A passive geophysical prospecting method and apparatus are provided. The apparatus has an antenna for detecting electromagnetic radiation naturally emanating from the Earth's surface and generating an electrical signal from the detected electromagnetic radiation while traversing the Earth's surface by a vehicle traveling over land or water or in an airplane. The electrical signal is filtered of frequencies below 65 Hz, preferably above 100 Hz and more preferably below 800 Hz, and above 12,000 Hz, preferably above 8,000 Hz, to generate a filtered signal. The filtered signal is converted to a prospecting voltage signal. The prospecting voltage signal is then compared to a set voltage and a difference signal is generated, recorded and analyzed.