We are trying to design a metal mineral deposit detection system using low frequency radio waves. I am not sure on the physics of why the ultra low frequency electromagnetic radiation can penetrate the earth and sea. We need to be able to scan to a depth of 3 miles from the surface.
Wikipedia classifies ELF (Extremely Low Frequency) radiation as between 3 Hz and 300 Hz, and ULF (Ultra Low Frequency) from 300 Hz to 3 kHz. Both ranges can penetrate the earth, but I don't know which would be better for our application. Which frequency would yield the most precise, accurate and reliable results for this depth?