Extremely low-frequency surface response caused by underground resource

Electromagnetic (EM) surface responses in the extremely low-frequency (ELF) band caused by possible underground resources are studied. Unlike traditional solutions, the authors apply the geodesic finite-difference time domain method to simulate ELF EM wave propagation in the whole Earth (including the crust)-ionosphere (E-I) system to study these responses. In this way, more rigorous analysis can be achieved by simulating the complex E-I environments, including the inhomogeneous crust and part of the ionosphere. Based on this method, they established a hypothetical model to study the effects of underground objects on received artificial signals. Through simulations, local surface responses and response perturbations caused by underground objects are studied and approximate formulas are obtained for predicting the behaviour of these responses. Finally, potential applications of the method and the results of this work are discussed.