rus
Issue #4/2024
D. V. Grigoriev, A. V. Koshurnikov
Procedure for Determination of the Electrical and Acoustic Properties of Frozen Soils During the Defrosting Cycle
Procedure for Determination of the Electrical and Acoustic Properties of Frozen Soils During the Defrosting Cycle
DOI: 10.22184/1993-7296.FRos.2024.18.4.332.338
This paper presents an original optical method for studying the specifications of frozen soil samples. The method for determination of the moisture quantitative composition in the frozen geological rocks is based on the measurement principles for the typical spectra of attenuated total reflection (ATR) using a dual-frequency modulation method for recording the optical signal emitted by the InGaAsSb-based semiconductor heterostructures. Further development of this method will make it possible to identify correlations between the reflected radiation parameters and specifications of the electrical and acoustic properties of frozen soils during the defrosting cycle.
This paper presents an original optical method for studying the specifications of frozen soil samples. The method for determination of the moisture quantitative composition in the frozen geological rocks is based on the measurement principles for the typical spectra of attenuated total reflection (ATR) using a dual-frequency modulation method for recording the optical signal emitted by the InGaAsSb-based semiconductor heterostructures. Further development of this method will make it possible to identify correlations between the reflected radiation parameters and specifications of the electrical and acoustic properties of frozen soils during the defrosting cycle.
Теги: acoustic emission properties (aep) attenuated total reflection (atr) electrical resistivity (er) frozen soils акустоэмиссионные свойства (аэ) мерзлые грунты нарушенное полное внутреннее отражение (нпво) удельное электрическое сопротивление (уэс)
Procedure for Determination of the Electrical and
Acoustic Properties of Frozen Soils During the Defrosting Cycle
D. V. Grigoriev, A. V. Koshurnikov
Lomonosov Moscow State University, Faculty of Geology, Department of Geocryology, Moscow, Russia
This paper presents an original optical method for studying the specifications of frozen soil samples. The method for determination of the moisture quantitative composition in the frozen geological rocks is based on the measurement principles for the typical spectra of attenuated total reflection (ATR) using a dual-frequency modulation method for recording the optical signal emitted by the InGaAsSb-based semiconductor heterostructures. Further development of this method will make it possible to identify correlations between the reflected radiation parameters and specifications of the electrical and acoustic properties of frozen soils during the defrosting cycle.
Key words: attenuated total reflection (ATR), frozen soils, electrical resistivity (ER), acoustic emission properties (AEP)
Article received: 28.03.2024
Article accepted: 03.05.2024
INTRODUCTION
The geophysical methods provide a set of informative data related to the structure, state and developmental dynamics of cryolitic zones that is important for practical implementation of the facility construction and operation activities in the permafrost area. The availability of ice in the permafrost formations establishes the contrast specifications of geophysical fields determined in the thawed and frozen states. The parameters of the electrical and acoustic characteristics of frozen rocks, determination of the moisture phase composition are necessary conditions for interpreting the results obtained.
This paper proposes the concept for determination of the moisture phase composition in the frozen soils, based on an analogue method for determining the moisture content in the freeze-dried products. It is based on the measurement method for the attenuated total reflection (ATR) index using a dual-frequency modulation method for recording an optical signal emitted by the InGaAsSb-based semiconductor heterostructures.
THEORY
Various research teams have been studying the dependencies of changes in the electrical and acoustic properties of frozen rocks within different temperature ranges. Most often, when studying the electric field specifications in the laboratory conditions, attention is paid to determination of the electrical resistivity values of rocks (ER – ρ, [Ohm∙m]) [1–3]. The main component that allows electrical current to pass through the rock is the pore liquid. When the soil is frozen, its significant part turns into ice that has a decisive influence on the resistance values.
The acoustic properties of rocks depend on the propagation nature of elastic waves that reflects the structural features of the medium, its thermodynamic state, physical and mechanical properties [2].
