Complex Dielectric Permittivity of Soil–Organic Mixtures (20 MHz–1.3 GHz)

Abstract

The dielectric permittivity of soils can be significantly modified by the presence of organic contaminant in the pore fluid. Thus, nondestructive techniques based on the propagation of electromagnetic waves may be used to detect contaminant plumes and to evaluate decontamination processes. Ground penetrating radar (GPR) and time domain reflectometry are two of the most relevant geophysical tools working in the frequency range of interest here. In this work, the complex dielectric permittivity of some soil–organic mixtures are measured in the frequency range of 20 MHz to 1.3 GHz. Tests are conducted in samples of silica sand, loess, and kaolinite mixed with varied amount of paraffin oil and lubricant oil. Additional tests are performed in soil–water samples for comparison. Mixtures formulas reported in the literature are extended from two to three and four phases in order to model the measured dielectric response of the contaminated soil samples. The results allow us to study the effect of the volumetric liquid content, organic type, mineral composition, and specific surface of soil particles on the dielectric permittivity of the mixtures. It is concluded in this work that the value of dielectric permittivity in soils is sensitive to the detection of contaminants when the organic concentration is high. On the other hand, the organic content can be determined providing that the total volume of fluid in the pores is known. The detection limits of organics in soils are discussed. Finally, a contamination process is monitored with GPR at the frequency of 1 GHz in a laboratory cell. The results show that organic contaminants are easily detected in dry sand, yet detection becomes very difficult in wet sand.