Abstract: An apparatus and method for passively determining physical characteristics of subterranean geological formation which for example bear oil or gas or commercial important ore deposits such as precious metals. The apparatus includes an antenna for acquiring low frequency signals naturally emanating from the earth. The received signal is first put through a low pass filter and buffer and then converted from analog to digital, stored in a memory buffer, converted to a frequency spectrum by a Fourier transform, and then further processed to display geophysical information versus the depth of such discontinuities.