利用冰雷达诊断南极冰盖底部环境的研究综述
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摘要
现今南极冰盖底部的环境对冰盖的动力演化有着重要的影响,准确的冰盖底部环境信息有利于更加准确地模拟南极冰盖的起源、发展以及未来的演化。采用无线电回波技术,可有效地对冰盖进行探测。通过定量地处理雷达波在冰盖内部的介电衰减,可以获取冰盖底部的反射率,从而对冰盖底部环境,特别是冰下水文环境进行诊断。综述了雷达波在冰盖内部衰减处理的三种方法,分析了它们的适用性及局限性,并对这些方法应用于诊断冰盖底部的环境以及方法本身可能的改进进行了讨论。
The subglacial environment in Antarctica has important effects on the evolution of the Antarctic ice sheet. Accurate information on the subglacial environment information will therefore enable more accurate simulation of the evolution of the ice sheet. Currently,radio echo sounding is the most effective technique available to investigate the development of ice sheet. The subglacial environment,in particular the hydrological environment,can be monitored based on bed reflectivity,which is measured by quantitative analysis of the dielectric attenuation of radar waves in the ice sheet. Three methods used to process radar wave attenuation in the ice sheet are reviewed,the applicability and limitations of these methods are analyzed,the subglacial environment is determined using each method,and potential improvements are discussed.
The subglacial environment in Antarctica has important effects on the evolution of the Antarctic ice sheet. Accurate information on the subglacial environment information will therefore enable more accurate simulation of the evolution of the ice sheet. Currently,radio echo sounding is the most effective technique available to investigate the development of ice sheet. The subglacial environment,in particular the hydrological environment,can be monitored based on bed reflectivity,which is measured by quantitative analysis of the dielectric attenuation of radar waves in the ice sheet. Three methods used to process radar wave attenuation in the ice sheet are reviewed,the applicability and limitations of these methods are analyzed,the subglacial environment is determined using each method,and potential improvements are discussed.
引文
1 Fretwell P,Pritchard H D,Vaughan D G,et al.Bedmap2:improved ice bed,surface and thickness datasets for Antarctica[J].The Cryosphere,2013,7(1):375—393.
2 唐学远,张占海,孙波.南极冰盖数值模拟研究进展与展望[J].极地研究,2006,18(4):290—300.
3 Lythe M B,Vaughan D G.BEDMAP:a new ice thickness and subglacial topographic model of Antarctica[J].Journal of Geophysical Research:Solid Earth(1978—2012),2001,106(B6):11335—11351.
4 Fujita S,Maeno H,Uratsuka S,et al.Nature of radio echo layering in the Antarctic Ice Sheet detected by a two-frequency experiment[J].Journal of Geophysical Research:Solid Earth(1978—2012),1999,104(B6):13013—13024.
5 Fujita S,Mae S.Causes and nature of ice-sheet radio-echo internal reflections estimated from the dielectric properties of ice[J].Annals of Glaciology,1994,20(1):80—86.
6 Wang B,Tian G,Cui X,et al.The internal COF features in Dome A of Antarctica revealed by multi-polarization-plane RES[J].Applied Geophysics,2008,5(3):230—237.
7 Vaughan D G,Anderson P S,King J C,et al.Imaging of firn isochrones across an Antarctic ice rise and implications for patterns of snow accumulation rate[J].Journal of Glaciology,2004,50(170):413—418.
8 Matsuoka T,Fujita S,Mae S.Effect of temperature on dielectric properties of ice in the range 5—39 GHz[J].Journal of Applied Physics,1996,80 (10):5884—5890.
9 Eisen O,Hamann I,Kipfstuhl S,et al.Direct evidence for continuous radar reflector originating from changes in crystal-orientation fabric[J].The Cryosphere,2007,1(1):1—10.
10 唐学远,孙波,崔祥斌.南极冰盖内部等时层研究进展综述[J].极地研究,2015,27(1):104—114.
11 Drews R,Eisen O,Weikusat I,et al.Layer disturbances and the radio-echo free zone in ice sheets[J].The Cryosphere,2009,3(2):195—203.
12 Rignot E,Mouginot J,Scheuchl B.Ice flow of the Antarctic ice sheet[J].Science,2011,333(6048):1427—1430.
13 崔祥斌,孙波,田钢,等.冰雷达探测研究南极冰盖的进展与展望[J].地球科学进展,2009,24(4):392—402.
