Electrical conductivity problems associated with time-domain reflectometry (TDR) measurement in geotechnical engineering

The effect of electrical conductivity (EC) of a dielectric material on its TDR-measured dielectric constant ε_a (called the apparent dielectric constant) was investigated both theoretically and experimentally. The effect of EC on the apparent dielectric constant ε_a was more pronounced at low frequency of the electromagnetic pulse applied to measure ε_a. The assumption of neglecting electrical losses in measuring the dielectric constant by TDR to represent ε_r was not valid for dielectric materials containing free charges, such as electrolyte solutions, bentonite clay, and saline soils. A theory is proposed to separate the real part of the dielectric constant ε_r from the TDR-measured ε_a in lossy dielectrics. An EC - ε_r relationship can be developed that would provide ε_a (=ε_r) at EC = 0 to be used to determine soil-water contents in conductive soils. Na-bentonite mixed with distilled water resulted in considerable magnitude of bulk EC due to the adsorbed cations on the surface of the particles. This EC caused energy loss of the TDR pulse that depended on the length of the probe rod. For a 4.5 cm long probe, the reflected TDR waveform was completely attenuated for EC between 3.05 dS m⁻¹ and 3.72 dS m⁻¹. The waveforms generated in this range of EC were not suitable for analysis by TDR-support software for dielectric constant. With the adsorbed cations, Na-bentonite behaved as a lossy or dispersive material. This revised version was published online in July 2006 with corrections to the Cover Date.