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A special issue of Sensors (ISSN 1424-8220).

Deadline for manuscript submissions: closed (21 April 2019) | Viewed by 41594

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Prof. Wojciech Skierucha

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Institute of Agrophysics, Polish Academy of Sciences, Doświadczalna 4, 20-290 Lublin, Poland
Interests: TDR and FRD techniques in broadband dielectric spectroscopy; moisture content of soil and materials/products of agricultural origin; agrophysical metrology; sensors of non-electrical quantities
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Dr. Agnieszka Szypłowska

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Institute of Agrophysics, Polish Academy of Sciences, ul. Doświadczalna 4, 20-290 Lublin, Poland
Interests: dielectric properties of soil; measurement of soil moisture and salinity; FDR; dielectric models; microwave measurement methods
Special Issues, Collections and Topics in MDPI journals

Dr. Andrzej Wilczek

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Guest Editor

Institute of Agrophysics, Polish Academy of Sciences, Doświadczalna 4, 20-290 Lublin, Poland
Interests: dielectric spectroscopy; dielectric aquametry; TDR; TDT; dielectric sensors development; HF electromagnetic simulations

Special Issue Information

Dear Colleagues,

The 12^th International Conference on Electromagnetic Wave Interaction with Water and Moist Substances—ISEMA 2018—will take place in Lublin, Poland, 4–7 June 2018, and is organized this time by the Institute of Agrophysics, Polish Academy of Sciences and Foundation of the Polish Academy of Sciences.

The conference will provide an interdisciplinary platform for sharing experience and discussing latest scientific results in understanding, development and application of electromagnetic moisture measurement techniques.

Conference Topics

Electromagnetic determination of physical properties of materials and standardization of measuring methods.
Moisture content determination and monitoring in soil, snow, agricultural materials, waste disposals and other nonhomogeneous materials.
Progress in measurement instrumentation and methods of broadband dielectric spectroscopy.
Electromagnetic sensors in time- and frequency-domain for moisture content determination.
Theory and applications of electromagnetic mixing rules and formulas.
Dielectric relaxation properties of water in heterogeneous materials, including biological substances and tissues.
Applications of broadband dielectric spectroscopy in precision agriculture, civil engineering, industry, etc.
Remote sensing for Earth’s water monitoring.
Computational methods of electromagnetic wave propagation in dispersive and lossy dielectrics.
Integrated techniques using RF and/or microwave dielectric measurements with other methods such as impedance spectroscopy, THz spectroscopy, Raman spectroscopy, infrared spectroscopy, NMR, etc.

Prof. Wojciech Skierucha
Dr. Agnieszka Szypłowska
Dr. Andrzej Wilczek
Guest Editors

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Keywords

Dielectric spectroscopy
Soil moisture
Soil salinity
Microwave moisture measurements
Dielectric properties of materials
Remote sensing
Electromagnetic moisture sensors
Dielectric sensors
TDR
FDR
Dielectric mixing rules
Moisture monitoring

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Published Papers (11 papers)

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Research

13 pages, 6276 KiB

Open AccessArticle

Penetration Depth of a Soil Moisture Profile Probe Working in Time-Domain Transmission Mode

by Marcin Kafarski, Jacek Majcher, Andrzej Wilczek, Agnieszka Szyplowska, Arkadiusz Lewandowski, Alicja Zackiewicz and Wojciech Skierucha

Sensors 2019, 19(24), 5485; https://doi.org/10.3390/s19245485 - 12 Dec 2019

Cited by 18 | Viewed by 3057

Abstract

Soil moisture is one of the most important soil parameters. Knowledge of volumetric water content (VWC) of the root zone as well as the VWC dynamics in the soil profile is especially important for agriculture. Monitoring VWC at several depths in the soil profile can be performed using several soil moisture sensors placed at various depths. However, the use of a profile probe is more convenient, because the installation of a single probe is less disturbing to the soil, as well as less laborious and more cost-effective. The objective of the paper is to present the design and performance of a novel profile probe working in the time-domain transmission mode (P-TDT probe) with emphasis put on the penetration depth and sensitivity zone. The performance of the probe was assessed with the use of finite element method (FEM) simulations in the frequency domain, transient simulations in the time domain and laboratory experiments with the use of a vector network analyzer (VNA) working in the 10 MHz–10 GHz frequency range. It was concluded that the effective soil volume measured by the profile probe of a given geometry is equivalent to a soil thickness of about 20 mm around the tested probe. The internal part of the probe body had a negligible effect on the measurement results, as it does not change with soil moisture. Moreover, the transmitted signal amplitude was related to the soil electrical conductivity. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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21 pages, 4565 KiB

