基于双频极化雷达技术的东南极冰盖内部结构特性研究
摘要
南极冰盖在全球气候系统中扮演着重要角色,南极冰盖的物质平衡状态和变化趋势对全球海平面变化和温盐环流变异具有举足轻重的影响。因此理解冰流机制和动力过程对于解释冰盖过去、现在和未来的变化规律非常重要。双频极化雷达技术提供了一种间接推测冰介质物性的技术手段。本文基于双频极化雷达技术的现状和需要,开展了以下三个方面工作:首先,从Maxwell方程出发,推导出横向各向异性介质电磁波时域有限差分(FDTD)公式,并使用前人试验测得的冰介质介电参数制作模型,验证了前人关于冰内雷达反射层性质的推断。其次,总结双频极化雷达技术的资料处理方法,针对目前缺乏相应处理软件的现状以及中国21次南极考察冰雷达数据处理的需求,笔者开发出一套处理软件,极大地提高了处理效率并改善了处理效果。最后,应用这套软件处理中国21次南极考察冰雷达数据,分析和解释处理结果,取得了一些列重要成果。其中包括:在Dome A顶部发现超过冰厚3000m的深谷,为中国深冰芯计划确定了目标靶区。获得了Dome A——中山站冰盖厚度和冰下地形数据。根据双频极化雷达资料推断解释Dome A——中山站之间冰盖内部冰晶组构特性等。
The Study on the Inner Structure and Tomography of East Antarctic Based on Two-Frequency and Multi-Polarization Plane Method of Radio-Echo-Sounding (RES)
Ph.D Supervisor: Prof. Gang Tian and Prof. Bo Sun Major: Geo-exploration and Information TechnologyAntarctic ice sheets play a crucial role in global climatic system, the state and tendency of mass balance influence the changes of global sea level and thermo-salt circumfluence significantly. Therefore, it is indispensable to understand ice stream mechanisms and dynamic processes for interpreting the past, present, and future changing law of ice sheet. The two-frequency polarization radar provides an indirect technical method to demonstrate the character of ice medium.Radio-Echo Sounding (RES) was used to detect the thickness of glaciers and ice sheets since 1960s, and had achieved great success as an important technique in ice-sheet exploration due to its convenience, high efficiency and accuracy. In 1990s, the four-year plan of BEDMAP implemented by BAS put forward special data-base of ice thickness and sub-glacial terrain covered Antarctic continent, and thereinto, 90% data came from radar sounding. RES not only can be used in ice-sheet thickness and sub-glacial terrain exploration, but also used in researching of space change and physical process of crystal-orientation fabrics. In 1999, summarizing former's achievements, based on the study of mass character of ice cores, Fujita suggested to use the two-frequency RES to distinguish reflecting layers caused by dielectric permitivity or conductivity. In 2003, Matsuka used dual-frequency and multi-polarization plane radar to research the space change of crystal-orientation fabrics in Shirase glaciers.
During the 21st South Pole ice-sheet investigation, the advanced two-frequency RES system was employed to survey the area between ZhongShan and Dome A, and got a set of valuable radar data. This paper, reviewed the past, present and future of the development of RES firstly, and then aimed at the technical characteristic and research progress of two-frequency RES, made further research and detailed discussion finally. We discussed on three topics primarily: forward simulation of electromagnetic wave response of anisotropic, processing software development and data process and interpretation of ZhongShan-Dome A radar data.
1. At present the commonly viewpoint is: internal reflections in ice sheet caused mostly by dielectric permittivity changes due to density fluctuations and(or) crystal-orientation fabrics changes and electrical conductivity changes due to changes in acidity(related to volcanic eruptions). Thro ugh experiments, some scholars found that heavy-acidity or anisotropic ice can cause detectable radar reflection. The paper, started off the equation of Maxwell, deduced the FDTD formula fit to the anisotropic and worked out the corresponding program. We made a set of arranged models which parameters were derived from the others experiment result, to simulate the various phenomenons of radar reflection in real Antarctic sheet. It includes:
A. The simulating result shows that the distinct reflection appeared if only changing the layer conductivity and keeping the permittivity in the model. B. The responding output of anisotropic medium was simulated in the case of antenna rotation. The simulating result shows that the strength of output voltage variated with rotation angle and had 180°period. C. The simulation results of the output differences of the three types of antenna commonly used in ice radar show that the type C is more sensitive to the difference between anisotropic and isotropic. D. The output differences in different antennas type were compared when the permittivity of the anisotropic medium changed. The result show that, to the type B, the arriving time from bed in the type A has "bring-forward" or "delay" phenomenon, and it depend on the wave impedance difference between the upper and the below layer. Based on these experiments, we presented a simple method for distinguishing the direction of ice flow by comparing the arriving time difference of the ice bed reflection in the two antenna type.
