Soil Permittivity Research Articles

Polarimetric Simulations of SAR at L-Band Over Bare Soil Using Scattering Matrices of Random Rough Surfaces From Numerical Three-Dimensional Solutions of Maxwell Equations

We have performed simulations of random rough surface scattering using 3-D numerical solution of Maxwell equations (NMM3D) using surface size up to 32 $\times$ 32 squared wavelengths. The rough surfaces are characterized by exponential correlation functions. The simulation results of cross- and copolarization backscattering coefficients were in good agreement with experimental measurements of bare soils at L-band. Because in numerical solutions of Maxwell equations the electric fields of the scattered wave are calculated for each realization, scattering matrices can be simulated by NMM3D, and such simulations are performed in this paper. For a given RMS height, correlation length, soil permittivity, and incident angle, we calculated the radar scattering matrix up to 958 independent realizations. For each realization, the components of the scattering matrix, namely, $S_{\rm HH}$ , $S_{\rm VV}$ , $S_{\rm HV}$ , and $S_{\rm VH}$ , are calculated. Using the simulated scattering matrices, we calculate the polarimetric speckle statistics (amplitude and phase difference), followed by a comparison with theoretical distributions. For fully developed speckle from the homogeneous rough surface, the results are examined and validated to ensure the simulated data quality as far as polarimetric properties are concerned. By taking ensemble averages, we calculate the coherency matrix from which the eigenvalues, entropy, anisotropy, and alpha angle in coherent target decomposition are then calculated. In particular, characterization of polarimetric descriptors for rough surface is presented. Issues of scattering symmetry characteristics are also discussed.

Factors affecting soil permittivity and proposals to obtain gravimetric water content from time domain reflectometry measurements

Time domain reflectometry (TDR) measures the apparent relative dielectric permittivity (ARDP) of a soil and is commonly used to determine the volumetric water content (VWC) of the soil. ARDP is affected by several factors in addition to water content, such as the soil’s electrical conductivity, temperature, and density. These relationships vary with soil type and are very soil-dependent, and despite previous research, they are still not fully understood. A multivariate statistical approach (principal component analysis, PCA) is used to describe a range of soils from two separate sites in the UK (clay and silty sand – sandy silt). The advantage of a PCA is that it considers several variables at a time, giving an immediate picture of their underlying relationships. It was found that for the studied soils, ARDP was positively correlated with VWC and bulk electrical conductivity, but did not show any dependence on some other geotechnical parameters. TDR has recently been used in geotechnical engineering for measuring the gravimetric water content (GWC) and dry density. However, the current approaches require a custom-made TDR probe and an extensive site specific empirical laboratory calibration. To extend the potential use of TDR in the geotechnical industry, three relatively simple methods are proposed to estimate the GWC from VWC (derived from the measured ARDP values) and dry density depending on the amount of information known about the soil. Examples of possible applications of these methods include continuous monitoring of consolidation adjacent to a structure, the effect of seasonal weather and climate change on ageing earthwork assets, and the shrink–swell potential adjacent to trees. All three methods performed well, with between 83% and 98% of the data lying within a ±5% GWC envelope, with the data for clay soils performing better than those for silty sands – sandy silts. This is partly due to the fact that the applied relationship converting ARDP to VWC performs better for clays than silty sands – sandy silts, as well as less variation of the estimated bulk density that is needed to derive the dry density.

수분함유량에 따른 토양의 전기적 파라미터의 주파수의존성

The electrical parameters of soils are highly dependent on the various factors such as types of soil, chemical compositions, moisture content, temperature, frequency, and so on. The analysis of soil parameters is of fundamental importance in design of grounding systems. In this paper, we present the experimental results of frequency-dependent impedance, resistivity, permittivity of soils as functions of types of soil and moisture content. The impedance and resistivity of soils are decreased as the moisture content and the frequency increase. In particular, the variation of the soil resistivity with the frequency is pronounced in the conditions of high resistivity and low moisture content. On the contrary, the permittivity of soils are sharply decreased with increasing the frequency below 10㎑ and the frequency-dependent permittivity of soils are highly changed in the conditions of high moisture and low resistivity.