Open AccessArticle

Spatio-Temporal Mapping of L-Band Microwave Emission on a Heterogeneous Area with ELBARA III Passive Radiometer

by Łukasz Gluba, Mateusz Łukowski, Radosław Szlązak, Joanna Sagan, Kamil Szewczak, Helena Łoś, Anna Rafalska-Przysucha and Bogusław Usowicz

Sensors 2019, 19(16), 3447; https://doi.org/10.3390/s19163447 - 7 Aug 2019

Cited by 5 | Viewed by 3293

Abstract

Water resources on Earth become one of the main concerns for society. Therefore, remote sensing methods are still under development in order to improve the picture of the global water cycle. In this context, the microwave bands are the most suitable to study land–water resources. The Soil Moisture and Ocean Salinity (SMOS), satellite mission of the European Space Agency (ESA), is dedicated for studies of the water in soil over land and salinity of oceans. The part of calibration/validation activities in order to improve soil moisture retrieval algorithms over land is done with ground-based passive radiometers. The European Space Agency L-band Microwave Radiometer (ELBARA III) located near the Bubnów wetland in Poland is capable of mapping microwave emissivity at the local scale, due to the azimuthal and vertical movement of the horn antenna. In this paper, we present results of the spatio-temporal mapping of the brightness temperatures on the heterogeneous area of the Bubnów test-site consisting of an area with variable organic matter (OM) content and different type of vegetation. The soil moisture (SM) was retrieved with the L-band microwave emission of the biosphere (L-MEB) model with simplified roughness parametrization (SRP) coupling roughness and optical depth parameters. Estimated soil moisture values were compared with in-situ data from the automatic agrometeorological station. The results show that on the areas with a relatively low OM content (4–6%—cultivated field) there was good agreement between measured and estimated SM values. Further increase in OM content, starting from approximately 6% (meadow wetland), caused an increase in bias, root mean square error (RMSE), and unbiased RMSE (ubRMSE) values and a general drop in correlation coefficient (R). Despite a span of obtained R values, we found that time-averaged estimated SM using the L-MEB SRP approach strongly correlated with OM contents. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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18 pages, 5559 KiB

Open AccessArticle

Physical Meanings of Fractal Behaviors of Water in Aqueous and Biological Systems with Open-Ended Coaxial Electrodes

by Shin Yagihara, Rio Kita, Naoki Shinyashiki, Hironobu Saito, Yuko Maruyama, Tsubasa Kawaguchi, Kohei Shoji, Tetsuya Saito, Tsuyoshi Aoyama, Ko Shimazaki, Keisuke Matsumoto, Minoru Fukuzaki, Haruchika Masuda, Shinichiro Hiraiwa, Koji Asami and Masayuki Tokita

Sensors 2019, 19(11), 2606; https://doi.org/10.3390/s19112606 - 8 Jun 2019

Cited by 13 | Viewed by 3185

Abstract

The dynamics of a hydrogen bonding network (HBN) relating to macroscopic properties of hydrogen bonding liquids were observed as a significant relaxation process by dielectric spectroscopy measurements. In the cases of water and water rich mixtures including biological systems, a GHz frequency relaxation process appearing at around 20 GHz with the relaxation time of 8.2 ps is generally observed at 25 °C. The GHz frequency process can be explained as a rate process of exchanges in hydrogen bond (HB) and the rate becomes higher with increasing HB density. In the present work, this study analyzed the GHz frequency process observed by suitable open-ended coaxial electrodes, and physical meanings of the fractal nature of water structures were clarified in various aqueous systems. Dynamic behaviors of HBN were characterized by a combination of the average relaxation time and the distribution of the relaxation time. This fractal analysis offered an available approach to both solution and dispersion systems with characterization of the aggregation or dispersion state of water molecules. In the case of polymer-water mixtures, the HBN and polymer networks penetrate each other, however, the HBN were segmented and isolated more by dispersed and aggregated particles in the case of dispersion systems. These HBN fragments were characterized by smaller values of the fractal dimension obtained from the fractal analysis. Some examples of actual usages suggest that the fractal analysis is now one of the most effective tools to understand the molecular mechanism of HBN in aqueous complex materials including biological systems. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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16 pages, 4436 KiB