14 Christner B C,Priscu J C,Achberger A M,et al.A microbial ecosystem beneath the West Antarctic ice sheet[J].Nature,2014,512(7514):310 —313.
15 唐学远,孙波,李院生,等.冰穹A冰川学研究进展及深冰芯计划展望[J].极地研究,2012,24(1):77—86.
16 Morse D L,Blankenship D D,Waddington E D,et al.A site for deep ice coring in West Antarctica:results from aerogeophysical surveys and thermo-kinematic modeling[J].Annals of Glaciology,2002,35(1):36—44.
17 Tang X Y,Sun B,Li Y S,et al.Dome Argus:ideal site for deep ice drilling[J].Advances in Polar Science,2012,23(1):47—54.
18 崔祥斌,孙波,张向培,等.极地冰盖冰雷达探测技术的发展综述[J].极地研究,2009,21(4):322—335.
19 Tang X Y,Zhang Z H,Sun B,et al.Antarctic ice sheet GLIMMER model test and its simplified model on 2-dimensional ice flow[J].Progress in Natural Science,2008,18(2):173—180.
20 唐学远,孙波,李院生,等.南极冰盖研究最新进展[J].地球科学进展,2009,24(11):1210—1218.
21 Siegert M J.Antarctic subglacial lakes[J].Earth-Science Reviews,2000,50(1—2):29—50.
22 Siegert M J,Carter S,Tabacco I,et al.A revised inventory of Antarctic subglacial lakes[J].Antarctic Science,2005,17(3):453—460.
23 Robin G D Q,Swithinbank C W M,Smith B M E.Radio echo exploration of the Antarctic ice sheet[C]//International Symposium on Antarctic Glaciological Exploration(ISAGE).Hanover,NH,USA:International Association of Scientific Hydrology Publication,1970:97—115.
24 Oswald G K A,Robin G D Q.Lakes beneath the Antarctic ice sheet[J].Nature,1973,245(5423):251—254.
25 Kapitsa A P,Ridley J K,Robin G D Q,et al.A large deep freshwater lake beneath the ice of central East Antarctica[J].Nature,1996,381(6584):684—686.
26 Siegert M J,Dowdeswell J A,Gorman M R,et al.An inventory of Antarctic sub-glacial lakes[J].Antarctic Science,1996,8(3):281—286.
27 Wright A,Siegert M.A fourth inventory of Antarctic subglacial lakes[J].Antarctic Science,2012,24(6):659—664.
28 Fujita S,Maeno H,Matsuoka K.Radio-wave depolarization and scattering within ice sheets:a matrix-based model to link radar and ice-core measurements and its application[J].Journal of Glaciology,2006,52(178):407—424.
29 Matsuoka K,Wilen L,Hurley S P,et al.Effects of birefringence within ice sheets on obliquely propagating radio waves[J].IEEE Transactions on Geoscience and Remote Sensing,2009,47(5):1429—1443.
30 Mac Gregor J A,Winebrenner D P,Conway H,et al.Modeling englacial radar attenuation at Siple Dome,West Antarctica,using ice chemistry and temperature data[J].Journal of Geophysical Research-Earth Surface,2007,112(F3):F03008.
31 Matsuoka K,Furukawa T,Fujita S,et al.Crystal orientation fabrics within the Antarctic ice sheet revealed by a multipolarization plane and dual-frequency radar survey[J].Journal of Geophysical Research:Solid Earth(1978—2012),2003,108(B10):2499.
32 Bentley C R,Lord N,Liu C.Radar reflections reveal a wet bed beneath stagnant Ice Stream C and a frozen bed beneath ridge BC,West Antarctica[J].Journal of Glaciology,1998,44(146):149—156.
33 Fujita S,Holmlund P,Matsuoka K,et al.Radar diagnosis of the subglacial conditions in Dronning Maud Land,East Antarctica[J].The Cryosphere Discussions,2012,6(3):1781—1837.
34 Matsuoka K,Morse D,Raymond C F.Estimating englacial radar attenuation using depth profiles of the returned power,central West Antarctica[J].Journal of Geophysical Research:Earth Surface(2003—2012),2010,115(F2):F02012.
