Cell Size for Water Content-Dielectric Constant Calibrations for Time Domain Reflectometry

Chaney, RC; Demars, KR; Suwansawat, Suchatvee; Benson, Craig H.

doi:10.1520/gtj11311j

Cited by 16 publications

(4 citation statements)

References 29 publications

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“…The unbound granular material was compacted within the calibration box in two horizontal layers. The electromagnetic field around the sensor did not exceed 30 mm (18), which is smaller than the distance of the outer rod to the wall of the box.…”

Section: Calibration Boxmentioning

confidence: 89%

Use of Time Domain Reflectometry to Estimate Moisture and Density of Unbound Road Materials: Laboratory Calibration and Field Investigation

Bhuyan

Scheuermann

Bodin

et al. 2017

Transportation Research Record: Journal of the Transportation R

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Soil moisture content and dry density of unbound granular pavement materials are important properties for compaction control that influence pavement performance under cyclic loading. Under these loading conditions, increasing moisture content can accelerate significant changes in density. Time domain reflectometry (TDR) is a method for measuring the moisture content and density of soils with rod probe sensors. This paper introduces new calibration functions for TDR measurements using these rod probe sensors embedded in the soil. TDR measurements were taken in the laboratory for a typical road base material at two basically different conditions: at constant moisture content with different dry densities and at constant dry density with different moisture contents. In this study, a relationship was developed between the voltage drop occurring for the passage of an electromagnetic wave through the soil and the bulk density. The permittivity of the soil sample obtained from the travel time of TDR signals was used to calculate the volumetric moisture content. Finally, the gravimetric moisture content was obtained from the volumetric moisture content and bulk density relationship. For the validation of the calibration functions, rod probe sensors were installed in a road to obtain in situ moisture content and density under field conditions. Laboratory results indicate that the calibration functions are independent of moisture and density, and the field test shows the applicability of the method. The newly developed calibration functions allow for the monitoring of the long-term pavement performance, leading to a better understanding of the time-dependent evolution of, for example, rutting of roads.

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Section: Calibration Boxmentioning

confidence: 89%

Use of Time Domain Reflectometry to Estimate Moisture and Density of Unbound Road Materials: Laboratory Calibration and Field Investigation

Bhuyan

Scheuermann

Bodin

et al. 2017

Transportation Research Record: Journal of the Transportation R

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“…Figure 3 shows the calibration box (with inner dimensions of 16.2 × 18.2 × 20.1 cm 3 ) and the procedure used to establish the relationship between the sensor output (mA) and volumetric water content (θ = V w /V, where V w is the volume of water and V is the total volume of soil). This specific size of the calibration box was chosen to minimize boundary effects on the moisture sensor signal, as suggested by Suwansawat et al [18]. To replicate field conditions as closely as possible, the soil sample was re-compacted in the calibration box to achieve the same dry unit weight as that of an undisturbed sample.…”

Section: Laboratory Testing Of Soilmentioning

confidence: 99%

Investigating Correlations and the Validation of SMAP-Sentinel L2 and In Situ Soil Moisture in Thailand

Jotisankasa,

Torsri,

Supavetch

et al. 2023

Sensors

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Soil moisture plays a crucial role in various hydrological processes and energy partitioning of the global surface. The Soil Moisture Active Passive-Sentinel (SMAP-Sentinel) remote-sensing technology has demonstrated great potential for monitoring soil moisture with a maximum spatial resolution of 1 km. This capability can be applied to improve the weather forecast accuracy, enhance water management for agriculture, and managing climate-related disasters. Despite the techniques being increasingly used worldwide, their accuracy still requires field validation in specific regions like Thailand. In this paper, we report on the extensive in situ monitoring of soil moisture (from surface up to 1 m depth) at 10 stations across Thailand, spanning the years 2021 to 2023. The aim was to validate the SMAP surface-soil moisture (SSM) Level 2 product over a period of two years. Using a one-month averaging approach, the study revealed linear relationships between the two measurement types, with the coefficient of determination (R-squared) varying from 0.13 to 0.58. Notably, areas with more uniform land use and topography such as croplands tended to have a better coefficient of determination. We also conducted detailed soil core characterization, including soil–water retention curves, permeability, porosity, and other physical properties. The basic soil properties were used for estimating the correlation constants between SMAP and in situ soil moistures using multiple linear regression. The results produced R-squared values between 0.933 and 0.847. An upscaling approach to SMAP was proposed that showed promising results when a 3-month average of all measurements in cropland was used together. The finding also suggests that the SMAP-Sentinel remote-sensing technology exhibits significant potential for soil-moisture monitoring in certain applications. Further validation efforts and research, particularly in terms of root-zone depths and area-based assessments, especially in the agricultural sector, can greatly improve the technology’s effectiveness and usefulness in the region.

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“…It can be found that when the rod lengths are 10 cm, 15 cm, and 20 cm, the errors of h will be 0.8 %, 0.5 %, and 0.4 %, respectively. On the other hand, Suwansawat and Benson (1999) indicated that increasing of the rod length may lead to greater loss and attenuation of the TDR signal. Therefore, 15 cm is chosen as the length of the rods into the soil.…”

Section: Design Of the Rodsmentioning

confidence: 99%

A Newly Designed TDR Probe for Soils with High Electrical Conductivities

Chen

Wang

Chen

et al. 2014

Geotechnical Testing Journal

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Time domain reflectometry (TDR) is a fast, accurate, and safe technology for field monitoring of soil moisture. Commonly used information in TDR signals includes the apparent dielectric constant and electrical conductivity. Because general TDR principles are not available for apparent dielectric constant measurements by travel time methods in soils with high electrical conductivities caused by the significant signal attenuation, the conventional commercial probes lose their purposes. For this reason, a new probe has been designed for measuring dielectric constants in highly conductive soils on the basis of the surface reflection coefficients method. This new probe can make the reflection at the soil surface more distinct. Experiments were conducted to verify the accuracy of measuring dielectric constants in different soils using this new probe. Finally, the probe was used to measure water content and dry density in the field. The results show that the probe has good integrity and high strength. This probe is capable of obtaining the dielectric constant in soils with high electrical conductivities using surface reflection coefficients methods with reasonable accuracy. In addition, it indicates that the dielectric constant measured by this approach matches well with that determined by travel time methods in the relative error range of 10 % in lowly conductive soils. Compared to oven-dry methods, the relative errors of water content and dry density determined using this new probe are less than 10 % and 3 %, respectively.

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Cell Size for Water Content-Dielectric Constant Calibrations for Time Domain Reflectometry

Cited by 16 publications

References 29 publications

Use of Time Domain Reflectometry to Estimate Moisture and Density of Unbound Road Materials: Laboratory Calibration and Field Investigation

Use of Time Domain Reflectometry to Estimate Moisture and Density of Unbound Road Materials: Laboratory Calibration and Field Investigation

Investigating Correlations and the Validation of SMAP-Sentinel L2 and In Situ Soil Moisture in Thailand

A Newly Designed TDR Probe for Soils with High Electrical Conductivities

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