Fun fact: magnetic resonance (a la hospital MRI machines) is also a common method of underground imaging. MRI is quite neat in that the time-space resolution t

Fun fact: magnetic resonance (a la hospital MRI machines) is also a common method of underground imaging. MRI is quite neat in that the time-space resolution tradeoff is extremely flexible, and you can get quite good pictures if you leave a cheap imager long enough. Instead of using massive superconducting magnets they just use the Earth's field. It's especially good for imaging underground water, as well as other hydrogen-containing materials like hydrates or hydroxides. I think there may be a DIY version on hackaday, but I know for a fact theres a project where someone took a DIY MRI of an apple. The hardware for a low-field MRI actually has a fair amount in common with GPR- the hydrogen atoms emit lowish frequency radio waves when they realign with the magnetic field. https://hackaday.io/project/5030-low-field-mri https://hackaday.io/project/5030-low-field-mri