The technique of making measurements in drill holes with probes designed to measure the physical and chemical properties of rocks and their contained fluids. Over a million holes are drilled annually around the world to probe for oil, gas, solid minerals, and fresh water, and to better understand the subsurface geology. Much information can be obtained from samples of rock brought to the surface in cores or bit cuttings, or from other clues while drilling, such as penetration rate; but the greatest amount of information comes from well logs.