A mud logger, or mud logger in the modern oil field drilling operation determines positions of hydrocarbons with respect to depth, identifies downhole lithology, monitors natural gas entering the drilling mud stream, and draws well logs for use by oil company geologists. Rock cuttings circulated to the surface in drilling mud are sampled and analyzed. (Under some circumstances compressed air's employed as a circulating fluid. And although most commonly used in petroleum exploration, mud logging… (More on Mudlogger)