TABLE OF CONTENTS A significant number of researchers have extensively investigated the dielectric properties of earth materials. They have shown experimentally that for most materials which constitute the shallow sub-surface of the earth, which in this case is taken to be a zone of depths of 100 m or less, the attenuation of electromagnetic radiation rises with frequency and that at a given frequency wet materials exhibit a higher loss than dry ones. From this generalisation a number of predictions can be made relating to the performance of a surface-penetrating radar system. Before this can be done it is necessary to understand those characteristics of materials which affect both the velocity of propagation and attenuation.
An order of magnitude indication of the basic dielectric characteristics of various materials can be gauged from Table 4.1, which shows their conductivity and relative permittivity measured at a frequency of 100 MHz. The velocity of propagation is primarily governed by the relative permittivity of a material, which depends primarily upon its water content. At low microwave frequencies, including the range over which surface-penetrating radar systems operate, water has a relative permittivity of ~80, while the solid constituents of most soils and man-made materials have, when dry, a relative dielectric constant ? r in the range 2 to 9. The measured values of ? r for soils and building materials lie mainly in the range 4 to 40. The absolute permittivity also varies with frequency, but is generally sensibly constant for most materials over...
