Shallow sounding of crustal regions using electromagnetic surface waves

A specific physical interpretation of the effect of relative permittivities ε r and conductivities σ upon the amplitude and phase of propagating electromagnetic surface waves is given, where ϵr and σ characterize the layered material regions of a planetary lithosphere. The interpretation is made possible by the availability of simple formulas for the fields of vertical and horizontal unit electric dipoles near interfaces between regions of matter; the earth's lithosphere at its interface with salt water is used as an example. The ranges of ε r , σ, frequency ƒ, and radial distance ρ over which the formulas apply are shown to be quite broad. The E 1ρ component (the radial electric field in region 1, characterized by complex wave number K 1 and adjoined at a planar interface to region 2, characterized by k 2 , where |k 1 | 2 » |k 1 | 2 ) is discussed in detail. The attenuation of 20 log 10 |E 1ρ | as a function of ρ and ρ/δ 2 , Where δ is the skin depth, is illustrated for conductivity ratios σ 2 /σ 1 = 10 −1 to 10 −3 , over a range of ƒ from 1 Hz to 10 5 Hz. |E 1ρ | is shown to be significant for determining permittivities ε 2 at the higher frequencies in this range. The use of |E 1ρ | to infer conductivities σ 2 at lower frequencies in shallow sounding of the earth's sea floor is discussed in terms of specific lithological and sediment‐related parameters. The technology required to transmit and receive 20 log 10 |E 1ρ | at freshwater and saltwater interfaces with the earth's crust is addressed. A specific experiment is proposed for testing a specialized insulated antenna for seafloor use in shallow coastal waters at 1 kHz.