To determine the electric resistivity values in the experimental conditions, the vertical electrical sounding method is used in a four-electrode system [3]. The acoustic properties of rock samples are determined by the ultrasonic translucence method that is applied, among other things, to measure the elastic wave velocities in the samples [4]. To confirm reliability of the results obtained, the conventional methods are supplemented with a procedure to determine the moisture phase composition by measuring the moisture content due to the unfrozen water using the galvanic and cryoscopic methods [5]. These measurements are indirect. Therefore, we propose to consider a method for determining the moisture phase composition based on the direct measurement method. For this purpose, we have used the control method for the attenuated total reflection (ATR) spectra using the Fourier transform spectroscopy technique [6]. Spectral control is a direct measurement method, since it is actually based on linking the results to the fundamental physical constants.
The issues of determining the moisture content of various dispersed components are widespread in the field of infrared spectroscopy. When conducting such studies, the water absorption spectrum in the liquid phase is determined that has a typical absorption band in the range of 3 μm. The standard values of the resulting spectrum are interpreted that makes it possible to determine the phase composition of moisture contained in the frozen soil sample under study. The frozen soils represent a comprehensive multiphase dynamic geological system that always contains some amount of ice and water in the liquid phase. Any changes in the ratio of these two components cause the following changes in the physical, mechanical, acoustic and electrical properties of the rock [7]. The parameters determined by the ATR methods have a various nature of spectral dependence of the reflected signal.
SPECTRAL PHOTOMETRIC METHOD FOR MEASURING MOISTURE CONTENT USING THE ATR SIGNALS
When using the attenuated total reflection (ATR) spectroscopy, the light is passed through an optical material (ATR sensor). The optical material specifications, namely the optical transparency and a refractive index exceeding the refractive index of the environment, allow us to consider a sample of the optical material as a waveguide that internally reflects the optical radiation.
The development of noncooled semiconductor radiation detectors and mid-IR emitters based on the InGaAsSb compounds [8] has contributed to the creation of devices that record not only the moisture content of products, but also many other components. In 1989–1992, the meters were developed to control humidity during the food freeze-drying process by J. V. Intercomplex. The application of this method with the simultaneous registration ATR signals for geological research will significantly reduce the cost of equipment by eliminating the recording Fourier transform spectrometer and create the mobile devices adapted for application in the field conditions.
The moisture meter for dispersed media during its dehydration process contained two light-emitting diodes (LEDs) with the wavelengths of 2.9 and 2.7 microns and two photodiodes (PDs) with the linear sensitivity within the same ranges. The LED emission was modulated by a pulse current of 100 mA in accordance with the time chart (Fig. 1a). The LED was mounted on a leucosapphire substrate with the evaporated electrical contacts (Fig. 1b). The PD recording the LED radiation power was also located on this substrate. This substrate was placed in the focus of a spherical reflector (Fig. 2) with a diameter of 18 mm and a focal length of 5 mm, made of quartz glass with aluminum sputtering on the reflecting surface. A plane-parallel LED radiation beam illuminated the test product surface at an angle of 45 degrees, and the measuring PD that recorded the radiation scattered by the surface, was located normally to the test sample.
Subsequently, the signals were processed by normalizing the signals of the measurement channels Pmeas and Pref by the relevant radiant powers P1 and P2 of the LEDs. The ratio logarithm of these signals: ln was a value proportional to the humidity of the product under study. These calculations were based on the optical radiation absorption principles according to the Bouguer-Lambert-Beer law.
The tests were performed with the samples of various humidity levels. When the test material samples were transilluminated, the signals of the LED radiation reflected from them were recorded. The moisture content of the samples was determined by the gravimetric method. At the final stage, a calibration curve was plotted. This method was applied to study the moisture content of granulated sugar and common salt. The method made it possible to determine the humidity of samples with a relative humidity measurement error of no more than 0.1%. The dynamic range of measurements was 0–2% humidity.
Based on the results of these studies, the following methodology is proposed to determine correlation between the electrical and acoustic properties of frozen soils and their moisture content due to the unfrozen water. The ATR elements made of an optically transparent material equivalent to an optical waveguide are introduced into the soil sample under study. The probe material is n-type polycrystalline silicon, transparent within the range of 3 µm and having a fairly high refractive index (n = 3.456) that allows for favorable conditions for the radiation propagation along the waveguide. The material is well developed in the electronics industry and has a low cost. It is proposed to make the waveguide in the form of a cylindrical rod with a diameter of 8 mm and a length of 50 mm. One end is supposed to be made reflective by sputtering a gold mirror, and the opposite end shall have a spherical lens that provides the radiation input. Thus, we get a round-trip multiple ATR element. It is proposed to use the InGaAsSb-based LED emitters as the light sources. The photodetectors shall be used that are sensitive to the relevant spectral range. The radiation input and output devices in relation to the probes require separate consideration.
