In geothermal drilling operations, lost circulation of drilling fluid is a huge issue
That's because the downhole rocks are so brittle, their natural fractures alone cause significant fluid loss. This affects mud pulse technology (where data signals are sent from the bottom of the hole to the surface with the help of a continuous fluid column). If telemetry signals disappear because of a fluid loss, the operator is suddenly working "blind" since drilling data signals have been interrupted.
Mud pulse telemetry
This is the most common method of data transmission used by MWD (Measurement While Drilling) tools. Downhole a valve is operated to restrict the flow of the drilling mud (slurry) according to the digital information to be transmitted. This creates pressure fluctuations representing the information. The pressure fluctuations propagate within the drilling fluid towards the surface where they are received from pressure sensors. On the surface, the received pressure signals are processed by computers to reconstruct the information. The technology is available in three varieties - positive pulse, negative pulse, and continuous wave. wikipedia