Electromagnetic characterization of a computational asymmetric analysis for wireless networks

While sensor networks research for terrestrial applications has made significant strides in the past few years, the literature has relatively few examples of papers that have validated the feasibility and reliability issue, for the performance of Wireless Sensor Networks (WSNs) for underground applications. The propagation characteristics of electromagnetic (EM) waves in soil, and significant differences from propagation in air, prevent straightforward characterization of the underground wireless channel. Fundamentals, such as the dielectric permittivity and magnetic permeability, are required to solve the propagation of EM waves in soil. To this end, in this paper, we proposed a new model, Asymmetric Mixture Homogenization, based on the representative volume element concept, to characterize the dielectric permittivity of soil according to the composition of soil, and layout the foundation for efficient communication in this new environment. Further, in this paper we validated that with the latest wireless network system being used for terrestrial applications, challenges exists for the WSN to be used for underground applications. Experiments are performed to examine the link-level quality and received signal strength parameter between under-soil and above soil sensor nodes. The theoretical analysis and the experimental results prove the feasibility for the realization of WSN in underground applications.

Estimation of Soil Permittivity in Presence of Antenna-Soil Interactions

This work deals with a technique for estimating the soil permittivity starting from Ground-Penetrating-Radar (GPR) measurements collected under a reflection mode single view (one transmitting antenna) multistatic (plurality of receivers) configuration. The technique consists of two steps: i) first the soil Fresnel reflection coefficient is estimated; ii) then the soil electromagnetic parameters are inferred from the retrieved reflection coefficient. In real scenarios, the transmitting antenna couples with the soil as it is in close proximity of the air-soil interface so that mutual interactions arise. Accordingly, the transmitting antenna plane-wave spectrum depends on the electromagnetic properties of the medium under investigation. This strongly complicates the problem as the first step becomes a non-linear inverse problem. To overcome such a drawback, a pre-processing stage consisting of a suitable time gating procedure is here introduced. This allows to restore linearity of the Fresnel coefficient estimation. Moreover, it is also able to mitigate negative effect due to not matched transmitting antenna. Numerical examples are presented to assess the feasibility of the proposed technique.

Numerical 3-D FEM and Experimental Analysis of the Open-Ended Coaxial Line Technique for Microwave Dielectric Spectroscopy on Soil

Open-ended coaxial line probes (OCs) are systematically analyzed by means of numerical 3-D finite element calculations in combination with experimental investigations for microwave dielectric spectroscopy on fine grained soils. The probes, based on conventional coaxial lines and connectors (N, SMA), are broadband characterized in the frequency range from 1 MHz to 10 GHz. The sensitive region for dielectric measurements is ±7-mm lateral and 7-mm perpendicular to the midpoint of the sensor aperture. The spatial spreading of the sensitive zone is stable for the investigated low-loss and high-loss strongly dispersive standard liquids, as well as the saturated and unsaturated soils. Dielectric spectra are determined based on a bilinear relationship between effective permittivity and complex reflection coefficient of the probe after probe-calibration with known standards. The mean relative error of the real part of the complex permittivity from 100 MHz to 10 GHz is smaller than 3.5% and is less than 10% for the imaginary part. A lower limit of the measurement range of 50 MHz with the used procedure and materials is suggested. Complex effective permittivity of saturated fine-grained soils is determined with the developed probes and procedure. The soil dielectric spectra were analyzed with a broadband relaxation model, as well as a novel, coupled hydraulic-dielectric mixture approach. The results demonstrate the suitability of the investigated OCs for the determination of high resolution soil dielectric spectra.

A combined equation to estimate the soil pore-water electrical conductivity: calibration with the WET and 5TE sensors

Affordable, commercial dielectric sensors of the frequency domain reflectometry (FDR) and capacitance–conductance (CC) types estimate the dielectric permittivity (εb) and electrical conductivity (σb) of bulk soil. In this work, an equation was obtained to estimate the pore-water electrical conductivity (σp), which is closely related to the soil salinity in contact with plant roots, from εb and σb data, by combining the simplified dielectric mixing (SDM) model that relates εb to the soil volumetric water content (θ), with the Rhoades equation that relates θ and σb to σp. This equation was calibrated with measurements of εb and σb obtained with the Delta-T WET (FDR) and the Decagon 5TE (CC) sensors, in 20 pots filled with a clay loam soil and arranged as combinations of four levels of soil moisture with five levels of soil salinity. The calibrations were performed against reference θ and σp values. The σp was calculated with the chemical equilibrium model SALSOLCHEMEC and used as a more reliable reference than the electrical conductivity of the soil wetting water. For both sensors, the SDM model on the one hand, and the Rhoades equation on the other, provided the most accurate estimations using the least number of parameters regarding their respective alternatives, i.e. the third-order polynomial and the Hilhorst equation. The combined equation for estimation of σp subsequently provided root mean square deviations of 3.1 (WET) and 4.1 (5TE) dS m–1, which decreased to 1.5 and 2.6 dS m–1 for θ >0.22 m3 m–3, and σb <3 (WET) and 3.7 (5TE) dS m–1. A new combined equation has been proposed for reliable estimations of σp with these sensors in clayey soils for θ >0.22 m3 m–3 and σb <3.7 dS m–1.

