THE electrical conductivity structure of the Earth may be explored by measuring the electric and magnetic field variations induced by magnetic storms. At any point on the surface of the Earth five parameters that vary with time may be observed: the northward (X), eastward (Y) and vertically down (Z) magnetic field components and the northward (A) and eastward (B) surface electric field components. In the magnetotelluric (MT) method only the horizontal components X, Y, A and B are observed, whereas in the geomagnetic depth sounding (GDS) method only the magnetic components X, Y and Z are observed. As these methods are equivalent only under certain special circumstances, there is usually an advantage in observing all five parameters.