Objectives   Transmitting extreme low frequency (ELF) signals with a high signal-noise ratio (SNR) to cover Chinese territory and major territorial sea areas can provide a radiating signal service for trial and exploratory research projects such as underground resource sounding, seismic prediction, etc.

  Methods   This paper explains the primary principle of horizontal low-profile antennae in radiating ELF signals, introduces ELF transmission system composition, analyzes key technologies such as ELF power amplification, horizontal antennae tuning, detection and control of strong power electromagnetic fields, composite lightning-proof measures, etc., and designs and constructs the first-ever civilian ELF transmission station in the world, through which long-range field tests and trials with a coverage of thousands of kilometers are accomplished.

  Results   The test and trial results indicate that ELF signals can cover Chinese territory and major territorial sea area 10-20 dB higher than natural signals, with a sounding depth of up to 10 km.

  Conclusions   The ELF transmission station can provide a brand-new open public platform for research on geological exploration and seismic prediction, thereby contributing to the development of primary disciplines in China including low-frequency radioactivity, space physics, geophysics, etc.