 Christofilos found that the lower the electrical conductivity of the underlying rock, the deeper the current will go, and the larger the effective loop area.Radio frequency current will penetrate into the ground to a depth equal to the "skin depth" of the ground at that frequency, which is inversely proportional to the square root of ground conductivity .The ground dipole forms a loop with effective area of , where is the total length of the transmission lines and is the skin depth.