Pavel P. Bobrov

Soil dielectric spectroscopic parameters dependence on humus content

The purpose of this paper is to apply generalized refractive mixing dielectric model (GRMDM) based on the Debye relaxation formula to soils with various humus contents. With this approach, the soil types containing 6.6% and 0.6% of natural humus. Complex dielectric constant or complex refractive index were measured as a function of moisture at the frequencies of 0.55; 1.1; 1.8, 3.0, 11.5, 13.5 and 16.3 GHz with the temperature being of 20-24 /spl deg/C. Using measured data only at 1.8, 3.0, 4.3 GHz, the GRMDM parameters - relaxation time, static dielectric constant, and conductivity for both types of water in soil were attained for both types of soil. Though variation in the relaxation parameters with humus content is moderate, this factor has to be taken into account when soil moisture remote sensing algorithms are being designed for agricultural areas. For this purpose, the GRMDM can be applied, with a soil humus content being one of its physical parameters.

Wideband Frequency Domain Method of Soil Dielectric Property Measurements

The measuring methods of the complex relative permittivity (CRP) of the sample in the single coaxial cell at frequencies from tens of hertz to several gigahertz are represented. The frequency range is covered by two instruments: by an LCR meter (frequencies up to 5 MHz) and by a vector network analyzer (VNA) (frequencies from 0.3 MHz to several gigahertz). In the high band (0.1-8.5 GHz), the cell is a coaxial line segment filled with the investigated substance. To determine the CRP, the complex transmission coefficient (scattering matrix parameter S12) of this cell is measured by using a VNA. For measurements in the middle range (0.3-100 MHz), the cell is included into the break of the central conductor of a coaxial line of a large cross section. The admittance of the cell is determined by measuring the scattering matrix parameter S12 of the coaxial line of the large cross section. At frequencies below 1 MHz, the cell admittance is measured in the usual way by the LCR meter. Thus, CRP measurements of a substance placed in the same cell are carried out in the whole frequency range from 42 Hz to 8.5 GHz. The examples of CRP measurement of soils with different values of the dielectric constants and loss factors are given. It is shown that the CRP measurement error depends on the values of CRP and frequency. The measurement result error of relative dielectric permittivity obtained with the aid of this method is from 0.4% to 2%.

Multirelaxation Generalized Refractive Mixing Dielectric Model of Moist Soils

In this letter, a multirelaxation generalized refractive mixing dielectric model (GRMDM) for moist soil is proposed and substantiated in the frequency range from 0.04 to 26.5 GHz. This model is based on the methodology of a single-relaxation GRMDM which accounts only for the dipole relaxation of water molecules in the gigahertz frequency range. The proposed multirelaxation GRMDM takes into account both the dipole (Debye) and ionic (Maxwell-Wagner) relaxations of soil water molecules. For this purpose, it uses a two-frequency Debye relaxation equation for the dielectric spectra of bound water. The spectroscopic parameters of the multirelaxation GRMDM were derived by fitting the spectra calculated by this model to the respective measured ones. The main advantage of this model is that it predicts the complex dielectric constant of moist soils throughout the megahertz and gigahertz frequency ranges with the same error as the single-relaxation GRMDM does only in the gigahertz range.

Data Processing Technique for Deriving Soil Water Spectroscopic Parameters in Microwave

In this paper, the technique employed in the generalized refractive mixing dielectric model (GRMDM) (1), (2) for soil water spectroscopic analysis is extended to include fitting the refractive mixing dielectric model to moist soil complex dielectric constant (CDC) data in the frequency domain, with a restricted number of volumetric moistures being available. To reach this goal, the refractive mixing dielectric model was fitted to the moist soil CDCs measured first at volumetric moistures in the range below the maximum bound water fraction (MBWF). At this phase of data processing, spectroscopic parameters, that is, static dielectric constant, relaxation time, and conductivity, relating to the bound soil water were derived, along with the values of CDCs relating to a dry soil. Further fitting in frequency domain to the CDCs of moist soil, measured in the range of free soil water moistures, yielded the values of MBWF, as well as the spectroscopic parameters related to the free soil water. After this final stage of data processing, a full set of the GRMDM spectroscopic parameter became available to insure CDCs prediction as a function of soil moisture and frequency. The technique was validated on the basis of correlating the predicted moist soil dielectric data to those measured.

The effect of clay and organic matter content on the dielectric permittivity of soils and grounds at the frequency range from 10 MHz to 1 GHz

In this paper, the results of measurements in the frequency range from 10 MHz to 1.0 GHz of the complex dielectric permittivity for some moist artificial soil, that is, mixtures consisting of quartz sand and bentonite clay components, with varying particle-size distributions, are discussed. The measurements were conducted with the use of coaxial line and capacitor probes in conjunction with a vector network analyzer. The complex permittivity was found to increase with the frequency decreasing and moisture increasing. In this case, the greatest variations in dielectric constant with frequency, in the limit of factor 3, were observed for the samples containing a largest percentage of clay.

