Surface Impedance Mapping Using Sferics

Naturally occurring radio emissions from discrete sferics in the frequency range 500 Hz to 30 kHz have the potential for shallow conductivity profiling beneath the surface of the earth. A dual-channel time-domain receiver was constructed and used over selected geophysical targets. The instrumentation was validated through a comparison with very low-frequency surface impedance measurements from a distant navigation transmitter and 2-D modeling using the impedance method. The multifrequency results were then verified using this 2-D modeling. The skin effect shows the frequency dependence of electromagnetic field strength dissipation as a signal enters a multilayered earth. Single-frequency methods offer fixed depth conductivity profiles whereas multiple frequencies reveal multiple depth conductivity profiles that can help with 3-D modeling of subsurface features and anomalies. Several electromagnetic techniques offer multiple-frequency operation capitalizing upon this effect however these techniques generally use a local artificial signal source, which leads to near-field distortion effects in the measured data. This method shows significant promise for cost-effective, high-speed, ground-level surface impedance measurements targeting subsurface features down to 100 m.