I think, not from the surface, but have a look here [1], where the author referenced from the IIEE article has build a submergible sensor and detected (a know) boat.

[1] https://alexmumm.de/pgProtonMagMarine_en.htm

You learn a lot more making this.

The sensitivity When I play with phypbox [1] there is a sensitivity in the µT range. From the web page [2] the device build has a 0.1 nT resolution and 50 ppm absolute accuracy.

[1] https://phyphox.org/download/

[2] https://alexmumm.de/pgProtonMagnetometer_en.htm

The magnetometer in your phone is a MEMS sensor which measures mechanical deflection of a current-carrying element. The deflection is caused by the Lorentz Force, i.e. force induced by an electron current flow in a magnetic field (in this case, the earth's magnetic field).[1] The magnetometer in the linked article senses (EDIT: corrected, hopefully) oscillation in the magnetic field of protons, a result of Larmor Precession[2]. Remarkably, the oscillation frequency is proportional to the ambient magnetic field strength, and the frequency is in the audible range. The circuit works by rotating protons in the fluid so that their magnetic axis align, this results in a synchronised bulk magnetic field oscillation that is large enough to be sensed by a simple tuned amplifier circuit.[3]

Further, the magnetometer in your phone is a 3-axis device that measures the orientation of the magnetic field, whereas the magnetometer in the linked article detects only the strength of the magnetic field (in fact, is tuned to detect only a single strength/precession frequency).

[1] https://en.wikipedia.org/wiki/MEMS_magnetic_field_sensor

[2] https://en.wikipedia.org/wiki/Larmor_precession

[3] https://en.wikipedia.org/wiki/Proton_magnetometer

The schematic in the linked article shows an NE5532.

You can drive even 8 ohm headphones to unpleasantly loud levels with any opamp and a pair of transistors to beef up the output, along with a resistor to sort out the biasing. I did something like this as a headphone driver amp for "desktop mobile" radios used as part of a communications centre for a large festival. Motorola had a device that would do it, for about 500 quid each. I built the thing in the PDF at the bottom (I must have rerendered this at some point, it was definitely not done in 2022, more like 2012).

Using cheap bag-of-1000-for-a-fiver Chinese transistors off eBay I was able to get incredibly quiet output, to the point that I needed to add a muting gate because the radio was objectionably noisy. I notice that the exact transistors are not mentioned but any small-signal NPN and PNP ones will do - I used BC548 and BC558s, like I use in everything.

It will be way quieter and way more stable than an LM386.

Edit: I'm a lot better at drawing things in Kicad these days, and would have left the capacitors at the input a lot tidier.

https://onlyfandans.com/headphone.pdf

This channel has a more detailed coverage of what goes down in the field

https://m.youtube.com/playlist?list=PLJ04J62_rG0AwQ57wb8Rxp7...

This particular vid is a sort of FAQ

https://m.youtube.com/watch?v=wg4GSXtpQzQ