Open-hole logging is the process by which the oil company lowers measuring devices into a drilled well to measure rock properties and fluid properties to determine if oil or gas is present in commercial quantities. This is done at the end of a well but before the production casing is placed in the well. The tools look like a “log” and the process of recording the data in the olden days was referred to as “logging the well.” The tools are extremely sophisticated and run in the well by specialists trained to do nothing but this. The oil company hires a logging company to provide this service. The oil company geologist works closely with the logging company engineer to interpret the recorded data. This a bit like running an “EKG” on a well to check out the “heart.” The data is recorded digitally and is presented on long sheets of paper looking like squiggly lines, much like an EKG or a lie detector test. Companies that provide this service are Schlumberger (industry leader), Halliburton, Baker Hughes, Weatherford International, Precision Well Logging, and several others.