Oil and gas field logging is the practice of making a detailed record of the geologic formations penetrated by a borehole. The log is based either on visual inspection of samples brought to the surface or on physical measurements made by instruments lowered into the hole. A logging procedure consists of lowering a logging tool on the end of a wireline into an oil well to measure the rock and fluid properties of the formation. An interpretation of these measurements is then made to locate and quantify potential depth zones containing oil and gas.