Measurement While Drilling (MWD), also known as Logging While Drilling (LWD), is a measurement taken of the wellbore (the hole) inclination from vertical, and also magnetic direction from north. Using basic trigonometry, a three-dimensional plot of the path of the well can be produced.

Essentially, a MWD Operator measures the trajectory of the hole as it is drilled (for example, data updates arrive and are processed every few seconds or faster). This information is then used to drill in a pre-planned direction into the formation which contains the oil, gas, water or condensate. Additional measurements can also be taken of natural gamma ray emissions from the rock; this helps broadly to determine what type of rock formation is being drilled, which in turn helps confirm the real-time location of the wellbore in relation to the presence of different types of known formations (by comparison with existing seismic data).

Density and porosity, rock fluid pressures and other measurements are taken, some using radioactive sources, some using sound, some using electricity, etc.; this can then be used to calculate how freely oil and other fluids can flow through the formation, as well as the volume of hydrocarbons present in the rock and, with other data, the value of the whole reservoir and reservoir reserves.

An MWD downhole tool is also "lined-up" with the bottom hole drilling assembly, enabling the wellbore to be steered in a chosen direction in 3D space known as directional drilling. Directional drillers rely on receiving accurate, quality tested data from the MWD engineer to allow them to keep the well safely on the planned trajectory.

Directional survey measurements are taken by three orthogonally mounted accelerometers to measure inclination, and three orthogonally mounted magnetometers which measure direction (azimuth). Gyroscopic tools may be used to measure Azimuth where the survey is measured in a location with disruptive external magnetic influences, inside "casing", for example, where the hole is lined with steel tubulars (tubes). These sensors, as well as any additional sensors to measure rock formation density, porosity, pressure or other data, are connected, physically and digitally, to a logic unit which converts the information into binary digits which are then transmitted to surface using "mud pulse telemetry" (MPT, a binary coding transmission system used with fluids, such as, combinatorial, Manchester encoding, split-phase, among others).

This is done by using a downhole "pulser" unit which varies the drilling fluid (mud) pressure inside the drill-string according to the chosen MPT: these pressure fluctuations are decoded and displayed on the surface system computers as wave-forms; voltage outputs from the sensors (raw data); specific measurements of gravity or directions from magnetic north, or in other forms, such as sound waves, nuclear wave-forms, etc.

Surface (mud) pressure transducers measure these pressure fluctuations (pulses) and pass an analogue voltage signal to surface computers which digitize the signal. Disruptive frequencies are filtered out and the signal is decoded back into its original data form. For example, a pressure fluctuation of 20psi (or less) can be “picked out” of a total mud system pressure of 3,500psi or more.

Downhole electrical and mechanical power is provided by downhole turbine systems, which use the energy of the “mud” flow, battery units (lithium), or a combination of both.