Logging while drilling (LWD) is a technique of conveying well logging tools into the well borehole downhole as part of the bottom hole assembly.
LWD tools work with its (MWD) system to transmit partial or complete measurement results to the surface via typically a drilling mud pulser or other improved techniques, while LWD tools are still in the borehole, which is called "real-time data". Complete measurement results can be downloaded from LWD tools after they are pulled out of hole, which is called "memory data".
LWD technology was developed originally as an enhancement to the earlier MWD technology to completely or partially replace wire line logging operation. With the improvement of the technology in the past decades, LWD is now widely used for drilling (including geostring), and formation evaluation (especially for real time and high angle wells).