Determination of electrical conductivity using time domain reflectometry: Soil and water experiments in coaxial lines

The simultaneous measurement of water content and electrical conductivity of soils and KCl solutions was achieved using time domain reflectometry (TDR). Coaxial transmission lines varying in length from 90 to 300 mm contained either KCl solutions or soil of varied water and salt content. The water content of soil or dielectric constant of the water solutions was determined from the travel time. The measured dielectric constant of KCl solutions was unchanged from that of pure water (81) at those concentrations where there was sufficient reflected signal for measurement. Two analyses were used for determination of electrical conductivity, one based on signal attenuation after one “round-trip” and the second based on a thin sample approximation for the signal reflection and attenuation. Reference measurements of conductivity were made on the same samples using low-frequency conductance bridge measurements. These analyses of the TDR traces showed that for water solution both the thin sample analysis and the analysis after a signal had traversed one round-trip yielded conductivity in agreement with bridge conductivity values. This indicated that the imaginary part of the complex dielectric constant was negligible. For soils the thin sample analysis was in general agreement with the bridge measurements. From the analysis of signal after one round-trip in soils there was indication that the imaginary part of the dielectric constant should not be assumed negligible. Further investigation of the frequency dependence of the dielectric constant and attenuation will be required to identify the relative contributions of the real and imaginary parts of the dielectric constant to measurement by TDR. The effect of impedance-matching transformers on conductivity measurements in the field has yet to be ascertained.