A student sent me an email the other day. He wondered what kind of low frequency signals would be picked up if he stuck two copper rods in the ground and connected them to a sensitive differential amplifier. It is an interesting question. Sure, you would expect a lot of signals associated with the 60Hz AC line frequency and maybe some odd order harmonics of 60Hz but would there be anything else? I know that some bedrock containing quartz can induce sizable earth ground currents during small earthquakes. Such a circuit might detect those kind of signals and perhaps distant lightning strikes as well. Also, suppose you injected a low frequency signal, such as 40KHz, into the ground and used that signal to monitor changes in the resistance between two probes in the ground. Could you detect footsteps or vehicular traffic between the probes, as the ground was compressed? Has anyone ever tried this? It could make for an interesting science fair project.
David A. Johnson, P.E. --- Consulting Engineer