Theory And Technology For Measurement Of Soil Water Content By Time Domain Reflectometry Surface Reflections

Soil water content is an important parameter for soil engineering properties assessment and compaction quality control in geotechnical engineering. Time domain reflectometry (TDR) is becoming a soil misture mearsuring technique. It has been shown to be a reliable, fast and safe technology.Commonly used information in TDR signals includes the apparent dielectric constant and the electrical conductivity. The apparent dielectric constant is generally measured by analyzing the travel time of electromagnetic waves reflected from the end of the soil probe. Soil water content was obtained by measuring the apparent dielectric. In soils with high electrical conductivities, the attenuation of the signal can eliminate the reflection from the end of the probe which limits the application of TDR to these materials. This primary objective of this study is to explore the use of TDR technology to identify water content of soils with high electrical conducitivities. Two methods were present to determine dielectric constants using TDR measurements. They make use of information of contained in the TDR signal from the reflection at the surface of the soil rather than the reflection from the end of the probe, one is analyzing in frequency domain, and the other is analyzing in time domain.The frequecncy spectral analysis method employs the numerical model for the propagation of TDR signals in nonuniform transmission line embedded in adispersive material. The system parameters are the first determined from a calibration process. After the system is calibrated, a two parameter dielectric model is used to estimate the dielectric constant of soils by matching the redicted surface reflection versus the measured signal.The time domain analysis method correlates the reflection coefficient of the soil surface with the dielectric constant of the soil. Due to the multiple reflection and dielectric dispersion, the reflection coefficient in the TDR signal is not represent the true values of reflection coefficient at the mismatch interfaces in the transmission line. A special designed probe with 375mm air gap and a coefficient-corrected method were used to get the true reflection coefficient of the soil surface. The dielectric constant of the soil can be calculated with the known reflection coefficient and a calibration parameter. Results indicate that the dielectric constant can be determined with reasonable accuracy by the proposed approach for soils with high electrical conductivity, where the conventional travel time analysis fails due to significant signal attenuation.