The Penetration of Rock by Electromagnetic Waves and Audio Frequencies

This paper aims to throw some light upon the much discussed topic of radio penetration and the path of radio waves. Conclusive experimental evidence at the Mammoth Cave of Kentucky, free from all metallic conductors, confirms the hypothesis that radio waves penetrated through the groand and did not follow air openings or conductors. There is further evidence that audio-frequency magnetic fields are not greatly damped through an overburden to 300 ft. of sandstone and limestone having a resistivity of about 50,000 ohms per cm³.