Summary Measurement while drilling (MWD) provides a means for monitoring drilling and formation parameters as drilling progresses. The technique, based on the use of advanced downhole sensor and telemetry technology, makes possible the real-time acquisition of data that was previously impossible without stopping the drilling previously impossible without stopping the drilling operation. Some downhole data, such as weight and torque on bit (WOB and TOB) can be obtained only while drilling. In Summer 1976, Teleco Oilfield Services began an experimental program to demonstrate the feasibility and usefulness of a multisensor MWD tool for real-time measurement of drilling mechanics, formation evaluation, and well-control parameters. A package incorporating six sensors was designed and implemented in a downhole tool capable of transmitting data to the surface by means of mud-pulse telemetry. In this paper we discuss the experimental 6-sensor tool and representative results obtained in a series of 12 field tests in various pails of the world, both onshore and offshore. Introduction The MWD tool was introduced in Aug. 1976 on a limited basis. It was designed to provide the directional-drilling industry with real-time measurements of hole deviation, azimuth, and tool facing. Full commercial directional service was made available in the Gulf of Mexico and the North Sea in Sept. 1978. In a parallel effort begun in 1976, an experimental system was built to demonstrate the broader potential of down hole MWD technology for enhancing the safety and efficiency of dulling while simultaneously obtaining formation-evaluation data. Two prototype MWD tools incorporating six downhole sensors and using mud-pulse telemetry were built. More than 1,200 hours of continuous mud-pulse data transmission were accumulated in the field test program. Telemetry System Mud-pulse telemetry is a wireless means of communicating data from downhole to the surface. Information in the form of binary bits is communicated through a sequence of pressure pulses generated in the mud column within the drill pipe by a valve located in a special sub near the bit. This information, in the form of pressure waves, propagates through the drilling mud and is detected and displayed at the surface. This technique imposes no restrictions on the drilling operation, is rugged, reliable, safe, and only marginally affected by mud composition or depth. So far, MWD technology has been applied primarily to directional surveying. Its use in other applications, such as formation evaluation and drilling efficiency, offers great promise to the oil and gas well drilling industry. In the balance of this paper, some of these applications are discussed on the basis of an extensive series of tests with the experimental six-sensor MWD tool. Additional discussions of MWD technology and applications are found in Refs. 1 through 4. The essential elements of the telemetry system consist of the transmitter and surface receiving equipment. JPT P. 899