Open AccessArticle

Application of the TDR Soil Moisture Sensor for Terramechanical Research

by Jarosław Pytka, Piotr Budzyński, Mariusz Kamiński, Tomasz Łyszczyk and Jerzy Józwik

Sensors 2019, 19(9), 2116; https://doi.org/10.3390/s19092116 - 7 May 2019

Cited by 14 | Viewed by 3945

Abstract

This paper presents examples of the application of the TDR (Time-Domain Reflectometry) sensor in terramechanical research. Examples include the determination of soil moisture content during off-road vehicle mobility tests, the determination of snow density before and after the wheeling of a snow grooming machine and an airplane, as well as the monitoring of turf moisture on a grassy airfield for the analysis and prediction of safe and efficient flight operations (takeoff and landing). A handheld TDR meter was used in these experiments. Soil moisture data were correlated with the vehicle mobility index and a simple model for this correlation was derived. Using grassy airfield research, soil moisture data were related to meteorological impacts (precipitation, sunlight, etc.). Generally, it was concluded that the TDR meter, in its handheld version, was a useful tool in the performed research, but a field sensor that operates autonomically would be an optimal solution for the subject applications. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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11 pages, 2589 KiB

Open AccessArticle

Complete Meniscus Removal Method for Broadband Liquid Characterization in a Semi-Open Coaxial Test Cell

by Michał Kalisiak and Wojciech Wiatr

Sensors 2019, 19(9), 2092; https://doi.org/10.3390/s19092092 - 6 May 2019

Cited by 10 | Viewed by 3129

Abstract

We present a new technique for broadband liquid characterization using a semi-open, vertically oriented test cell that is measured with a calibrated vector network analyzer in three states: the empty one and filled with two different volumes of the liquid under test. Using the measurements, we de-embed a transfer matrix representing a volume increment of the liquid sample and determine its column height with a novel closed-form formula. Thanks to the de-embedding, the method enables one, for the first time, to completely remove effects caused by a reproducible meniscus forming the top surface of a liquid tested in the cell and determine not only the propagation constant, but also characteristic impedance of the liquid sample, from which its permittivity and permeability are calculated. The results are highly consistent, because all the measurements are performed without disassembling the cell. We validate experimental results of the meniscus removal method by comparing them with reference data and outcomes of the Nicolson–Ross–Weir method. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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11 pages, 2588 KiB

Open AccessArticle

Combined Thickness and Permittivity Measurement of Thin Layers with Open-Ended Coaxial Probes

by Kjetil Folgerø, Kjetil Haukalid, Jan Kocbach and Andreas Soto Peterson

Sensors 2019, 19(8), 1765; https://doi.org/10.3390/s19081765 - 12 Apr 2019

Cited by 5 | Viewed by 4159

Abstract

This paper presents a method to simultaneously determine the thickness and permittivity of thin layers from multi-frequency reflection coefficient measurements using an open-ended coaxial probe. This is achieved by exploiting that the probe becomes radiating at frequencies higher than the probe’s typical operating range. Permittivity information is extracted from measurements in the typical frequency range, whereas thickness information is obtained from high frequency measurements by exploiting resonances that occur when the radiated waves are reflected at the layer boundary. A finite element model of the measurement set-up is made in COMSOL Multiphysics^TM, and a matrix of simulations spanning the relevant layer thicknesses and permittivity range is generated. The measured permittivity spectra of unknown samples are compared to the simulation matrix to estimate layer thickness and permittivity. The method is verified by measurements of water–ethanol mixtures. An application example where the water fraction and layer thickness of a gas hydrate deposition layer is estimated from permittivity measurements in a multiphase flow loop is also presented. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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12 pages, 3657 KiB

Open AccessArticle

A Seven-Rod Dielectric Sensor for Determination of Soil Moisture in Well-Defined Sample Volumes

by Justyna Szerement, Aleksandra Woszczyk, Agnieszka Szypłowska, Marcin Kafarski, Arkadiusz Lewandowski, Andrzej Wilczek and Wojciech Skierucha

