10:45 AM - 11:00 AM
[ACC29-07] Improvement of the dielectric tensor measurement as a method to measure density, 3D porous structure and crystal orientation fabrics of ice cores: realization of high spatial resolution measurement and its impacts
Keywords:ice core, permittivity, density
Major physical properties of ice cores include (i) densification from snow to ice, and (ii) evolution of 3D porous structure composed of ice matrix and porous space. In addition, (iii) crystal orientation fabrics (COF) is an important property to reveal degree of deformability (or viscosity) of polycrystalline ice. These three items are measurable with a method “dielectric tensor measurement (hereinafter, DTM)” for which we use millimeter wave open resonators. The method is operatable with five advantageous conditions: (i) non-destructive, (ii) high-spatial resolution (a cylinder with ~15mm in diameter), (iii) continuous, (iv) rapid and (v) safe. The authors have developed the DTM method of ice core analyses for the tentorial values of relative permittivity (references given in our Japanese version abstract). Recently, we established a system with which we can measure thick sections up to about 70 mm with a diameter of about 15 mm, by a new design of the open resonator and higher power of the millimeter wave signal generator. In addition, by this achievement, we realized commonization of samples with samples used for Continuous Flow Analysis (CFA); CFA is the modern and major analytical method for ice cores to analyze major elements, water stable isotope ratio, dusts, gas components and so on with high-spatial resolution, continuously and rapidly. We explain here differences in experimental situations between this new DTM method and the cases when we rely on conventional methods of physical analysis.
(I) Density measurement
Using the DTM, we can derive density of samples mediating relative permittivity. As for conventional non-destructive continuous method, there are Gamma-ray transmission method and X-ray transmission method, in which we measure attenuation of the ray. However, when we use relatively low power of such rays because of safety control for hazardous rays, required measurement time becomes longer (days long), consuming time. In contrast, if we attempt to make measurement time shorter, we need rays with higher energy. Then we need much more strict control and care for safety to handle high energy radiation hazardous rays. Then, it is impossible to satisfy the 5 conditions as mentioned above at the same time.
(II) Anisotropy of the 3D porous structure
Using the DTM, we can extract orientation dependence of the permittivity. The method is based on interactions between electrical field vector of the electromagnetic wave and 3D geometrical structure of firn, composed of ice matrix and porous structure. Based on orientation dependence of the permittivity, we can derive degree of elongations of the structure. X-ray Computer Tomography (known as X-ray CT) is also often used for investigation of 3D geometrical structure of firn or snow. Although the method is advantageous for high-spatial-resolution, it is difficult to make it as non-destructive, continuous, or rapid.
(III) Crystal Orientation Fabrics
Again, by extracting orientation dependence of the permittivity, we can derive preferred orientation of c-axes in the polycrystalline ice. We can determine principal components of the COF in three orthogonal orientations. It is known as normalized eigenvalues. In comparison, a conventional method to detect COF is an optical ice fabric analyzer. In it, we observe orientation of c-axis for each crystal grains observing through crossed polaroids. Based on statistics from hundreds or thousands of grains data, we then derive the normalized eigenvalues. The DTM is advantageous because the COF information of extracted from thick sections instead of thin sections; final eigenvalues of data are statistically far more significant. In addition, the DTM is advantageous in terms of 5 conditions as mentioned above.
In this way, with the DTM, we can acquire basic physical information of ice cores very efficiently compared with any conventional methods. In addition, a big advantage is that samples can be common with CFA. As a result, we can reduce both amount of ice core consumption and preparation work to make samples (cutting, microtoming and/or ice core management work). Because both CFA and the DTM use common ice samples, we can maximize information extraction from ice cores, making synergistic effect between multiple kinds of data. We expect that the DTM will be one of standard and routine methods for the future studies of ice cores. Based on the innovation of the method, we can acquire high-resolution and high-quality data rapidly. In addition, we can enhance better understanding the physical structures and mechanisms within polar ice sheets and glaciers.
References are given in the Japanese version abstract.
(I) Density measurement
Using the DTM, we can derive density of samples mediating relative permittivity. As for conventional non-destructive continuous method, there are Gamma-ray transmission method and X-ray transmission method, in which we measure attenuation of the ray. However, when we use relatively low power of such rays because of safety control for hazardous rays, required measurement time becomes longer (days long), consuming time. In contrast, if we attempt to make measurement time shorter, we need rays with higher energy. Then we need much more strict control and care for safety to handle high energy radiation hazardous rays. Then, it is impossible to satisfy the 5 conditions as mentioned above at the same time.
(II) Anisotropy of the 3D porous structure
Using the DTM, we can extract orientation dependence of the permittivity. The method is based on interactions between electrical field vector of the electromagnetic wave and 3D geometrical structure of firn, composed of ice matrix and porous structure. Based on orientation dependence of the permittivity, we can derive degree of elongations of the structure. X-ray Computer Tomography (known as X-ray CT) is also often used for investigation of 3D geometrical structure of firn or snow. Although the method is advantageous for high-spatial-resolution, it is difficult to make it as non-destructive, continuous, or rapid.
(III) Crystal Orientation Fabrics
Again, by extracting orientation dependence of the permittivity, we can derive preferred orientation of c-axes in the polycrystalline ice. We can determine principal components of the COF in three orthogonal orientations. It is known as normalized eigenvalues. In comparison, a conventional method to detect COF is an optical ice fabric analyzer. In it, we observe orientation of c-axis for each crystal grains observing through crossed polaroids. Based on statistics from hundreds or thousands of grains data, we then derive the normalized eigenvalues. The DTM is advantageous because the COF information of extracted from thick sections instead of thin sections; final eigenvalues of data are statistically far more significant. In addition, the DTM is advantageous in terms of 5 conditions as mentioned above.
In this way, with the DTM, we can acquire basic physical information of ice cores very efficiently compared with any conventional methods. In addition, a big advantage is that samples can be common with CFA. As a result, we can reduce both amount of ice core consumption and preparation work to make samples (cutting, microtoming and/or ice core management work). Because both CFA and the DTM use common ice samples, we can maximize information extraction from ice cores, making synergistic effect between multiple kinds of data. We expect that the DTM will be one of standard and routine methods for the future studies of ice cores. Based on the innovation of the method, we can acquire high-resolution and high-quality data rapidly. In addition, we can enhance better understanding the physical structures and mechanisms within polar ice sheets and glaciers.
References are given in the Japanese version abstract.