Estimating the Attenuation of ELF-Band Radio Waves in the Earth’s Crust by Q-Bursts

Q-bursts are extremely low-frequency (ELF) wave packets spawned by exceptionally powerful lightning strokes. As the Earth’s surface bears finite conductivity, ELF-band radio waves can also be detected under the ground. Q-bursts detected in the magnetic field near the surface and at a depth of 140 m were compared to characterize signal attenuation in the Earth’s crust. The applied analysis incorporates data from permanent ELF measuring sites and the World Wide Lightning Location Network so that simultaneous surface and underground measurements were not needed. The analysis in the 2–140 Hz frequency band yielded
$\sim \!70 ~\Omega \mathrm{m} $
bulk resistivity. The corresponding skin depths, 1470, 1100, and 920 m for the lowest three Schumann resonance (SR) modes at 7.8, 14.1, and 20.0 Hz, respectively, were found to decrease at a rate proportional to the inverse of the square root of the frequency. A well conductive clay-rich ground layer under the surface can explain the obtained relatively low resistivity compared to known values of local andesitic bedrock. Incorporating independently obtained near-surface ground resistivity values in the evaluation of magnetotelluric (MT) surveys is suggested to improve results for the upper crust layers. The advantages of using Q-bursts in surveying the properties of the Earth’s crust by electromagnetic (EM) methods are pointed out.