Dielectric Properties of Soils of India at Radio and Microwave Frequencies: A Review

When electromagnetic (EM) energy is incident on a dielectric material, some part of energy is reflected, some is transmitted and rest is absorbed by the material. All these phenomenon of reflection, transmission and absorption are represented in terms of dielectric properties of that materials. As most of the soils have negligible trace of magnetic material so permeability of soil is almost same as that of free space. However, permittivity which results due to interaction of electrical energy with material is different than that of free space [1]. Thus, for soils, dielectric properties arise only due to electrical part of EM energy and are represented in the form of complex permittivity. Complex permittivity has two parts, out of which real part is called dielectric constant and imaginary part is called dielectric loss factor. Dielectric constant represents the energy storage in a dielectric medium whereas dielectric loss factor represents dissipation of EM energy. Dielectric properties of soils are important in the field of geophysical exploration, agriculture and remote sensing as these directly affect the performance of electromagnetic sensors. Soil vary from region to region due to variation in texture, temperature, demography and environment so area wise dielectric properties of soil differ significantly and depend upon frequency, moisture content, bulk density, salinity. India has a wide variety of soils and numerous researchers have studied dielectric properties of many states of India. However, due to difference in technique used, frequency of interest and soil type, it is difficult to generalize dielectric behaviour of soils of India as a whole. Out of numerous available techniques, some popular techniques are coaxial probe method, free space method, cavity perturbation method, parallel plate method and waveguide method Choice of dielectric property measurement depends upon many factors like frequency of interest, nature of the material to be investigated, required accuracy and precision etc. [2]. Existing methods can be broadly divided into two categories namely Time Domain Reflectometry (TDR) and Frequency Domain methods. In this review, we present a systematic state wise analysis of all reported dielectric data on soils of India at radio and microwave frequencies. Dielectric data of states of Gujarat, Rajasthan, Maharashtra, Karnataka, Haryana, Chhattisgarh are plotted as a function of moisture content as enough studies have been performed by different researchers for these states. Dielectric constant of soils of Haryana and Rajasthan was found to be higher than other states. However, reported values of dielectric loss factor were higher for soils of Chhattisgarh in comparison to other states of India. It is difficult and less reliable to compare dielectric data of a particular state/region with other if single or limited number of samples are studied for that particular state. So, enough number of representative samples and correct sampling procedure is necessary to study and compare dielectric properties any region over a particular frequency range.