Final answer:
A neutron porosity log evaluates the porosity of geological formations using neutron logging. It is distinct from tools that measure natural gamma radiation, which help determine lithology, rather than porosity.
Step-by-step explanation:
The answer to the student's question, 'Which of the following best describes a neutron porosity log?' is C) Evaluates the porosity of formations using neutron logging. A neutron porosity log is a well logging tool used primarily in the oil and gas industry to determine the porosity of a geological formation. This technique involves emitting neutrons into the formation and then detecting the resulting gamma rays that are scattered back. The amount of scattering is related to the hydrogen atoms present, which are mostly found in the pore spaces filled with fluids like oil, gas, or water.
Natural gamma radiation logs, on the other hand, are useful for determining lithology, such as distinguishing between clay and sand layers, but they do not evaluate porosity directly. Tools analyzing lithostratigraphy, such as those mentioned in Figure 11.14 from the IODP expedition, are different from neutron porosity logs but can contribute to a broader understanding of the geological context and physical properties of the sediment or rock.