Abstract: An apparatus and method for passively determining the depth and thickness of a subterranean geologic formation bearing hydrocarbons, e.g., oil and/or gas, or commercially important ore deposits, e.g., precious metals. The apparatus has an antenna to pick up extremely low frequency signals naturally emanating from the Earth's surface. In a preferred embodiment, the received signal is amplified and filtered. The signal is then modulated onto a carrier wave. The modulated signal is then filtered to eliminate one of the sidebands, for example, the lower sideband. An oscillator generates a tuning frequency which is then beat against the filtered, modulated signal in order to tune to a particular frequency. The oscillator sweeps through the range of frequencies, e.g., in the upper sideband portion of the filtered, modulated signal.