Through-Wall Tank & Silo Fill-Level Gauging

A column of liquid is a heavy obstacle for signal: paths crossing below the waterline slow down and fade, while paths above it barely notice. The liquid surface itself reflects like a lid whose height moves. Together, those two effects read the fill level with nothing ever touching the tank.

In the physics: The liquid column is a massive dielectric body: paths crossing the vessel below the fill line acquire large excess delay and attenuation (water eps_r~80, lossy), paths above the line barely change. The air/liquid interface is also a strong specular reflector whose height moves the geometry of a reflected tap. Fill level maps monotonically to which CIR taps are delayed/attenuated.