Build a DIY magnetometer with a couple of seasoning bottles

Hmmm.

> the listening circuit must also be tuned to resonate at the expected frequency of proton precession, which will depend on Earth’s magnetic field at your location

> the frequency of these tones matches the magnetic field at my location to about 1 percent

I don’t doubt the physics, but I’m not sure about the experiment design. Being able to hear the correct frequency may just mean you’ve built an oscillator and tuned it.

If I may recommend, replace output LM386 stage with any dual opamp (e.g. another NE5532 or TL072, slightly different schematic of course), they can drive 32 ohm headphone speakers without issue and have significantly (~100x) lower white noise.

I want to see pictures of the device and ideally a video of it in action. It would be stimulating.

Could one use something like this from the surface to detect steel submerged under 20-40 feet of water?

How is this different from the magnetometer accessible in a phone through and app like Phyphox?

Note the first comment.