Never tried but i would go for a geophone. The datasheet of this one https://www.sparkfun.com/products/11744 says that it has reasonable response along that range.

I got an infrasound microphone but all it did was demonstrate that the hum I can (barely) hear (and that my partner cannot hear) is real... and that indeed it doesn't go away when I cut power to everything within 500ft of my home (which I believed from my own observation, but wasn't completely sure of).

Get an audio spectrum analyzer app for your phone, and enable its log-X scale. Very useful for this sort of thing. The FFT will reveal sounds you don't hear yourself.

They are indeed cool, but no math magic can help you if your microphone is not picking up the signal. I don’t have datasheets for mobile phone microphones but lets look at some professional ones. The Senheiser MD 46 goes down to 40hz. The Behringer MC8000 goes down to 15hz. These are just two random samples I looked at to illustrate that its not obvious that a microphone will even have a good response down to 10hz.

Those ratings are 3-dB points, not brick walls. It doesn't need to have a good response down to 10 Hz (and it's safe to say that it won't).

The main drawback is that you may have to wait a while between updates.