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Frequency Domain Versus Travel Time Analyses of TDR Waveforms for Soil Moisture Measurements

Dep. of Civil Engineering, National Chiao Tung University, 1001 Ta-Hsueh Road, Hisnchu, Taiwan

View larger version (15K): [in a new window]   Fig. 1. Two tangent-line schemes for determining the reflection point of a TDR waveform.

View larger version (10K): [in a new window]   Fig. 2. A cascade of uniform lines represents a multi-section transmission line. Each uniform line (i) is characterized by its propagation constant (i), characteristic impedance (Zc,i), and length (li).

View larger version (29K): [in a new window]   Fig. 3. (a) The measured TDR waveform of Soil M3 compared with that predicted by the volumetric mixing model and Debye equation, and (b) the real part of a dielectric relaxation spectrum estimated by the volumetric mixing model and Debye equation.

View larger version (17K): [in a new window]   Fig. 4. Numerical simulations using parameters that fit the TDR waveform of Soil M3 but varying the probe length show that (a) the apparent dielectric constant is not invariant of the probe length, and (b) the apparent dielectric constant increase with rise time.

View larger version (26K): [in a new window]   Fig. 5. Numerical simulations using Debye equation show the effects of (a) dielectric relaxation (for dc = 10 and = 0) and (b) electrical conductivity on apparent dielectric constant (for dc = 10 and = 20). Measurements of Ethanol (dc = 25.2, = 4.52, frel = 0.782 GHz) and Butanol alcohol (dc = 17.7, = 3.3, frel = 0.274 GHz) are also shown.

View larger version (33K): [in a new window]   Fig. 6. Effects of (a) bulk dry density (for (Aes = 100 m2 g-1, bw = 15 S m-1, fw = 0.6 S m-1, and = 0.5) and (b) specific surface on dielectric permittivity based on four component volumetric mixing model (d = 1.56 g cm-3, bw = 15 S m-1, fw = 0.6 S m-1, and = 0.5).

View larger version (15K): [in a new window]   Fig. 7. Comparison of the measured data with conventional linear calibration equation and Topp's equation.

View larger version (13K): [in a new window]   Fig. 8. Effect of soil type on the density-compensating relationship between the apparent dielectric constant and water content (Eq. [11]).

View larger version (14K): [in a new window]   Fig. 9. The new calibration equation (Eq. [12]) for soil water content that reduces the effects of soil structure and bulk dry density.