AUDIO-MAGNETOTELLURIC SURVEY

The audio-magnetotelluric (AMT) method uses natural electromagnetic fields to investigate the electrical conductivity structure of the earth. Natural sources of AMT fields above about 1 Hertz are from thunderstorms worldwide. The electrical discharges from thunderstorms radiate powerful electromagnetic (EM) fields that propagate great distances. It is assumed that the EM fields at the surface of the earth ACT almost like plane waves, with most of their energy reflected but with a small amount propagating vertically downward into the earth. The amplitude, phase, and directional relation between electric (E) and magnetic (H or B) fields on the surface depend on the distribution of electrical conductivity in the subsurface (Vozoff, 1991).

For this study, the four channel Geometrics EH-4 Audio-magnetotelluric (AMT) system was used with a controlled source to fill in the middle frequencies (450-4,500 Hz) where the natural signal strength is often below the detection limits of the instrument. This instrument is designed to use lightning or atmospheric disturbances (sometimes called sferics) as an energy source. In some cases, atmospheric disturbances were close enough to forgo the use of the controlled source. The instrument measures the electrical field in the X direction (Ex) and the magnetic field in the Y direction (Hy). It also measures the orthogonal field set (Ey and Hx).