MATERIAL and METHODS
The methods for determining the electrical and acoustic properties of frozen rocks used in the geological studies have a number of inaccuracies and assumptions. The research is performed in a freezer compartment with a gradual temperature increase, and the air temperature determined during the experiment provides indirect ideas about the sample temperature that is not rather correct. In this regard, it is proposed to thaw a sample placed in a thermally insulated soil container outside the freezer compartment under the conditions of controlled heat inflow (Fig. 3). The heating resistor (9) and a temperature sensor (10) are installed on the lid of the soil container. The availability of heating resistor allows to set the temperature at which the sample thawing process will occur, and a therocouple-type temperature sensor controls the temperature conditions. The soil container is equipped with four thermocouples (7), availability of which allows to control the sample temperature during its thawing.
The measurement unit for determining ER values is equipped with four electrodes being the brass rods (8) [9]. The transmitting electrodes (AB) are required to pass electric current, and the other two (MN) are required to measure the potential difference.
To determine the propagation speed of compressional waves through the sample, an acoustic wave emitter and receiver in the form of piezoelectric elements (4) are fixed on the unit ends. It is proposed to supplement the unit with a device for photometric spectral measurement of moisture content.
The LED radiation generated by the reflector (11) enters a cylindrical waveguide (5) that is applied as an ATR element and has an edge beveled at an angle to the waveguide axis. To simplify understanding of the ongoing process, we will assume that the LED radiation is propagated in the waveguide along the spiral trajectories (6), while experiencing Fresnel reflection at the interface between the waveguide and the soil under study [6]. The other end of the waveguide has a spherical shape and collects the transmitted radiation onto a photodiode (12). The ATR element is made of polycrystalline silicon or germanium.
CONCLUSIONS
Any change in the ratio of the crystalline and liquid water phases in the frozen soils has an impact on changes in their acoustic and electrical properties, and additional measurement by the ATR method of the optical reflection parameters of thawing samples ensures reliability of the results obtained by using the emitter wavelength as a reference meter.
The proposed method makes it possible to impartially control changes in the moisture phase composition in the samples during their thawing process, as well as to make changes in the temperature, electrical conductivity and acoustic properties. It is scheduled to determine the correlation dependencies of electrical, acoustic and optical specifications that can greatly simplify and speed up the engineering and geocryological surveys. Further developments of a technology for using this method will keep the track of selecting the ATR element optimal geometry, convenient for use in the geological experimental units, and allow elaborating a calibration method for the moisture meter being created.
Acoustic Properties of Frozen Soils During the Defrosting Cycle
D. V. Grigoriev, A. V. Koshurnikov
Lomonosov Moscow State University, Faculty of Geology, Department of Geocryology, Moscow, Russia
This paper presents an original optical method for studying the specifications of frozen soil samples. The method for determination of the moisture quantitative composition in the frozen geological rocks is based on the measurement principles for the typical spectra of attenuated total reflection (ATR) using a dual-frequency modulation method for recording the optical signal emitted by the InGaAsSb-based semiconductor heterostructures. Further development of this method will make it possible to identify correlations between the reflected radiation parameters and specifications of the electrical and acoustic properties of frozen soils during the defrosting cycle.
Key words: attenuated total reflection (ATR), frozen soils, electrical resistivity (ER), acoustic emission properties (AEP)
Article received: 28.03.2024
Article accepted: 03.05.2024
INTRODUCTION
The geophysical methods provide a set of informative data related to the structure, state and developmental dynamics of cryolitic zones that is important for practical implementation of the facility construction and operation activities in the permafrost area. The availability of ice in the permafrost formations establishes the contrast specifications of geophysical fields determined in the thawed and frozen states. The parameters of the electrical and acoustic characteristics of frozen rocks, determination of the moisture phase composition are necessary conditions for interpreting the results obtained.
