基于变密度模型的位场界面反演
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摘要
在波数域中用重力反演莫霍面深度时通常假定壳幔密度差为一常数,但这只是一种近似的密度模型,本文采用了密度随深度呈指数变化的变密度模型来反演莫霍面深度,给出了利用指数密度模型在波数域中计算重力异常的正演公式及界面深度的反演公式.利用指数密度模型及重力资料反演了青藏高原莫霍面的深度,分析了莫霍面的特征.结果表明,青藏高原莫霍面呈现出边缘浅、中部深的特点,边缘变化快、梯度大,中间变化梯度趋缓.中心地带的羌塘地体莫霍面深度达74 km,向四周慢慢变浅至67 km左右,边缘地区突然变浅至50km左右.通过常密度模型、变密度模型及地震反演得到的莫霍面的比较,证实变密度模型更适合于莫霍面结构的反演.
The constant density model for conventional Moho depths inversion in wave-number domain from gravity data is just an approximate density model.A variable density model with exponential density contrast is used for inverting Moho depths instead of constant density model.The method is introduced and some formulae are derivated for forward calculating gravity anomaly and Moho depths inversion.The Moho depths of Tibet plateau are inverted from gravity data with exponential density model.The inversion result demonstrates that the Moho depths of Tibet plateau are shallow around the edges,and deep at the center.The gradients at the edges are steeper than at the center.The Moho depth of Qiangtang block situated at the central Tibet plateau is 74 km,and changed to 67 km slowly outward,but changed to 50km rapidly at the margins.The variable density model is more suitable for inversion of Moho depths structures than constant density model by comparison of Moho depth from seismic inversion.
The constant density model for conventional Moho depths inversion in wave-number domain from gravity data is just an approximate density model.A variable density model with exponential density contrast is used for inverting Moho depths instead of constant density model.The method is introduced and some formulae are derivated for forward calculating gravity anomaly and Moho depths inversion.The Moho depths of Tibet plateau are inverted from gravity data with exponential density model.The inversion result demonstrates that the Moho depths of Tibet plateau are shallow around the edges,and deep at the center.The gradients at the edges are steeper than at the center.The Moho depth of Qiangtang block situated at the central Tibet plateau is 74 km,and changed to 67 km slowly outward,but changed to 50km rapidly at the margins.The variable density model is more suitable for inversion of Moho depths structures than constant density model by comparison of Moho depth from seismic inversion.
引文
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[11]Braitenberg C,Zadro M,Fang J,et al.The gravity and iso-static Moho undulations in Qinghai-Tibet plateau[J].Journalof Geodynamics,2000,30(5):489~505.
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[2]Oldenburg D W.The inversion and interpretation of gravity a-nomalies[J].Geophysics,1974,9(4):526~536.
[3]冯锐.三维物性分布的位场计算[J].地球物理学报,1986,29(4):399~406.
[4]熊绍柏,滕吉文,尹周勋.西藏高原地区的地壳厚度和莫霍界面的起伏[J].地球物理学报,1985,28(增刊Ⅰ):16~27.
[5]王谦身,安玉林.青藏高原东部玛多-沙马地区的重力场与深部构造[J].地球物理学进展,2001,16(4):4~10.
[6]赵金仁,李松林,张先康,等.青藏高原东北缘莫霍界面的三维空间构造特征[J].地球物理学报,2005,48(1):78~85.
[7]滕吉文,曾融生,闫雅芬,等.东亚大陆及周边海域Moho界面深度分布及基本构造格局[J].中国科学(D辑),2002,32(2):89~100.
[8]张中杰,李英康,王光杰,等.藏北地壳东西向结构与“下凹”莫霍面-来自宽角反射剖面的启示[J].中国科学(D辑),2001,31(11):881~888.
[9]丁志峰,曾融生,吴大铭.青藏高原的Pn波速度和Moho面的起伏[J].地震学报,1992,14(增刊):592~599.
[10]柯小平,王勇,许厚泽.用遗传算法反演地壳的变密度模型[J].武汉大学学报.信息科学版,2004,29(11):981~984.
[11]Braitenberg C,Zadro M,Fang J,et al.The gravity and iso-static Moho undulations in Qinghai-Tibet plateau[J].Journalof Geodynamics,2000,30(5):489~505.
[12]孟令顺,高锐,周富祥,等.利用重力异常研究亚东-格尔木地壳构造[J].中国地质科学院院报,1990,21:149~161.
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