西南印度洋中脊三维地震探测中炮点与海底地震仪的位置校正
摘要
在海上实施三维地震探测过程中,人工震源枪阵中心与船上GPS的距离及地震探测作业中的船行方向造成炮点实际位置与预设位置有一定偏差;自由落体投放的OBS由于海流的影响会偏离原定设计位置(投放点),因此,炮点与海底地震仪(OBS)的位置校正是三维地震结构研究中的基本环节.本文利用艏向信息校正了炮点位置;采用蒙特卡洛和最小二乘法方法对海底地震仪的位置进行了校正,并探讨了直达水波曲线特征.结果表明OBS位置一般偏离设计点1km左右,其误差范围在20m以内,校正后的OBS记录剖面展示了真实的记录情况.该研究结果为下一步西南印度洋的三维层析成像研究提供了坚实数据基础,同时为今后南海的三维深部地壳结构探测提供经验与借鉴.
In a three-dimensional (3-D) seismic experiment carried out at sea, due to the distance between GPS antenna on ship and the center of air gun sources in sea water, and some environmental factors such as wind and sea current, the survey ship would adjust its heading direction which causes the shot's real position to deviate from presupposed shot track; meanwhile, real locations of OBSs on the seafloor would drift from designed points (deployed locations) since OBSs are of free-fall type and are usually affected by sea current during descending. So, the calibration of air gun shot and Ocean Bottom Seismometer (OBS) locations are basic steps for later studies on fine seismic velocity tomography. Our research focuses on the correction of shot and OBS positions with latest data from a new 3D seismic experiment in the SW Indian Ocean Ridge. The shot positions were corrected with geometric relationship among ship s heading, GPS antenna and the air gun array. The correction of OBS positions was based on travel times of direct water waves, and Monte Carlo and Least Square Methods were adopted. In addition, we discuss the relationship between reduced travel time curve of direct water wave and OBS deviation. Our results show that OBS locations generally drifted from the design points about 1 km, with an error range of 0～20 m and shots were adjusted to their actual positions. The research results do not only supply a fundamental database for the 3D seismic tomography in SW Indian Ocean Ridge, but also provide valuable experiences for 3D seismic survey in the South China Sea in future.
In a three-dimensional (3-D) seismic experiment carried out at sea, due to the distance between GPS antenna on ship and the center of air gun sources in sea water, and some environmental factors such as wind and sea current, the survey ship would adjust its heading direction which causes the shot's real position to deviate from presupposed shot track; meanwhile, real locations of OBSs on the seafloor would drift from designed points (deployed locations) since OBSs are of free-fall type and are usually affected by sea current during descending. So, the calibration of air gun shot and Ocean Bottom Seismometer (OBS) locations are basic steps for later studies on fine seismic velocity tomography. Our research focuses on the correction of shot and OBS positions with latest data from a new 3D seismic experiment in the SW Indian Ocean Ridge. The shot positions were corrected with geometric relationship among ship s heading, GPS antenna and the air gun array. The correction of OBS positions was based on travel times of direct water waves, and Monte Carlo and Least Square Methods were adopted. In addition, we discuss the relationship between reduced travel time curve of direct water wave and OBS deviation. Our results show that OBS locations generally drifted from the design points about 1 km, with an error range of 0～20 m and shots were adjusted to their actual positions. The research results do not only supply a fundamental database for the 3D seismic tomography in SW Indian Ocean Ridge, but also provide valuable experiences for 3D seismic survey in the South China Sea in future.