This paper proposes the concept for determination of the moisture phase composition in the frozen soils, based on an analogue method for determining the moisture content in the freeze-dried products. It is based on the measurement method for the attenuated total reflection (ATR) index using a dual-frequency modulation method for recording an optical signal emitted by the InGaAsSb-based semiconductor heterostructures.
THEORY
Various research teams have been studying the dependencies of changes in the electrical and acoustic properties of frozen rocks within different temperature ranges. Most often, when studying the electric field specifications in the laboratory conditions, attention is paid to determination of the electrical resistivity values of rocks (ER – ρ, [Ohm∙m]) [1–3]. The main component that allows electrical current to pass through the rock is the pore liquid. When the soil is frozen, its significant part turns into ice that has a decisive influence on the resistance values.
The acoustic properties of rocks depend on the propagation nature of elastic waves that reflects the structural features of the medium, its thermodynamic state, physical and mechanical properties [2].
To determine the electric resistivity values in the experimental conditions, the vertical electrical sounding method is used in a four-electrode system [3]. The acoustic properties of rock samples are determined by the ultrasonic translucence method that is applied, among other things, to measure the elastic wave velocities in the samples [4]. To confirm reliability of the results obtained, the conventional methods are supplemented with a procedure to determine the moisture phase composition by measuring the moisture content due to the unfrozen water using the galvanic and cryoscopic methods [5]. These measurements are indirect. Therefore, we propose to consider a method for determining the moisture phase composition based on the direct measurement method. For this purpose, we have used the control method for the attenuated total reflection (ATR) spectra using the Fourier transform spectroscopy technique [6]. Spectral control is a direct measurement method, since it is actually based on linking the results to the fundamental physical constants.
The issues of determining the moisture content of various dispersed components are widespread in the field of infrared spectroscopy. When conducting such studies, the water absorption spectrum in the liquid phase is determined that has a typical absorption band in the range of 3 μm. The standard values of the resulting spectrum are interpreted that makes it possible to determine the phase composition of moisture contained in the frozen soil sample under study. The frozen soils represent a comprehensive multiphase dynamic geological system that always contains some amount of ice and water in the liquid phase. Any changes in the ratio of these two components cause the following changes in the physical, mechanical, acoustic and electrical properties of the rock [7]. The parameters determined by the ATR methods have a various nature of spectral dependence of the reflected signal.
SPECTRAL PHOTOMETRIC METHOD FOR MEASURING MOISTURE CONTENT USING THE ATR SIGNALS
When using the attenuated total reflection (ATR) spectroscopy, the light is passed through an optical material (ATR sensor). The optical material specifications, namely the optical transparency and a refractive index exceeding the refractive index of the environment, allow us to consider a sample of the optical material as a waveguide that internally reflects the optical radiation.
The development of noncooled semiconductor radiation detectors and mid-IR emitters based on the InGaAsSb compounds [8] has contributed to the creation of devices that record not only the moisture content of products, but also many other components. In 1989–1992, the meters were developed to control humidity during the food freeze-drying process by J. V. Intercomplex. The application of this method with the simultaneous registration ATR signals for geological research will significantly reduce the cost of equipment by eliminating the recording Fourier transform spectrometer and create the mobile devices adapted for application in the field conditions.
The moisture meter for dispersed media during its dehydration process contained two light-emitting diodes (LEDs) with the wavelengths of 2.9 and 2.7 microns and two photodiodes (PDs) with the linear sensitivity within the same ranges. The LED emission was modulated by a pulse current of 100 mA in accordance with the time chart (Fig. 1a). The LED was mounted on a leucosapphire substrate with the evaporated electrical contacts (Fig. 1b). The PD recording the LED radiation power was also located on this substrate. This substrate was placed in the focus of a spherical reflector (Fig. 2) with a diameter of 18 mm and a focal length of 5 mm, made of quartz glass with aluminum sputtering on the reflecting surface. A plane-parallel LED radiation beam illuminated the test product surface at an angle of 45 degrees, and the measuring PD that recorded the radiation scattered by the surface, was located normally to the test sample.
