Method and apparatus for monitoring the thickness of a coal?

Question

New advanced Method? Plasma Waves,

Answer 1

The thickness of a coal layer is typically measured by generating a seismic signal on the surface and measuring the response at other points on the surface, using accelerometers to gauge the response of the layer. Because wave propogation in coal differs from that in solid rock, the vibration profile provides information about the thickness and composition of the layers.

I didn't find any information to suggest that plasma waves are used for this purpose.