As we just saw, even with today’s advanced technologies of seismic imaging, it’s difficult for drill operators to know exactly what they'll encounter when digging an oil well. And until the 1980s, it was also difficult to know details about what was going on with the drill bit as the hole was being dug. This challenge was overcome by measurement-while-drilling (MWD) technology.
MWD allows operators to receive real-time information on the status of drilling, as well as the ability to steer the well in other directions. It relates information such as gamma rays, temperature and pressure, as well as the density and magnetic resonance of the rock formations. This serves a myriad of functions. It helps operators drill more efficiently while preventing blowouts and tool failures. It also helps operators show that they're not drilling into unauthorized areas.
What’s perhaps most amazing is how this information is transmitted up to the surface. Because it isn’t practical to string a wire or cable down the well from the surface to the drill bit, MWD instead relies on mud pulse telemetry. A mud slurry that is sent down into the well to carry debris back up (through the outer column of the well) provides a convenient acoustic channel to send mud pulses up in a binary code that is decoded on the surface.