引文
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[18] Shiobara H,Nakanishi A,Shimamura H,et al.Precise Positioning of Ocean Bottom Seismometer by Using Acoustic Transponder and CTD.Marine Geophysical Researches,1997,19:199～209
[19] Creager K C,Dorman L M.Location of Instruments on the Seafloor by Joint Adjustment of Instrument and Ship Positions.J.Geophys.Res.,1982,87(B10) :8379～8388
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[22] 赵明辉,丘学林,李家彪等.慢速、超慢速扩张洋中脊热液区三维地震结构研究展望.热带海洋学报,2010,29(6) :1～7 Zhao M H,Qiu X L,Li J B,et al.Research development and prospect on three-dimensional seismic structures of slow and,ultraslow spreading ocean ridges.Journal of Tropical Oceanography(in Chinese),2010,29(6) :1～7
[23] 阮爱国,丘学林,李家彪等.中国海洋深地震探测与研究进展.华南地震,2009,29(2) :10～18 Ruan A G,Qiu X L,Li J B,et al.Wide Aperture Seismic Sounding in the Margin Seas of China.South China Journal of Seismology(in Chinese),2009,29(2) :10～18
[24] 丘学林,施小斌,阎贫等.南海北部地壳结构的深地震探 测和研究新进展.自然科学进展,2003,13(3) :231～236 Qiu X L,Shi X B,Yan P,et al.Recent progress of deep seismic experiments and studies of crustal structure in northern South China Sea.Progress in Natural Science,2003,13(7) :481～488
[25] 王家映.地球物理资料非线性反演方法讲座(二),蒙特卡洛方法.工程地球物理学报.2007,4(2) :81～85 Wang J Y.Lecture on Non-linear inverse methods in geophysics(second),Monte Carlo Method.Chinese Journal of Engineering Geophysics(in Chinese),2007,4(2) :81～85
[26] Zelt C A,Forsyth D A.Modeling wide-angle seismic data for crustal structure:Southeastern Grenville Province.J.Geophys.Res.,1994,99(B6) :11687～11704
[2] Dunn R A,Toomey D R,Solomon S C.Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rsie at 9°30′N.J.Geophys.Res.,2000,105(B10) :23537～23555
[3] Lees M J.Seismic tomography of magmatic systems.Journal of Volcanology and Geothermal Research,2007,167:37～56
[4] Canales J P,Sohn R A,deMartin B J.Crustal structure of the Trans-Atlantic Geotraverse(TAG)segment(Mid-Atlantic Ridge,26°10′N):Implications for the nature of hydrothermal circulation and detachment faulting at slow spreading ridges.Geochem.Geophys.Geosyst.,2007,8(8) :doi:10． 1029/2007GC001629
[5] 孙枢.将我国深海大洋研究推向新阶段.地球科学进展,2003,18(5) :653 Su S.Pushing the deep ocean research into a new stage.Advance in Earth Science(in Chinese),2003,18(5) :653
[6] 田丽艳,林间.全球大洋中脊研究十年科学规划(2004～2013) .海洋地质动态,2004,20(3) :10～15 Tian L Y,Lin J.A science and structure plan for ridge research(2004～2013) .Marine Geology Letters(in Chinese),2004,20(3) :10～15
[7] 中国大洋钻探学术委员会.中国加入综合大洋钻探(IODP)科学计划(2003～2013) .海洋地质动态,2004,20(1) :14～17 China Academic Committee for Ocean Drilling.China's Scientific Program to Take Part in IODP(2003～2013) .Marine Geology Letters(in Chinese),2004,20(1) :14～17
[8] 丘学林,陈颙,朱日祥等.大容量气枪震源在海陆联测中的应用:南海北部试验结果分析.科学通报,2007,52(4) :463～469 Qiu X L,Chen Y,Zhu R X,et al.The application of large volume airgun sources to the onshore-offshore seismic surveys:implication of the experimental results in northern South China Sea.Chinese Science Bulletin,2007,52(4) :553～560
[9] 赵明辉,丘学林,夏少红等.大容量气枪震源及其波形特征. 地球物理学报,2008,51(2) :558～565 Zhao M H,Qiu X L,Xia S H,et al.Large volume air-gun sources and its seismic waveform characters.Chinese J.Geophys.(in Chinese),2008,51(2) :558～565
[10] Peirce C,Day A J.Ocean-bottom seismograph tomographic experiments-a consideration of acquisition geometries vs.resources.Geophys.J.Int.,2002,151:543～565
