Water quality monitoring. Communicate back to the surface with ultrasonic pulses.

And in case people think ultrasonic signaling isn’t a real thing, it very much is. In the oil and gas industry, it’s how measurements taken while drilling a hole are sent back to the surface through the drilling mud.

	▲	doodlebugging 2 hours ago \| parent [-]
		Downhole tools used in MWD/LWD do not use ultrasonic frequencies. Drilling is a very noisy environment. The first tool that I used employed a downhole pulser with a surface transducer and operated at about 0.5-1 Hz. That's not ultrasonic. It was a pretty low frequency signal. Data transmission rates limit the volume of information that can be transmitted from the downhole tool to the surface where they are decoded. Your tool has to set up a pressure wave in the mud column that is detectable miles away from the tool while the well is being drilled. This is a good recent paper authored by an experienced drilling engineer about MWD/LWD drilling history and pulser designs and data rates that can be expected. https://www.aade.org/application/files/1917/4604/2319/AADE-2...