2. There are great differences between dual-frequency-polarization RES andtraditional RES in the collection method and proceeding and interpretationmethod, since dual-frequency-polarization RES is a new ice radar technique. Wecan obtain four sets of radar data in case of a single profile at least. These mayinclude data from two kinds of frequency (for example, 60 and 179MHz) and twopolarization directions. The data contain abundant information of the inner layers.We can extract them by subtracting between the different data, therefore we caninterpret the forming cause of the inner reflection layer (from the change ofdensity or acid or COF). We gave entire summarization on the proceeding methodof the dual-frequency-polarization RES.
The present actuality of the RES data proceeding technique exist in two points. The first is that the quantity of the RES data is huge. The proceeding works need a lot of time and the period of the data transformation is relatively long. The second is that the resource and superiority of the collective division and cooperation isn't used fully. Summarizing the technique of the dual-frequency-polarization RES, I developed a software applied in RES data proceeding of the China 21th Antarctic Expedition, and improved the efficiency greatly.
3. Based on the RES result of the China 21th Antarctic Expedition proceedingby my software, we interpreted some data obtained during the expedition. Firstly,we found a north-east deep channel according to 3D RES exploration result inDome A, its ice thickness exceeded 3000m. This provided the target area forsearch the oldest ice and the china deep ice core project. Secondly, We analyzedand interpreted the multi-polarization plane result, and compared it with the deepcore record in Dome Fuji. According to the comparing result, we proposed thatthe inner ice at the survey point in Dome A may be composed of elongated single-pole COF. In addition, based on the RES time-depth-conversion result, we obtained the ice-bed elevation and tomography combined with the surface elevation data. The RES result has higher precision than the data from BEDMAP database.
Based on the dual-frequency comparison, we analyzed the inner composition of the ice sheet in the Dome A- ZhongShan station section with large scale. Then a certain section with dual-frequencies and dual-polarizations was selected. We compared the results with diferent frequencies and polarizations and analyzed some corresponding points. According to the conclusion derived from simulation in chapter 4, the arriving time from the bottom of anisotropy layer has difference in the different antenna type. We chose some corresponding points with different polarization to estimate the ice flow direction. The statistic result showed that most of the deduced results were consistent with the real case.
The Study on the Inner Structure and Tomography of East Antarctic Based on Two-Frequency and Multi-Polarization Plane Method of Radio-Echo-Sounding (RES)
Ph.D Supervisor: Prof. Gang Tian and Prof. Bo Sun Major: Geo-exploration and Information TechnologyAntarctic ice sheets play a crucial role in global climatic system, the state and tendency of mass balance influence the changes of global sea level and thermo-salt circumfluence significantly. Therefore, it is indispensable to understand ice stream mechanisms and dynamic processes for interpreting the past, present, and future changing law of ice sheet. The two-frequency polarization radar provides an indirect technical method to demonstrate the character of ice medium.Radio-Echo Sounding (RES) was used to detect the thickness of glaciers and ice sheets since 1960s, and had achieved great success as an important technique in ice-sheet exploration due to its convenience, high efficiency and accuracy. In 1990s, the four-year plan of BEDMAP implemented by BAS put forward special data-base of ice thickness and sub-glacial terrain covered Antarctic continent, and thereinto, 90% data came from radar sounding. RES not only can be used in ice-sheet thickness and sub-glacial terrain exploration, but also used in researching of space change and physical process of crystal-orientation fabrics. In 1999, summarizing former's achievements, based on the study of mass character of ice cores, Fujita suggested to use the two-frequency RES to distinguish reflecting layers caused by dielectric permitivity or conductivity. In 2003, Matsuka used dual-frequency and multi-polarization plane radar to research the space change of crystal-orientation fabrics in Shirase glaciers.
