Small cheap electric field detectors could provide emergency warning of an about-to-collapse mine, according to Stuart Hoenig at the University of Arizona. Working under a National Science Foundation grant, the electrical engineering professor is working to develop detectors that could be placed throughout mines to signal when the wall or roof was about to give way.
Transient lunar events, flashes of light seen around the moon, gave him the first idea that the electrical charge of a rock under stress could be used to detect dangerous weakness in mine walls. Under stress, moisture-bearing rock generates a charge. This happens as moisture is forced from an area of compression toward an area of tension— the area where the rock is about to fall. The water carries with it metal ions, atoms with some electrons missing. The ions make the area under tension positively charged.