Abstract: The locations of compositional discontinuities in salt deposits and the like are determined by measuring the travel time of electromagnetic waves reflected off of boundary surfaces between underground geological regions of differing mineralogical characteristics. A spark transmitter is used to generate pulses each composed of a plurality of cycles of electromagnetic waves. The electromagnetic waves have a frequency between 1 and 500 MHz. The pulses have a pulse duration of 0.1 to 1 microsecond and a pulse repetition frequency of 10 to 1000 Hz. The thusly generated pulses of electromagnetic waves are radiated into the solid body of a mountain or other geological structure from within the interior of an underground excavation, so that the transmitted radiation will be reflected back from one or more boundary surfaces with different respective travel times. The reflected-back radiation is received and recorded. The travel times are measured, in order to determine the locations of the boundary surfaces.