Frequency Dependence of Permittivity of Free and Bound Water in Soils for Different Textures

In microwave methods of the soil moisture remote sensing it is necessary to have models for determining the complex dielectric permeability (CDP) dependence on moisture, frequency, mineralogy and particle-size distribution. The soil permeability measurement shows that the basic contribution to the CDP is introduced by free and bound water, and permeability of soil water depends on a soil type. To establish the influence of particle-size distribution of soil particles on CDP of the bound and free soil water we carried out the measurements of the frequency and moisture dependencies of refraction index of artificial sandy-clay mixes having different mineral structure and particle-size distribution. The carried out research at the frequencies from 30MHz up to 4 GHz has shown that the refraction index of free soil water depends on the quartz granules sizes, as well as on the clay fraction relative content. The characteristic feature of the free soil water is the increase of refraction index at frequency reduction. This dependence is the stronger the more clay the sample has. The similar properties are also observed in the bound water.

Dynamic radiobrightness for drying soils as a function of humus content

In this investigation, brightness observations were carried out in the course of drying over bare clayey soils, which have different content of humus, using a dual-frequency (2.7 and 8.2 GHz) radiometer. The observation site was located in the steppe area of West Siberia near the city of Omsk, and the time of observation stretched from May through September in 2003 and 2004. The normalized difference index (NDI), designated as the difference of brightness temperatures at 2.7 and 8.2 GHz divided by the sum of those, was studied in 14 cycles of drying, with the data being sampled in the day hours. In all drying cycles, substantial difference between the NDIs relating to poor and rich in humus soils was observed in the period from 5 to 30 hours after irrigation, depending on its intensity. At this time, the NDIs were shown to lie in the ranges from 0.3 to 0.0 and from – 0.5 to – 0.25 for the soils having 0.6% and 6.6% humus content, respectively. The soil moisture gradients with the depth in drying soil were shown to depend on the porous/aggregate structure of soil and suggested to account for the NDI dynamics of the soils under study. As a result, the 0.02 contrast in NDI magnitudes was found to be applicable for distinguishing between the poor and rich in humus soils, using brightness temperature measurements conducted in the course of post irrigation/precipitation drying of

Impact of the Soil Moisture Distribution in the Top Layer on the Accuracy Moisture Retrieval by Microwave Radiometer Data

This paper investigates the impact of the soil moisture distribution in the top layer on the accuracy of soil moisture retrieval by microwave remote sensing methods. We modeled soil emission at L-band by coherent and noncoherent models for the different moisture distributions in the top layer. As a result, it is found that, at high moisture gradients, the difference between average moisture within the sensing depth at L-band and the moisture retrievable from remote sensing data can be more than 20% in absolute terms. In addition, high differences between Soil Moisture and Ocean Salinity (SMOS) Level 2 data and the in situ measurements were revealed in cases of high gradients. Such high gradients may be observed during some time in the top layer of the drying soil after rainfall. These differences are significantly more than the accuracy declared by SMOS development team. We proposed a simple method that allows the assessment of the type of soil moisture profile by SMOS and Global Change Observation Mission-Water “SHIZUKU” (GCOM-W1) satellites data. The procedure for simple processing of data of the two satellites is described. In addition, we compared the type of soil moisture profile retrieved from satellite data and the soil moisture profile found by in situ measurements.

Microwave spectroscopic dielectric model of moist soils using physical and hydrological characteristics as input parameters

In this paper, the generalized refractive mixing dielectric model (GRMDM) has been modified to incorporate the major advantage of the semiempirical dielectric mixing model (SDMM) in providing for the complex dielectric constant of moist soil predictions in the microwave band, using only physical and hydrological characteristics of the soil as input parameters. First, the GRMDM spectral parameters were obtained for a set of 14 types of soils having various texture and mineralogy characteristics. Second, the regression equations linking the GRMDM spectral parameters to the physical and hydrological ones were deduced to ensure predictions of the complex dielectric constant of moist soils as a function of wave frequency, volumetric moisture, soil sand, clay, and organic component percentages, dry soil density, and maximum hygroscopicity soil. Through this methodology, a modified refractive mixing dielectric model (MRMDM) was developed, which provided for prediction of complex dielectric constant of moist soils, using their physical and hydrological characteristics as input parameters.

Microwave dielectric spectroscopy of moist soils for a forest-tundra region

− In this paper, the measured microwave dielectric data are presented for some soils collected in the forest-tundra area located at 64° N and 100° E, which is near the city of Tura in East Siberia. The measurements were developed in the range from 0.3 to 12.5 GHz at the temperature of 24°C. The Debye spectroscopic parameters related to the bound soil water (BSW) and free soil water (FSW) were derived with the use of the generalized refractive mixing dielectric model (GRMDM) [1], [2]. The foresttundra soils analyzed were found to contain the smaller percentage of bound water, with its complex dielectric constant (CDC) being less than that of the soils in the agricultural zones of Siberia [3]-[5], in spite of the fact that the forest-tundra soils demonstrated smaller clay and humus percentage as compared to the agricultural soils. The results obtained can be considered as a substantial contribution to the soil dielectric data base for the northern circumpolar region, which is a compulsory element of physically based both the remote sensing models and retrieving algorithms.