Laboratory measurements of RF complex permittivity have been made on a variety of 'rocks' encountered in mining, tunneling, and engineering works. An RF impedance bridge and a parallel-plate capacitance test cell were employed at frequencies of 1, 5, 25, and 100 Mhz. The results predict that low-loss propagation will be possible in certain granites, limestones, coals, and dry concretes. Existing VHF mining radar equipment should be capable of exploring into such rocks to distances of up to hundreds of feet.Useful but shorter probing distances are predicted for other coals, gypsums, oil shales, dry sandstones, high-grade tar sands, and schists. Radar probing distances of less than 10 ft are predicted for most shales, clays, and fine-grained soils. Uncombined moisture content is evidently the governing factor. Efforts were made throughout the experiments to preserve or simulate the original moisture content of the 'rocks' in place.