35 Shabtaie S,Bentley C R.West Antarctic ice streams draining into the Ross Ice Shelf:Configuration and mass balance[J].Journal of Geophysical Research:Solid Earth(1978—2012),1987,92(B2):1311—1336.
36 Gades A M,Raymond C F,Conway H,et al.Bed properties of Siple Dome and adjacent ice streams,West Antarctica,inferred from radio-echo sounding measurements[J].Journal of Glaciology,2000,46(152):88—94.
37 Catania G A,Conway H B,Gades A M,et al.Bed reflectivity beneath inactive ice streams in West Antarctica[J].Annals of Glaciology,2003,36(1):287—291.
38 Raymond C F,Catania G A,Nereson N,et al.Bed radar reflectivity across the north margin of Whillans Ice Stream,West Antarctica,and implications for margin processes[J].Journal of Glaciology,2006,52(176):3—10.
39 Winebrenner D P,Smith B E,Catania G A,et al.Radio-frequency attenuation beneath Siple Dome,West Antarctica,from wide-angle and profiling radar observations[J].Annals of Glaciology,2003,37(1):226—232.
40 Matsuoka K,Mac Gregor J A,Pattyn F.Using englacial radar attenuation to better diagnose the subglacial environment:a review[C]//Proceedings of the IEEE 2010 13th International Conference on Ground Penetrating Radar(GPR).Lecce:IEEE,2010:1—5.
41 Jacobel R W,Welch B C,Osterhouse D,et al.Spatial variation of radar-derived basal conditions on Kamb Ice Stream,West Antarctica[J].Annals of Glaciology,2009,50(51):10—16.
42 Fujita S,Matsuoka T,Ishida T,et al.A summary of the complex dielectric permittivity of ice in the megahertz range and its applications for radar sounding of polar ice sheets[J].Physics of Ice Core Records,2000:185—212.
43 Matsuoka K,Mac Gregor J A,Pattyn F.Predicting radar attenuation within the Antarctic ice sheet[J].Earth and Planetary Science Letters,2012,359 —360:173—183.
44 Mac Gregor J A,Matsuoka K,Studinger M.Radar detection of accreted ice over Lake Vostok,Antarctica[J].Earth and Planetary Science Letters,2009,282(1—4):222—233.
45 Matsuoka K,Pattyn F,Callens D,et al.Radar characterization of the basal interface across the grounding zone of an ice-rise promontory in East Antarctica[J].Annals of Glaciology,2012,53(60):29—34.
46 Carter S P,Blankenship D D,Young D A,et al.Using radar-sounding data to identify the distribution and sources of subglacial water:application to Dome C,East Antarctica[J].Journal of Glaciology,2009,55(194):1025—1040.
47 唐学远,孙波,张占海,等.东南极冰盖Dome A的内部等时层结构[J].中国科学:地球科学,2010,40(11):1504—1509.
48 Suzuki T,Iizuka Y,Matsuoka K,et al.Distribution of sea salt components in snow cover along the traverse route from the coast to Dome Fuji station1000 km inland at east Dronning Maud Land,Antarctica[J].Tellus B,2002,54(4):407—411.
49 Matsuoka K.Pitfalls in radar diagnosis of ice-sheet bed conditions:Lessons from englacial attenuation models[J].Geophysical Research Letters,2011,38(5):L05505.
50 Camplin G C,Glen J W.The dielectric properties of HF-doped single crystals of ice[M]//Whalley E,Jones S J,Gold L W.Physics and Chemistry of Ice.Ottawa:Royal Society of Canada,1973:256—261.
51 Johari G P,Charette P A.The permittivity and attenuation in polycrystalline and single-crystal ice Ih at 35 and 60 MHz[J].Journal of Glaciology,1975,14(71):293—303.
52 Takei I,Maeno N.Electric characteristics of point defects in HCl-doped ice[J].Le Journal de Physique Colloques,1987,48(C1):C1-121-C1-126.
53 Matsuoka K,Fujita S,Matsuoka T,et al.Measurements of the complex permittivity of acid-doped ice from1 k Hz to 30 MHz-New data set for developing ice radar and dielectricanalysis of ice cores[C]//Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology.Tokyo,Japan:National Institute of Polar Research,1996:25—35.
