly affected by density, the ratio of counts in the high energy window to the counts in the low energy window can be used to obtain information on changes in Z. This ratio increases when the probe passes through zones containing high-Z materials. Thus the log can be considered as a heavy element indicator, and can be calibrated in some conditions to produce an assay tool for quantitative determination of the heavy element concentration in situ along the borehole, without resorting to chemical assaying of the core (Killeen and Mwenifumbo, 1988).

The SGG sample volume is smaller than for natural gamma ray logging since the gamma rays must travel out from the probe, into the rock and back to the detector. A 10 to 15 cm radius around the probe is "seen". Data are acquired at a logging speed of 6.0 m/minute, with a sample time of 1 second giving a measurement every 10 cm.