Sensors 2019, 19(7), 1646; https://doi.org/10.3390/s19071646 - 6 Apr 2019

Cited by 14 | Viewed by 4273

Abstract

This paper presents a novel seven-rod sensor used for time-domain reflectometry (TDR) and frequency-domain reflectometry (FDR) measurements of soil water content in a well-defined sample volume. The probe directly measures the complex dielectric permittivity spectrum and for this purpose requires three calibration media: air, water, and ethanol. Firstly, electromagnetic simulations were used to study the influence of the diameter of a container on the sensitivity zone of the probe with respect to the measured calibration media and isopropanol as a verification liquid. Next, the probe was tested in three soils—sandy loam and two silt loams—with six water contents from air-dry to saturation. The conversion from

S_{11}

parameters to complex dielectric permittivity from vector network analyzer (VNA) measurements was obtained using an open-ended liquid procedure. The simulation and measurement results for the real part of the isopropanol dielectric permittivity obtained from four containers with different diameters were in good agreement with literature data up to 200 MHz. The real part of the dielectric permittivity was extracted and related to the moisture of the tested soil samples. Relations between the volumetric water content and the real part of the dielectric permittivity (by FDR) and apparent dielectric permittivity (by TDR) were compared with Topp’s equation. It was concluded that the best fit to Topp’s equation was observed in the case of a sandy loam. Data calculated according to the equation proposed by Malicki, Plagge, and Roth gave results closer to Topp’s calibration. The obtained results indicated that the seven-rod probe can be used to accurately measure of the dielectric permittivity spectrum in a well-defined sample volume of about 8 cm³ in the frequency range from 20 MHz to 200 MHz. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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13 pages, 6058 KiB

Open AccessArticle

Dielectric Spectroscopy Using Dual Reflection Analysis of TDR Signals

by Yin Jeh Ngui, Chih-Ping Lin and Tsai-Jung Wu

Sensors 2019, 19(6), 1299; https://doi.org/10.3390/s19061299 - 14 Mar 2019

Cited by 4 | Viewed by 3372

Abstract

Time-domain reflectometry (TDR) has been a powerful tool for measuring soil dielectric properties. Initiating from apparent dielectric constant (

K_{a}

) measurement up until apparent and complex dielectric spectroscopies, the embedded information in the TDR signal can be extracted to inspire our [...] Read more.

Time-domain reflectometry (TDR) has been a powerful tool for measuring soil dielectric properties. Initiating from apparent dielectric constant (

K_{a}

) measurement up until apparent and complex dielectric spectroscopies, the embedded information in the TDR signal can be extracted to inspire our understanding of the underlying dielectric behaviors. Multiple full waveform inversion techniques have been developed to extract complex dielectric permittivity (CDP) spectrum, but most of them involved prior knowledge of input function and tedious calibration. This rendered the field dielectric spectroscopy challenging and expensive to conduct. Dual reflection analysis (DRA) is proposed in this study to measure CDP spectrum from 10 MHz to 1 GHz. DRA is a simple, robust, model-free, and source-function free algorithm which requires minimal calibration effort. The theoretical framework of DRA is established and the necessary signal processing procedures are elaborated in this study. Eight materials with different dielectric characteristics are selected to evaluate DRA’s performance, by using both simulated and experimental signals. DRA is capable of measuring non-dispersive materials very well, whereas dispersive materials require the assistance of a long-time-window (LTW) extraction method to further extend the effective bandwidth. The DRA approach is suitable for field applications that can only record a limited amount of data points and in-situ dielectric spectroscopy. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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16 pages, 4930 KiB

Open AccessArticle

Determination of the Porosity Distribution during an Erosion Test Using a Coaxial Line Cell

by Tilman Bittner, Mathieu Bajodek, Thierry Bore, Eric Vourc’h and Alexander Scheuermann

