Measurement While Drilling. Refers to a technique of making downhole measurements of borehole position, toolface orientation, formation parameters and drilling information using sensors located in the bottom hole assembly adjacent to the drill bit. These measurements are made during drilling and can be recorded downhole and/or transmitted to surface.
MEASUREMENT WHILE DRILLING. Used to assist in horizontal drilling. Computerized devices are placed in the drill string just above the bit. These devices are called MWD tools. MWD tools monitor the drill string's orientation and transmit this information in sonar-like pulses through the drilling fluid to the surface, allowing the driller to navigate the bit very precisely. A MWD system usually consists of three fully integrated assemblies: a pulser, a battery power pack and a sensor system.