What does a Gamma Log primarily measure?

A Gamma Log primarily measures natural radiation emitted from the surrounding borehole material. This tool detects the gamma rays released by radioactive isotopes present in the geological formations, which are often a key indicator of certain lithologies or the presence of specific minerals. The intensity of the gamma radiation provides insights into the composition of the subsurface materials, which can help in identifying different rock types and understanding the distribution of minerals.

Understanding the material composition through gamma radiation is crucial for applications such as resource exploration and assessing environmental impacts. The data obtained can help geologists and engineers make educated decisions regarding drilling and groundwater management.

The other options pertain to different logging techniques. Temperature variations in aquifers are generally measured using dedicated temperature probes, while porosity is typically assessed through various methods, including density logging or sonic logging. The diameter of the borehole is determined through caliper logs rather than gamma logs. Each of these techniques serves different analytical purposes in subsurface investigations, but the primary function of a Gamma Log is distinctly tied to the measurement of natural radiation from the borehole materials.