Using acoustics to measure the composition, density, and hardness of layers?

Question

I am an engineering student at Alfred University and some fellow classmates and I are doing a competition, but one of the tasks we need to complete is autonomously measuring the composition, density, and hardness of layers of overburden. I was thinking that using acoustics would be possible to accomplish these things. What follows is exactly what we have to do: "2.Create a digital core that contains the following information: The number of overburden layers in their test station A sequence of the layers in order from softness to hardest The thickness of each layer The digital core should result from information garnered using system telemetry and not via placing a ruler down the hole. Teams may not touch the layers to determine hardness." The reading has to be continuous. For example another way we could do this is measure the resistance on our auger as it tries to dig a hole. This would provide a continuous reading, but might not be accurate because our frame holding the auger may not be solid. The question is "can we use sound waves to measure the composition, density, and hardness of each layer?" The total overburden depth will be between 0.5 and 0.8 m. I don't know if this is a large enough volume to measure all of these things using sound waves.

Answer 1

Acoustics as in seismology could be used to solve for the velocity of the materials, which could then help you deal with the identification. Because density is part of the wave equation, you could work back out the value. However you will have some difficulty depending on the arrangement of the layering as lower velocity layers will not reflect the signal (assuming a source receiver offset is on the surface). Due to the thin nature of the layers you would need to work out the frequency response. It is my guess that you would need some very high frequencies or you might be better served by switching to ground penetrating radar. You would need to work out the frequency and wavelength sizes first, to see if it fits within the individual layer and then see what your capabilities are.

Best of luck.