Subsequently, the signals were processed by normalizing the signals of the measurement channels Pmeas and Pref by the relevant radiant powers P1 and P2 of the LEDs. The ratio logarithm of these signals: ln was a value proportional to the humidity of the product under study. These calculations were based on the optical radiation absorption principles according to the Bouguer-Lambert-Beer law.
The tests were performed with the samples of various humidity levels. When the test material samples were transilluminated, the signals of the LED radiation reflected from them were recorded. The moisture content of the samples was determined by the gravimetric method. At the final stage, a calibration curve was plotted. This method was applied to study the moisture content of granulated sugar and common salt. The method made it possible to determine the humidity of samples with a relative humidity measurement error of no more than 0.1%. The dynamic range of measurements was 0–2% humidity.
Based on the results of these studies, the following methodology is proposed to determine correlation between the electrical and acoustic properties of frozen soils and their moisture content due to the unfrozen water. The ATR elements made of an optically transparent material equivalent to an optical waveguide are introduced into the soil sample under study. The probe material is n-type polycrystalline silicon, transparent within the range of 3 µm and having a fairly high refractive index (n = 3.456) that allows for favorable conditions for the radiation propagation along the waveguide. The material is well developed in the electronics industry and has a low cost. It is proposed to make the waveguide in the form of a cylindrical rod with a diameter of 8 mm and a length of 50 mm. One end is supposed to be made reflective by sputtering a gold mirror, and the opposite end shall have a spherical lens that provides the radiation input. Thus, we get a round-trip multiple ATR element. It is proposed to use the InGaAsSb-based LED emitters as the light sources. The photodetectors shall be used that are sensitive to the relevant spectral range. The radiation input and output devices in relation to the probes require separate consideration.
MATERIAL and METHODS
The methods for determining the electrical and acoustic properties of frozen rocks used in the geological studies have a number of inaccuracies and assumptions. The research is performed in a freezer compartment with a gradual temperature increase, and the air temperature determined during the experiment provides indirect ideas about the sample temperature that is not rather correct. In this regard, it is proposed to thaw a sample placed in a thermally insulated soil container outside the freezer compartment under the conditions of controlled heat inflow (Fig. 3). The heating resistor (9) and a temperature sensor (10) are installed on the lid of the soil container. The availability of heating resistor allows to set the temperature at which the sample thawing process will occur, and a therocouple-type temperature sensor controls the temperature conditions. The soil container is equipped with four thermocouples (7), availability of which allows to control the sample temperature during its thawing.
The measurement unit for determining ER values is equipped with four electrodes being the brass rods (8) [9]. The transmitting electrodes (AB) are required to pass electric current, and the other two (MN) are required to measure the potential difference.
To determine the propagation speed of compressional waves through the sample, an acoustic wave emitter and receiver in the form of piezoelectric elements (4) are fixed on the unit ends. It is proposed to supplement the unit with a device for photometric spectral measurement of moisture content.
The LED radiation generated by the reflector (11) enters a cylindrical waveguide (5) that is applied as an ATR element and has an edge beveled at an angle to the waveguide axis. To simplify understanding of the ongoing process, we will assume that the LED radiation is propagated in the waveguide along the spiral trajectories (6), while experiencing Fresnel reflection at the interface between the waveguide and the soil under study [6]. The other end of the waveguide has a spherical shape and collects the transmitted radiation onto a photodiode (12). The ATR element is made of polycrystalline silicon or germanium.
CONCLUSIONS
Any change in the ratio of the crystalline and liquid water phases in the frozen soils has an impact on changes in their acoustic and electrical properties, and additional measurement by the ATR method of the optical reflection parameters of thawing samples ensures reliability of the results obtained by using the emitter wavelength as a reference meter.
The proposed method makes it possible to impartially control changes in the moisture phase composition in the samples during their thawing process, as well as to make changes in the temperature, electrical conductivity and acoustic properties. It is scheduled to determine the correlation dependencies of electrical, acoustic and optical specifications that can greatly simplify and speed up the engineering and geocryological surveys. Further developments of a technology for using this method will keep the track of selecting the ATR element optimal geometry, convenient for use in the geological experimental units, and allow elaborating a calibration method for the moisture meter being created.
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