[11] 薛彬,阮爱国,李湘云等.SEDIS-Ⅳ型短周期自浮式海底地震仪数据校正方法.海洋学研究,2008,26(2) :98～102 Xue B,Ruan A G,Li X Y,et al.The seismic data corrections of short period auto-floating ocean bottom seismometer.Journal of Marine Sciences(in Chinese),2008,26(2) :98～102
[12] White R S,Smith L K,Roberts A W,et al.Lower-crustal intrusion on the North Atlantic continental margin.Nature.2008,452:460～465
[13] Minshull T A,Muller M R,White R S.Crustal structure of the Southwest Indian Ridge at 66°E:seismic constraints.Geophys.J.Int.,2006,166:135～147
[14] Christeson G L.OBSTOOL:Software for processing UTIG OBS data.Technical Report No.134． 1995,University of Texas Institute for Geophysics,Austin,TX
[15] 王彦林,阎贫,郑红波等.OBS记录的时间和定位误差校正.热带海洋学报,2007,26(5) :40～46 Wang Y L,Yan P,Zheng H B,et aI.Timing and positioning corrections of ocean bottom seismograph data.Journal of Tropical Oceanography(in Chinese),2007,26(5) :40～46
[16] Tong C H,Barton P J,White R S,et al.Influence of enhanced melt supply on upper crustal structure at a midocean ridge discontinuity:A three-dimensional seismic tomographic study of 9°N East Pacific Rise.J.Geophys.Res.,2003,108(B10) :
[17] Oshida A,Kubota R,Nishiyama E,et al.A new method for determining OBS positions for crustal structure studies,using airgun shots and precise bathymetric data.Exploration Geophysics,2008． 61． (Exploration Geophysics,2008,39:15～25;Butsuri-Tansa,2008,61:15～25;Mulli-Tamsa,2008,11:15～25)
[18] Shiobara H,Nakanishi A,Shimamura H,et al.Precise Positioning of Ocean Bottom Seismometer by Using Acoustic Transponder and CTD.Marine Geophysical Researches,1997,19:199～209
[19] Creager K C,Dorman L M.Location of Instruments on the Seafloor by Joint Adjustment of Instrument and Ship Positions.J.Geophys.Res.,1982,87(B10) :8379～8388
[20] Hobro J W D,Minshull T A,Singh S C,et al.A three-dimensional seismic tomographic study of the gas hydrate stability zone,offshore Vancouver Island.J.Geophys.Res.,2005,110(B09102)
[21] 阮爱国,李家彪,陈永顺等.国产I-4C型OBS在西南印度洋中脊的试验.地球物理学报,2010,53(4) :1015～1018 Ruan A G,Li J B,Chen Y S,et al.The experiment of broad I-4C type OBS in the Southwest India ridge.Chinese J.Geophys.(in Chinese),2010,53(4) :1015～1018
[22] 赵明辉,丘学林,李家彪等.慢速、超慢速扩张洋中脊热液区三维地震结构研究展望.热带海洋学报,2010,29(6) :1～7 Zhao M H,Qiu X L,Li J B,et al.Research development and prospect on three-dimensional seismic structures of slow and,ultraslow spreading ocean ridges.Journal of Tropical Oceanography(in Chinese),2010,29(6) :1～7
[23] 阮爱国,丘学林,李家彪等.中国海洋深地震探测与研究进展.华南地震,2009,29(2) :10～18 Ruan A G,Qiu X L,Li J B,et al.Wide Aperture Seismic Sounding in the Margin Seas of China.South China Journal of Seismology(in Chinese),2009,29(2) :10～18
[24] 丘学林,施小斌,阎贫等.南海北部地壳结构的深地震探 测和研究新进展.自然科学进展,2003,13(3) :231～236 Qiu X L,Shi X B,Yan P,et al.Recent progress of deep seismic experiments and studies of crustal structure in northern South China Sea.Progress in Natural Science,2003,13(7) :481～488
[25] 王家映.地球物理资料非线性反演方法讲座(二),蒙特卡洛方法.工程地球物理学报.2007,4(2) :81～85 Wang J Y.Lecture on Non-linear inverse methods in geophysics(second),Monte Carlo Method.Chinese Journal of Engineering Geophysics(in Chinese),2007,4(2) :81～85
[26] Zelt C A,Forsyth D A.Modeling wide-angle seismic data for crustal structure:Southeastern Grenville Province.J.Geophys.Res.,1994,99(B6) :11687～11704
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