During the 21st South Pole ice-sheet investigation, the advanced two-frequency RES system was employed to survey the area between ZhongShan and Dome A, and got a set of valuable radar data. This paper, reviewed the past, present and future of the development of RES firstly, and then aimed at the technical characteristic and research progress of two-frequency RES, made further research and detailed discussion finally. We discussed on three topics primarily: forward simulation of electromagnetic wave response of anisotropic, processing software development and data process and interpretation of ZhongShan-Dome A radar data.
1. At present the commonly viewpoint is: internal reflections in ice sheet caused mostly by dielectric permittivity changes due to density fluctuations and(or) crystal-orientation fabrics changes and electrical conductivity changes due to changes in acidity(related to volcanic eruptions). Thro ugh experiments, some scholars found that heavy-acidity or anisotropic ice can cause detectable radar reflection. The paper, started off the equation of Maxwell, deduced the FDTD formula fit to the anisotropic and worked out the corresponding program. We made a set of arranged models which parameters were derived from the others experiment result, to simulate the various phenomenons of radar reflection in real Antarctic sheet. It includes:
A. The simulating result shows that the distinct reflection appeared if only changing the layer conductivity and keeping the permittivity in the model. B. The responding output of anisotropic medium was simulated in the case of antenna rotation. The simulating result shows that the strength of output voltage variated with rotation angle and had 180°period. C. The simulation results of the output differences of the three types of antenna commonly used in ice radar show that the type C is more sensitive to the difference between anisotropic and isotropic. D. The output differences in different antennas type were compared when the permittivity of the anisotropic medium changed. The result show that, to the type B, the arriving time from bed in the type A has "bring-forward" or "delay" phenomenon, and it depend on the wave impedance difference between the upper and the below layer. Based on these experiments, we presented a simple method for distinguishing the direction of ice flow by comparing the arriving time difference of the ice bed reflection in the two antenna type.
2. There are great differences between dual-frequency-polarization RES andtraditional RES in the collection method and proceeding and interpretationmethod, since dual-frequency-polarization RES is a new ice radar technique. Wecan obtain four sets of radar data in case of a single profile at least. These mayinclude data from two kinds of frequency (for example, 60 and 179MHz) and twopolarization directions. The data contain abundant information of the inner layers.We can extract them by subtracting between the different data, therefore we caninterpret the forming cause of the inner reflection layer (from the change ofdensity or acid or COF). We gave entire summarization on the proceeding methodof the dual-frequency-polarization RES.
The present actuality of the RES data proceeding technique exist in two points. The first is that the quantity of the RES data is huge. The proceeding works need a lot of time and the period of the data transformation is relatively long. The second is that the resource and superiority of the collective division and cooperation isn't used fully. Summarizing the technique of the dual-frequency-polarization RES, I developed a software applied in RES data proceeding of the China 21th Antarctic Expedition, and improved the efficiency greatly.
3. Based on the RES result of the China 21th Antarctic Expedition proceedingby my software, we interpreted some data obtained during the expedition. Firstly,we found a north-east deep channel according to 3D RES exploration result inDome A, its ice thickness exceeded 3000m. This provided the target area forsearch the oldest ice and the china deep ice core project. Secondly, We analyzedand interpreted the multi-polarization plane result, and compared it with the deepcore record in Dome Fuji. According to the comparing result, we proposed thatthe inner ice at the survey point in Dome A may be composed of elongated single-pole COF. In addition, based on the RES time-depth-conversion result, we obtained the ice-bed elevation and tomography combined with the surface elevation data. The RES result has higher precision than the data from BEDMAP database.
Based on the dual-frequency comparison, we analyzed the inner composition of the ice sheet in the Dome A- ZhongShan station section with large scale. Then a certain section with dual-frequencies and dual-polarizations was selected. We compared the results with diferent frequencies and polarizations and analyzed some corresponding points. According to the conclusion derived from simulation in chapter 4, the arriving time from the bottom of anisotropy layer has difference in the different antenna type. We chose some corresponding points with different polarization to estimate the ice flow direction. The statistic result showed that most of the deduced results were consistent with the real case.
引文
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