Shallow sounding of crustal regions using electromagnetic surface waves

Abstract

A specific physical interpretation of the effect of relative permittivities εr 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 εr, σ, frequency ƒ, and radial distance ρ over which the formulas apply are shown to be quite broad. The E1ρ component (the radial electric field in region 1, characterized by complex wave number K1 and adjoined at a planar interface to region 2, characterized by k2, where |k1|2 » |k1|2) is discussed in detail. The attenuation of 20 log10 |E1ρ| as a function of ρ and ρ/δ2, Where δ is the skin depth, is illustrated for conductivity ratios σ2/σ1 = 10−1 to 10−3, over a range of ƒ from 1 Hz to 105 Hz. |E1ρ| is shown to be significant for determining permittivities ε2 at the higher frequencies in this range. The use of |E1ρ| to infer conductivities σ2 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 log10 |E1ρ| 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.