뇌격전류에 의한 수평접지극의 전위상승 계산

An electric potential rise of the grounding electrode developed by the lightning stroke currents should be calculated for the purpose of effective protection of electrical and electronic equipment. In this paper, the electromagnetic model was applied to calculate the harmonic impedance of grounding electrode. Also the empirical equation related to the permittivity and resistivity of soil was used. The lightning current waveforms, which are expressed by the Heidler"s equation, were used in order to calculate accurately transient electric potential rises. The transient voltage was obtained by using the simulated harmonic impedance and the lightning current in frequency domain. Finally, the transient voltages of horizontal grounding electrode(10m) under lightning stroke currents were calculated by IFFT(Inverse Fast Fourier Transform).

Wide-Band Modeling of Grounding Grid Based on the Fractional Order Differential Theory

Research on the wide-band modelling method of the grounding grid and the analysis of the electromagnetic transients on the grounding grid has significant practical meaning on the engineering problems, e.g. overvoltage protection, insulation coordination and electromagnetic interference and the increasing levels of capacity in the power and communication systems. This paper presents strategies which establish the equivalent circuit for computing the transient performance of grounding grid base on fractional order differential theory, which takes into account the fractional frequency characteristics of soil permittivity. In order to validate the presented method, an example is considered and the transient responses are compared with the results obtained by the method of moments.

Supercooled interfacial water in fine-grained soils probed by dielectric spectroscopy

Abstract. Water substantially affects nearly all physical, chemical and biological processes on the Earth. Recent Mars observations as well as laboratory investigations suggest that water is a key factor of current physical and chemical processes on the Martian surface, e.g. rheological phenomena. Therefore it is of particular interest to get information about the liquid-like state of water on Martian analogue soils for temperatures below 0 °C. To this end, a parallel plate capacitor has been developed to obtain isothermal dielectric spectra of fine-grained soils in the frequency range from 10 Hz to 1.1 MHz at Martian-like temperatures down to −70 °C. Two Martian analogue soils have been investigated: a Ca-bentonite (specific surface of 237 m2 g−1, up to 9.4% w / w gravimetric water content) and JSC Mars 1, a volcanic ash (specific surface of 146 m2 g−1, up to 7.4% w / w). Three soil-specific relaxation processes are observed in the investigated frequency–temperature range: two weak high-frequency processes (bound or hydrated water as well as ice) and a strong low-frequency process due to counter-ion relaxation and the Maxwell–Wagner effect. To characterize the dielectric relaxation behaviour, a generalized fractional dielectric relaxation model was applied assuming three active relaxation processes with relaxation time of the ith process modelled with an Eyring equation. The real part of effective complex soil permittivity at 350 kHz was used to determine ice and liquid-like water content by means of the Birchak or CRIM equation. There are evidence that bentonite down to −70 °C has a liquid-like water content of 1.17 monolayers and JSC Mars 1 a liquid-like water content of 1.96 monolayers.

Dielectric Spectroscopy of Dry and Wet Soils in Frequency Region from 20 Hz to 20 GHz

The complex permittivity of a soil is a very much useful parameter in remote sensing applications. The measurement of complex permittivity of soil at various microwave frequencies can be related with the moisture content in the soil, which is useful in Agriculture, Hydrology and Meteorology. The complex dielectric spectra of dry and wet soil samples were measured in the frequency range from 20 Hz to 20 GHz. For this, various instruments and different methods were used. The complex permittivity of the soils, in the frequency range from 20 Hz to 2 MHz, was measured using a precision LCR meter, and a newly designed and developed coaxial capacitor probe. The complex permittivity of the soils, in the frequency range from 75 MHz to 1.5 GHz, was measured using a Vector Network Analyzer. The complex permittivity of the soils at 5.65 GHz, 9.5 GHz and 19.3 GHz was measured using microwave bench set up. The results are presented in the paper.

Complex Permittivity of Soils of Western Rajasthan at Microwave Frequency

Complex permittivity (ε', ε'') of soil sample of Western Rajasthan region is measured at 9.385 GHz for various moisture content at a temperature of 200c. The dielectric constant of dry soil is found in good agreement with literature values. The dielectric constant and dielectric loss of moist soil increases with increase in moisture content in soil. The dielectric data is used to calculate ac conductivity and emissivity.

