Very Low-Frequency Electromagnetic Method: A Shallow Subsurface Investigation Technique for Geophysical Applications

Abstract

The very low-frequency (VLF) electromagnetic (EM) method is the simplest EM method to delineate shallow subsurface conducting structures. Since the approach utilizes signals transmitted from worldwide transmitters located in coastal areas in the 5–30 kHz frequency band, it is suitable to depict conducting structures up to 200 m depth in highly resistive terrain. Freely and readily available primary field signals anywhere around the Earth make the VLF method very convenient and efficient for field data collection. Further, VLF data processing using digital linear filtering is quite accurate and very efficient in depicting the qualitative information about subsurface conductors, even though quantitative interpretation of VLF data is as complex as other EM data interpretation. In the present study, various aspects of the VLF method such as basic theory, worldwide VLF transmitters, quantities measured, and interpretation procedures are discussed in detail. Finally, the efficacy of the VLF method for groundwater investigation, mineral investigation, and landslide and subsurface pollution monitoring studies has been demonstrated. Even though the VLF method is a rapid technique for subsurface investigation, use of complementary geophysical methods such as gravity, direct current (DC) resistivity, self-potential, radiometric, etc., reduces the ambiguity in the interpretation and yields reliable subsurface information.