54 Pattyn F.Antarctic subglacial conditions inferred from a hybrid ice sheet/ice stream model[J].Earth and Planetary Science Letters,2010,295(3—4):451—461.
55 Li X,Sun B,Siegert M J,et al.Characterization of subglacial landscapes by a two-parameter roughness index[J].Journal of Glaciology,2010,56(199):831—836.
56 Bingham R G,Siegert M J.Radar-derived bed roughness characterization of Institute and Mller ice streams,West Antarctica,and comparison with Siple Coast ice streams[J].Geophysical Research Letters,2007,34(21):L21504.
57 Bingham R G,Siegert M J.Quantifying subglacial bed roughness in Antarctica:implications for ice-sheet dynamics and history[J].Quaternary Science Reviews,2009,28(3—4):223—236.
58 Sun B,Moore J C,Zwinger T,et al.How old is the ice beneath Dome A,Antarctica[J].The Cryosphere Discussions,2014,8(1):289—305.
59 崔祥斌,孙波,田钢,等.东南极Dome A冰雷达探测:冰厚分布和冰下地形[J].科学通报,2010,55(3):268—273.
2 唐学远,张占海,孙波.南极冰盖数值模拟研究进展与展望[J].极地研究,2006,18(4):290—300.
3 Lythe M B,Vaughan D G.BEDMAP:a new ice thickness and subglacial topographic model of Antarctica[J].Journal of Geophysical Research:Solid Earth(1978—2012),2001,106(B6):11335—11351.
4 Fujita S,Maeno H,Uratsuka S,et al.Nature of radio echo layering in the Antarctic Ice Sheet detected by a two-frequency experiment[J].Journal of Geophysical Research:Solid Earth(1978—2012),1999,104(B6):13013—13024.
5 Fujita S,Mae S.Causes and nature of ice-sheet radio-echo internal reflections estimated from the dielectric properties of ice[J].Annals of Glaciology,1994,20(1):80—86.
6 Wang B,Tian G,Cui X,et al.The internal COF features in Dome A of Antarctica revealed by multi-polarization-plane RES[J].Applied Geophysics,2008,5(3):230—237.
7 Vaughan D G,Anderson P S,King J C,et al.Imaging of firn isochrones across an Antarctic ice rise and implications for patterns of snow accumulation rate[J].Journal of Glaciology,2004,50(170):413—418.
8 Matsuoka T,Fujita S,Mae S.Effect of temperature on dielectric properties of ice in the range 5—39 GHz[J].Journal of Applied Physics,1996,80 (10):5884—5890.
9 Eisen O,Hamann I,Kipfstuhl S,et al.Direct evidence for continuous radar reflector originating from changes in crystal-orientation fabric[J].The Cryosphere,2007,1(1):1—10.
10 唐学远,孙波,崔祥斌.南极冰盖内部等时层研究进展综述[J].极地研究,2015,27(1):104—114.
11 Drews R,Eisen O,Weikusat I,et al.Layer disturbances and the radio-echo free zone in ice sheets[J].The Cryosphere,2009,3(2):195—203.
12 Rignot E,Mouginot J,Scheuchl B.Ice flow of the Antarctic ice sheet[J].Science,2011,333(6048):1427—1430.
13 崔祥斌,孙波,田钢,等.冰雷达探测研究南极冰盖的进展与展望[J].地球科学进展,2009,24(4):392—402.
14 Christner B C,Priscu J C,Achberger A M,et al.A microbial ecosystem beneath the West Antarctic ice sheet[J].Nature,2014,512(7514):310 —313.
15 唐学远,孙波,李院生,等.冰穹A冰川学研究进展及深冰芯计划展望[J].极地研究,2012,24(1):77—86.
16 Morse D L,Blankenship D D,Waddington E D,et al.A site for deep ice coring in West Antarctica:results from aerogeophysical surveys and thermo-kinematic modeling[J].Annals of Glaciology,2002,35(1):36—44.
17 Tang X Y,Sun B,Li Y S,et al.Dome Argus:ideal site for deep ice drilling[J].Advances in Polar Science,2012,23(1):47—54.
18 崔祥斌,孙波,张向培,等.极地冰盖冰雷达探测技术的发展综述[J].极地研究,2009,21(4):322—335.
19 Tang X Y,Zhang Z H,Sun B,et al.Antarctic ice sheet GLIMMER model test and its simplified model on 2-dimensional ice flow[J].Progress in Natural Science,2008,18(2):173—180.
