Well logging refers to the process of collecting and recording data on the geologic formations encountered while drilling a borehole. This data is used to determine the characteristics of the formations and the properties of the fluids they contain, such as oil, gas, or water. Well logging can also help determine the location and size of hydrocarbon reservoirs in the subsurface. Synonyms for well logging include borehole logging, formation evaluation, well characterization, and subsurface imaging. These terms all describe the same process of gathering information about the earth's subsurface to better understand its geology and hydrocarbon reserves.