Borehole logging is the practice of making a detailed record (a well log) of the geologic formations penetrated by a borehole. The geophysical logs are the physical measurements made by instruments lowered into the hole. Logging tools measure the natural gamma ray, electrical, acoustic, stimulated radioactive responses, electromagnetic, nuclear magnetic resonance, pressure and other properties of the subsurface and their contained fluids. Well logging is performed in boreholes drilled for: