Positive electrification on rock surface in field observation and laboratory experiments

We have observed the vertical electrostatic field using a large plate antenna under ground. This observatory space (12 m x 15 m x 2.4 m) locates middle of a mine-gallery penetrating tuff and andesite bedrock in northeast Japan. This plate antenna (4 m x 4 m) is placed 1.2 m above the floor with insulator pillars and connected to the floor through a recorder with 1-MØmega impedance. When seismic waves reached the room floor, the antenna often detected the electric field upwards. One of the potential mechanisms is the positive electrification of the floor. Here, we inspected such electrification using air-dried brocks of igneous rocks in laboratory. As results, the rock samples that were pressed partially also showed the positive electrification on the surface of the unstressed volume. Because the rock samples were air-dried and normally good insulators against electrons, the assumption of positive holes is reasonable to explain the electrification. They can be activated from the deformation of positive hole pairs that are populous lattice defects also known as peroxy bonds. The activated positive holes in the stressed volume can spread away into the unstreesed volume through the mineral's valence band network. In case of the underground detection, the seismic wave would activate positive holes near the underground room floor.