20 唐学远,孙波,李院生,等.南极冰盖研究最新进展[J].地球科学进展,2009,24(11):1210—1218.
21 Siegert M J.Antarctic subglacial lakes[J].Earth-Science Reviews,2000,50(1—2):29—50.
22 Siegert M J,Carter S,Tabacco I,et al.A revised inventory of Antarctic subglacial lakes[J].Antarctic Science,2005,17(3):453—460.
23 Robin G D Q,Swithinbank C W M,Smith B M E.Radio echo exploration of the Antarctic ice sheet[C]//International Symposium on Antarctic Glaciological Exploration(ISAGE).Hanover,NH,USA:International Association of Scientific Hydrology Publication,1970:97—115.
24 Oswald G K A,Robin G D Q.Lakes beneath the Antarctic ice sheet[J].Nature,1973,245(5423):251—254.
25 Kapitsa A P,Ridley J K,Robin G D Q,et al.A large deep freshwater lake beneath the ice of central East Antarctica[J].Nature,1996,381(6584):684—686.
26 Siegert M J,Dowdeswell J A,Gorman M R,et al.An inventory of Antarctic sub-glacial lakes[J].Antarctic Science,1996,8(3):281—286.
27 Wright A,Siegert M.A fourth inventory of Antarctic subglacial lakes[J].Antarctic Science,2012,24(6):659—664.
28 Fujita S,Maeno H,Matsuoka K.Radio-wave depolarization and scattering within ice sheets:a matrix-based model to link radar and ice-core measurements and its application[J].Journal of Glaciology,2006,52(178):407—424.
29 Matsuoka K,Wilen L,Hurley S P,et al.Effects of birefringence within ice sheets on obliquely propagating radio waves[J].IEEE Transactions on Geoscience and Remote Sensing,2009,47(5):1429—1443.
30 Mac Gregor J A,Winebrenner D P,Conway H,et al.Modeling englacial radar attenuation at Siple Dome,West Antarctica,using ice chemistry and temperature data[J].Journal of Geophysical Research-Earth Surface,2007,112(F3):F03008.
31 Matsuoka K,Furukawa T,Fujita S,et al.Crystal orientation fabrics within the Antarctic ice sheet revealed by a multipolarization plane and dual-frequency radar survey[J].Journal of Geophysical Research:Solid Earth(1978—2012),2003,108(B10):2499.
32 Bentley C R,Lord N,Liu C.Radar reflections reveal a wet bed beneath stagnant Ice Stream C and a frozen bed beneath ridge BC,West Antarctica[J].Journal of Glaciology,1998,44(146):149—156.
33 Fujita S,Holmlund P,Matsuoka K,et al.Radar diagnosis of the subglacial conditions in Dronning Maud Land,East Antarctica[J].The Cryosphere Discussions,2012,6(3):1781—1837.
34 Matsuoka K,Morse D,Raymond C F.Estimating englacial radar attenuation using depth profiles of the returned power,central West Antarctica[J].Journal of Geophysical Research:Earth Surface(2003—2012),2010,115(F2):F02012.
35 Shabtaie S,Bentley C R.West Antarctic ice streams draining into the Ross Ice Shelf:Configuration and mass balance[J].Journal of Geophysical Research:Solid Earth(1978—2012),1987,92(B2):1311—1336.
36 Gades A M,Raymond C F,Conway H,et al.Bed properties of Siple Dome and adjacent ice streams,West Antarctica,inferred from radio-echo sounding measurements[J].Journal of Glaciology,2000,46(152):88—94.
37 Catania G A,Conway H B,Gades A M,et al.Bed reflectivity beneath inactive ice streams in West Antarctica[J].Annals of Glaciology,2003,36(1):287—291.
38 Raymond C F,Catania G A,Nereson N,et al.Bed radar reflectivity across the north margin of Whillans Ice Stream,West Antarctica,and implications for margin processes[J].Journal of Glaciology,2006,52(176):3—10.
39 Winebrenner D P,Smith B E,Catania G A,et al.Radio-frequency attenuation beneath Siple Dome,West Antarctica,from wide-angle and profiling radar observations[J].Annals of Glaciology,2003,37(1):226—232.