토양의 유전율 특성을 고려한 정보통신설비용 수평접지전극의 임피던스 계산

정보통신설비용 접지전극의 임피던스는 접지전극이 매설되어 있는 토양의 전기적 특성과 밀접한 관계가 있다. 특히 인가된 전계의 주파수에 따른 토양의 유전율과 도전율 특성이 접지전극의 임피던스에 직접적으로 영향을 준다. 접지전극의 임피던스를 계산할 수 있는 상용프로그램은 유전율과 도전율을 수정하여 시뮬레이션하는 것이 불가능하기 때문에 전자계 이론을 적용한 프로그램을 MATLAB으로 구현하였다. 토양의 유전율은 유전완화 모델인 디바이(Debye)식을 적용하였으며, 도전율은 실험에서 얻어진 수식을 다른 논문에서 인용하였다. 시뮬레이션 결과의 신뢰성을 확인하기 위해서 실험계를 구성하여 접지전극의 임피던스를 측정하고, 시뮬레이션 결과와 서로 비교하였다. 그 결과, 토양의 유전율과 도전율 특성을 고려한 본 논문 결과가 고려하지 않은 상용프로그램(NEC) 결과 보다 측정값과 더욱 잘 일치하는 것을 확인하였다.

Free Space Permittivity Determination Technique

ABSTRACTA new approach for free space determination of the complex valued permittivity of soil and other materials is presented. The method is tailored for ground penetrating radar purposes where an averaged value of the permittivity for a certain bandwidth is desired. Coming with less demands on calibration and specimen configuration, our approach represents a robust algorithm with good accuracy and capability for in situ measurements. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2888–2892, 2013

Computation of Radar Scattering From Heterogeneous Rough Soil Using the Finite-Element Method

A 2-D vector-element-based finite-element method (FEM) is used to calculate the radar backscatter from 1-D bare rough soil surfaces which can have an underlying heterogeneous substrate. Monte Carlo simulation results are presented for scattering at L-band (λ = 0.24 m). For homogeneous soils with rough surfaces, the results of FEM are compared with the predictions of the small perturbation method. In the case of heterogeneous substrates, soil moisture (and, hence, soil permittivity) is assumed to vary as a function of depth. In this case, the results of FEM are compared with those of the transfer matrix method for flat soil surfaces. In both cases, a good agreement is found. For homogeneous rough soils, it is found that polarimetric radar backscatter and copolarized phase difference have a nonlinear relationship with soil moisture. Finally, it is found that the nature of the soil moisture variation in the top few centimeters of the soil has a strong influence on the backscatter and, hence, on the inferred soil moisture content.

Retrieving Temperature Gradient in Frozen Active Layer of Arctic Tundra Soils From Radiothermal Observations in $L$-Band—Theoretical Modeling

Possibility of remote sensing of both the surface temperature and the temperature gradient in the permafrost active layer from L-band brightness temperature observations is theoretically investigated at a SMOS frequency of 1.4 GHz. Bare soil emission is simulated based on the semi-empirical L-MEB model. The brightness temperature is simulated using the soil density, surface roughness, temperature, and moisture profiles measured in situ at the Biosphere Station Franklin Bluffs, Alaska, USA (69°39'N, 148°43'W) from September 2, 1999, to August 23, 2001. The soil permittivity is calculated using the temperature-dependent generalized refractive mixing dielectric model for the organic rich soil sample collected in North Slope, Alaska (68°38'N, 149°35'W). This model predicts the complex dielectric constant of moist soil both thawed and frozen at temperatures from -30°C to +25°C and moistures from 0 to 0.94 g/g. The brightness temperatures simulated for field-of-view angles from 0 to 60° are inverted into the temperature profiles, and their deviations from the temperature profiles measured in situ are estimated. The error in reconstructing temperature profiles is found to be no greater than 1.8°C to depths of 0.15 m.

Temperature- and Texture-Dependent Dielectric Model for Moist Soils at 1.4 GHz

In this letter, a monofrequent dielectric model for moist soils taking into account dependences on the temperature and texture is proposed, in the case of an electromagnetic frequency equal to 1.4 GHz. The proposed model is deduced from a more general model proposed by Mironov and Fomin (2009) that provides estimations of the complex relative permittivity (CRP) of moist soils as a function of frequency, temperature, moisture, and texture of soils. The latter employs the physical laws of Debye and Clausius-Mossotti and the law of ion conductance to calculate the CRP of water solutions in the soil. The parameters of the respective physical laws were determined by using the CRPs of moist soils measured by Curtis (1995) for a wide ensemble of soil textures (clay content from 0% to 76%), moistures (from drying at 105 °C to nearly saturation), temperatures (10 °C -40 °C), and frequencies (0.3-26.5 GHz). This model has standard deviations of calculated CRPs from the measured values equal to 1.9 and 1.3 for the real and imaginary parts of CRP, respectively. In the model proposed in this letter, the respective standard deviations were decreased to the values of 0.87 and 0.26. In addition, the equations to calculate the complex dielectric permittivity as a function of moisture, temperature, and texture were represented in a simple form of the refractive mixing dielectric model, which is commonly used in the algorithms of radiometric and radar remote sensing to retrieve moisture in the soil.

