Final answer:
A natural gamma ray log tracks the radioactive properties of formations (option D) and is used in geology to differentiate rock types by their gamma radiation emissions. High gamma counts suggest clay or shale presence, while lower counts indicate sands and silts.
Step-by-step explanation:
The natural gamma ray log best describes the option D: Tracks the radioactive properties of formations. This tool is used primarily in the field of geology and is essential for conducting subsurface investigations in oil and gas exploration, as well as in hydrogeology. It measures the gamma radiation emitted by the natural isotopes within the rock formations and is displayed as counts per second (CPS). Gamma ray logs help geologists ascertain the types of rocks present, particularly differentiating between clays and shales versus sands and silts based on the gamma count. High counts typically indicate clay or shale, whereas low counts point to silt and sand.
In exercises such as correlating sediment cores, natural gamma radiation is critical for comparing the lithology between different cores. When you correlate cores using this data, you're essentially matching patterns that indicate layers with similar gamma radiation profiles. This information contributes to understanding the sedimentary environment and geo-history of the area being studied.