Sensors 2019, 19(3), 611; https://doi.org/10.3390/s19030611 - 1 Feb 2019

Cited by 16 | Viewed by 3785

Abstract

The detection of porosity changes within a soil matrix caused by internal erosion is beneficial for a better understanding of the mechanisms that induce and maintain the erosion process. In this paper, an electromagnetic approach using Spatial Time Domain Reflectometry (STDR) and a transmission line model is proposed for this purpose. An original experimental setup consisting of a coaxial cell which acts as an electromagnetic waveguide was developed. It is connected to a transmitter/receiver device both measuring the transmitted and corresponding reflected electromagnetic pulses at the cell entrance. A gradient optimization method based on a computational model for simulating the wave propagation in a transmission line is applied in order to reconstruct the spatial distribution of the soil dielectric permittivity along the cell based on the measured signals and an inversion algorithm. The spatial distribution of the soil porosity is deduced from the dielectric permittivity profile by physically based mixing rules. Experiments were carried out with glass bead mixtures of known dielectric permittivity profiles and subsequently known spatial porosity distributions to validate and to optimize both, the proposed computational model and the inversion algorithm. Erosion experiments were carried out and porosity profiles determined with satisfying spatial resolution were obtained. The RMSE between measured and physically determined porosities varied among less than 3% to 6%. The measurement rate is sufficient to be able to capture the transient process of erosion in the experiments presented here. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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12 pages, 1657 KiB

Open AccessArticle

Estimating Pore Water Electrical Conductivity of Sandy Soil from Time Domain Reflectometry Records Using a Time-Varying Dynamic Linear Model

by Basem Aljoumani, Jose A. Sanchez-Espigares and Gerd Wessolek

Sensors 2018, 18(12), 4403; https://doi.org/10.3390/s18124403 - 13 Dec 2018

Cited by 7 | Viewed by 4002

Abstract

Despite the importance of computing soil pore water electrical conductivity (σ_p) from soil bulk electrical conductivity (σ_b) in ecological and hydrological applications, a good method of doing so remains elusive. The Hilhorst concept offers a theoretical model describing a linear relationship between σ_b, and relative dielectric permittivity (ε_b) in moist soil. The reciprocal of pore water electrical conductivity (1/σ_p) appears as a slope of the Hilhorst model and the ordinary least squares (OLS) of this linear relationship yields a single estimate (

\hat{1 / σ_{p}}

) of the regression parameter vector (σ_p) for the entire data. This study was carried out on a sandy soil under laboratory conditions. We used a time-varying dynamic linear model (DLM) and the Kalman filter (Kf) to estimate the evolution of σ_p over time. A time series of the relative dielectric permittivity (ε_b) and σ_b of the soil were measured using time domain reflectometry (TDR) at different depths in a soil column to transform the deterministic Hilhorst model into a stochastic model and evaluate the linear relationship between ε_b and σ_b in order to capture deterministic changes to (1/σ_p). Applying the Hilhorst model, strong positive autocorrelations between the residuals could be found. By using and modifying them to DLM, the observed and modeled data of ε_b obtain a much better match and the estimated evolution of σ_p converged to its true value. Moreover, the offset of this linear relation varies for each soil depth. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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22 pages, 4012 KiB

Open AccessArticle

Spatial Retrieval of Broadband Dielectric Spectra

by Jan Bumberger, Juliane Mai, Felix Schmidt, Peter Lünenschloß, Norman Wagner and Hannes Töpfer

Sensors 2018, 18(9), 2780; https://doi.org/10.3390/s18092780 - 23 Aug 2018

Cited by 6 | Viewed by 4392

Abstract

A broadband soil dielectric spectra retrieval approach ( 1 MHz– 2 GHz) has been implemented for a layered half space. The inversion kernel consists of a two-port transmission line forward model in the frequency domain and a constitutive material equation based on a power law soil mixture rule (Complex Refractive Index Model - CRIM). The spatially-distributed retrieval of broadband dielectric spectra was achieved with a global optimization approach based on a Shuffled Complex Evolution (SCE) algorithm using the full set of the scattering parameters. For each layer, the broadband dielectric spectra were retrieved with the corresponding parameters thickness, porosity, water saturation and electrical conductivity of the aqueous pore solution. For the validation of the approach, a coaxial transmission line cell measured with a network analyzer was used. The possibilities and limitations of the inverse parameter estimation were numerically analyzed in four scenarios. Expected and retrieved layer thicknesses, soil properties and broadband dielectric spectra in each scenario were in reasonable agreement. Hence, the model is suitable for an estimation of in-homogeneous material parameter distributions. Moreover, the proposed frequency domain approach allows an automatic adaptation of layer number and thickness or regular grids in time and/or space. Full article

(This article belongs to the Special Issue Selected Papers from ISEMA 2018)

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