I know very little about audio engineering. I wonder what else I might be able to use to pick up the vibration signature?
I completely understand your drive to pick up those high-frequency vibrations! There's a whole secret world of vibrations out there, and we can analyze it!
Amplifying and digitizing the signal is straightforward for an experienced EE, but you might look into acquiring some Raspberry Shake hardware which has it all already done.
Could you use the accelerometer on a smart phone to 'hear' the vibrations?
However, what you're saying is completely legit. I read that I might want to lay a large, sturdy thing on the floor and put the mic on that, for the same reason that you gave. I used a large old pane of glass that I found in the cellar.
What do seismographs do? People also use acoustic pings to measure soil and rock density underground. What are those devices using for their “microphone”?
And yeah, would an accelerometer work? I don’t know what the temporal resolution is on your standard Amazon / AliExpress accelerometer is but it’s probably pretty decent?