A Reevaluation of TDR Travel Time Estimation in Soils and Geological Media.

Time domain reflectometry (TDR) is an established method for the determination of apparent dielectric permittivity and water content in soils. Using current waveform interpretation procedures, signal attenuation and variation in dielectric media properties along the transmission line can significantly increase sampling error in estimating the time, t₂, at which the pulse arrives at the end of the probe. Additionally, manual adjustment of waveform analysis parameters is frequently required in current software to accommodate changes in media properties when processing large time series of TDR measurements. An algorithm entitled “adaptive waveform interpretation with Gaussian filtering” (AWIGF) that circumvents difficulties with current methods was developed and evaluated. The algorithm filters signal noise using Gaussian kernels with an adaptively estimated standard deviation based on the maximum gradient of the reflection at the termination of the probe. Two fitted parameters are required to scale the smoothing level for a given step pulse generator. Additionally, the maximum second derivative is used to evaluate t₂. AWIGF determined t₂ was compared with TACQ, a standard waveform interpretation algorithm. The strategies of AWIGF permitted the determination of t₂ without parameter adjustment when the loss characteristics of the media changed, such as with an increase in soil water content and bulk electrical conductivity. Using the new method, the sampling error of t₂ was less than 0.06 ns over a wide range of media properties and less than or equal to that obtained with TACQ. In strongly attenuated waveforms, the water content sampling error determined with AWIGF was 0.005 m³ m^-3 compared with 0.038 m³ m^-3 obtained using TACQ.