Soil Permittivity Response to Bulk Electrical Conductivity for Selected Soil Water Sensors

Bulk electrical conductivity (σa) can dominate the low frequency dielectric loss spectrum in soils, causing changes in the permittivity and errors in estimated water content. We examined the dependence of measured apparent permittivity (Ka) on σa in contrasting soils using time‐domain reflectometry (TDR), a digital time‐domain transmission (TDT) sensor, and a capacitance sensor (5TE) during near saturated solute displacement experiments. Sensors were installed in columns packed with fine sand or a clay loam soil. Displacement experiments were completed by first equilibrating columns with 0.25 dS m−1 CaCl2, introducing a step pulse of ∼4.7 dS m−1 CaCl2 and, after equilibration, displacing the resident solution with 0.25 dS m−1 CaCl2. Using TDR, measured Ka increased with increasing σa; however, the slope of this response averaged 3.47 m dS−1 for clay loam compared with 0.19 m dS−1 for sand. The large response in the clay loam was attributed to relaxation losses that narrowed the effective bandwidth from 821 to an estimated 164 MHz. In contrast, the effective frequency in sand averaged 515 MHz. Permittivity measured using the TDT probe exhibited little or no sensitivity to σa (<0.32 m dS−1) in both media. Measured Ka using the 5TE probe declined with increasing σa up to 1 to 1.8 dS m−1 and then increased thereafter with net negative responses for sand (ΔKa/Δσa = −3.1 m dS−1) and net positive responses for the clay loam (ΔKa/Δσa = 2.9 m dS−1). Consideration of the Ka–σa response is required for accurate soil water content estimation (±0.03 m3 m−3) in the presence of solution EC variations using TDR in fine‐textured soils or the 5TE sensor in all media. Large differences in the sampling volumes between 5TE‐measured bulk EC and permittivity confounded the Ka–σa response in the presence of a concentration gradient.

Research on synthetic aperture radar imaging technology of one-dimensional layered rough surfaces

A quick and exact imaging method for one-dimensional layered rough surfaces is proposed in this paper to study the scattering characteristics of a layered medium that exists widely in nature. The boundary integral equations of layered rough surfaces are solved by using the propagation-inside-layer expansion combined with the forward and backward spectral acceleration method (PILE+FB-SA), and the back scattering data are obtained. Then, a conventional synthetic aperture radar (SAR) imaging procedure called back projection method is used to generate a two-dimensional (2D) image of the layered rough surfaces. Combined with the relative dielectric permittivity of realistic soil, the random rough surfaces with Gauss spectrum are used to simulate the layered medium with rough interfaces. Since the back scattering data are computed by using the fast numerical method, this method can be used to study layered rough surfaces with any parameter, which has a great application value in the detection and remote sensing areas.

Application of Multiphase Dielectric Mixing Models for Understanding the Effective Dielectric Permittivity of Frozen Soils

The time domain reflectometry (TDR)–measured effective permittivity in frozen soil conditions is affected by many complex factors including bound water effects on soil water permittivity, phase changes, soil microstructure and relative positions of soil constituents with respect to each other. The objective of this study was to improve understanding of some of the factors affecting the effective permittivity of frozen soils through the use of dielectric mixing models. Published datasets and frozen and unfrozen soil data measured on western Canadian soils were investigated with multiphase discrete and confocal ellipsoid models available in the literature. The results revealed that adjusting model parameters allowed the mixing models to describe the frozen soil permittivity equally well when bound water effects and temperature‐dependent water permittivity effects were included or not included. Measurement of freezing and thawing curves on western Canadian soils showed significant hysteresis and some mechanisms for this observed hysteresis and its influence on the interpretation of published datasets are discussed. When independent measurements of liquid water, ice and effective permittivity are available, it is possible to find one set of model parameters that reasonably predict effective permittivity for both frozen and unfrozen conditions. In frozen soils the predictive capability of the models is constrained to scenarios where the initial water content prior to freezing (i.e., the total water content) in the sampling volume is constant.