Development of a TDR dielectric penetrometer

Electrical properties of a soil include the electrical conductivity and dielectric permittivity. Conventional electrical probes measure only the resistivity (the reciprocal of the electric conductivity) of the soil. Interpretation of the resistivity alone for soil physical properties is difficult because it is sensitive to many factors, such as water content, soil types and ground water characteristics. The Time Domain Reflectomerry (TDR) is a geophysical test method based on electromagnetic waves. It can be used to make simultaneous measurements of electrical conductivity, apparent dielectric constant, and dielectric constant at different frequencies of a soil. The dielectric properties provide extra information related to soil physical properties. Cur-rent TDR probes can only apply to soils at surface. The paper describes the development of a TDR cone penetrometer that is capable of providing continuous TDR measurements during the cone penetration. Various probe configurations were experimentally studied. The data reduction method for the determination of apparent dielectric constant and electrical conductivity has been formulated and calibrated. The region of influence around the probe and the effect of penetration on TDR measurements are also studied.