40 Matsuoka K,Mac Gregor J A,Pattyn F.Using englacial radar attenuation to better diagnose the subglacial environment:a review[C]//Proceedings of the IEEE 2010 13th International Conference on Ground Penetrating Radar(GPR).Lecce:IEEE,2010:1—5.
41 Jacobel R W,Welch B C,Osterhouse D,et al.Spatial variation of radar-derived basal conditions on Kamb Ice Stream,West Antarctica[J].Annals of Glaciology,2009,50(51):10—16.
42 Fujita S,Matsuoka T,Ishida T,et al.A summary of the complex dielectric permittivity of ice in the megahertz range and its applications for radar sounding of polar ice sheets[J].Physics of Ice Core Records,2000:185—212.
43 Matsuoka K,Mac Gregor J A,Pattyn F.Predicting radar attenuation within the Antarctic ice sheet[J].Earth and Planetary Science Letters,2012,359 —360:173—183.
44 Mac Gregor J A,Matsuoka K,Studinger M.Radar detection of accreted ice over Lake Vostok,Antarctica[J].Earth and Planetary Science Letters,2009,282(1—4):222—233.
45 Matsuoka K,Pattyn F,Callens D,et al.Radar characterization of the basal interface across the grounding zone of an ice-rise promontory in East Antarctica[J].Annals of Glaciology,2012,53(60):29—34.
46 Carter S P,Blankenship D D,Young D A,et al.Using radar-sounding data to identify the distribution and sources of subglacial water:application to Dome C,East Antarctica[J].Journal of Glaciology,2009,55(194):1025—1040.
47 唐学远,孙波,张占海,等.东南极冰盖Dome A的内部等时层结构[J].中国科学:地球科学,2010,40(11):1504—1509.
48 Suzuki T,Iizuka Y,Matsuoka K,et al.Distribution of sea salt components in snow cover along the traverse route from the coast to Dome Fuji station1000 km inland at east Dronning Maud Land,Antarctica[J].Tellus B,2002,54(4):407—411.
49 Matsuoka K.Pitfalls in radar diagnosis of ice-sheet bed conditions:Lessons from englacial attenuation models[J].Geophysical Research Letters,2011,38(5):L05505.
50 Camplin G C,Glen J W.The dielectric properties of HF-doped single crystals of ice[M]//Whalley E,Jones S J,Gold L W.Physics and Chemistry of Ice.Ottawa:Royal Society of Canada,1973:256—261.
51 Johari G P,Charette P A.The permittivity and attenuation in polycrystalline and single-crystal ice Ih at 35 and 60 MHz[J].Journal of Glaciology,1975,14(71):293—303.
52 Takei I,Maeno N.Electric characteristics of point defects in HCl-doped ice[J].Le Journal de Physique Colloques,1987,48(C1):C1-121-C1-126.
53 Matsuoka K,Fujita S,Matsuoka T,et al.Measurements of the complex permittivity of acid-doped ice from1 k Hz to 30 MHz-New data set for developing ice radar and dielectricanalysis of ice cores[C]//Proceedings of the NIPR Symposium on Polar Meteorology and Glaciology.Tokyo,Japan:National Institute of Polar Research,1996:25—35.
54 Pattyn F.Antarctic subglacial conditions inferred from a hybrid ice sheet/ice stream model[J].Earth and Planetary Science Letters,2010,295(3—4):451—461.
55 Li X,Sun B,Siegert M J,et al.Characterization of subglacial landscapes by a two-parameter roughness index[J].Journal of Glaciology,2010,56(199):831—836.
56 Bingham R G,Siegert M J.Radar-derived bed roughness characterization of Institute and Mller ice streams,West Antarctica,and comparison with Siple Coast ice streams[J].Geophysical Research Letters,2007,34(21):L21504.
57 Bingham R G,Siegert M J.Quantifying subglacial bed roughness in Antarctica:implications for ice-sheet dynamics and history[J].Quaternary Science Reviews,2009,28(3—4):223—236.
58 Sun B,Moore J C,Zwinger T,et al.How old is the ice beneath Dome A,Antarctica[J].The Cryosphere Discussions,2014,8(1):289—305.
59 崔祥斌,孙波,田钢,等.东南极Dome A冰雷达探测:冰厚分布和冰下地形[J].科学通报,2010,55(3):268—273.