Downhole logging while drilling. Logging While Drilling is one form of "measurement while drilling" - to differentiate it from mud logging, wireline logging, etc. See Logging While Drilling
George E. King Engineering, 5000 Oilfield Terms: A Glossary of Petroleum Engineering Terms, Abbreviations and Acronyms, US 2010. http://gekengineering.com/Downloads/Free_Downloads /Glossary_of_Petroleum_Engineering_Terms_25_August _2010.pdf, (Oil - Amer) {593}