What does porosity measure in rock or soil?

Porosity measures the bulk volume occupied by interstices or void spaces in a rock or soil sample relative to its total volume. This characteristic is crucial because it indicates the capacity of the material to store fluids, such as water or oil. When evaluating subsurface materials, understanding porosity can help in assessing how much groundwater can be stored and how easily it can flow through the material.

The other options, while related to the context of hydrogeology, do not accurately define porosity. The percentage of water in a sample refers to moisture content, which is a different measurement and not solely indicative of the void space present. Permeability, on the other hand, assesses how well fluids can move through the material, which is a separate but linked characteristic to porosity. Lastly, density pertains to the mass per unit volume of the material itself and does not directly reflect the amount of space within it.