基于波场延拓和分解方法的渤海湾盆地沉积层及地壳结构研究

Bohai Bay Basin (BBB) located in Eastern North China Craton which was deposited in Paleozoic and grew into a Cenozoic rift basin by the Indosinian and Yanshan Movements. It covers a very thick sedimentary at the top which makes the unconsolidated sediment reverberation marking the crustal information. So it is difficult to use the traditional imaging method of the receiver function (H-k stacking，CCP stacking) to obtain the crustal structure in this area. This condition makes a barrier in studying the North China Craton thinning. In this study, we use the wavefied continuation and decomposition method and subsurface receiver function to imaging the sedimentary structure and crustal structure of BBB. The upgoing P, downgoing P, upgoing S and downgoing S can be calculated by decomposition of the wavefield at the depth of the sedimentary basement and Moho. Based on the principle that the upgoing S wavefield is absent in the half-space, we can search for the optimum velocity and thickness of the layers that give the minimum S-wave energy flux from the half-space to the layers, the results show the structure of the sedimentary and the crust. Then we propagate the surface records downwards to the sedimentary basement and decomposed the wavefield into upgoing P, downgoing P, upgoing S and downgoing S. The deconvolution of the upgoing P and upgoing S is the subsurface receiver function which is free from sedimentary reverberation. The traditional imaging method can be used in subsurface receiver function to help us obtain a high-resolution and reliable structure of BBB.

渤海湾盆地位于华北克拉通东部，经古生代沉积并在印支、燕山期运动的基础上发展为新生代断陷盆地，上覆沉积层较厚。由沉积层造成的波形信号较强，覆盖了相对较弱的地壳信号，无法通过传统的接收函数成像方法（H-k叠加方法，CCP叠加方法）获得地壳结构，对华北克拉通减薄的研究造成一定的障碍。本项目计划采用波场延拓分解方法和近地表接收函数计算方法（Tao et al, 2014），对渤海湾盆地沉积层结构和地壳结构进行计算。通过对Moho面和沉积层基底所在深度的波形进行波场延拓和分解，获得上行和下行的P、S波，分别对其上行S波能量进行最小化约束的网格搜索，得到沉积层结构和地壳结构；同时将获得的沉积层基底所在深度的波场进行分解，将得到的上行S波和P波进行反褶积，获得近地表接收函数。该接收函数大大削减了沉积层结构的影响，可以采用传统接收函数成像方法，获得分辨率较高且可信度较大的沉积层地区地壳结构。

渤海湾盆地位于华北克拉通东部陆块，是华北克拉通岩石圈改造最为强烈的区域，对该地区的地壳结构研究有助于深入了解华北克拉通减薄机制。接收函数在地壳结构的研究上有非常明显的优势，然而沉积层的存在，给研究造成了很大的困难。沉积层对地震信号的衰减作用以及沉积层基底的多次反射作用，使得Moho面转换波信号无法识别，使用传统的接收函数成像方法无法获取沉积层区域的地壳结构。波场延拓方法可以对地表信号进行反向传播，获取地下任意深度处的地震信号，并对此信号进行分解，得到的上行P波和上行S波进行反褶积运算后，可以得到地下任意深度处的近地表接收函数，如果这一深度在沉积层基底以下，则此处的近地表接收函数可以有效的去除沉积层信号，使得地壳的转换波信号得以重新识别。沉积层的计算本项目采用了类似H-κ叠加方法H-β叠加方法进行计算，得到的渤海湾盆地沉积层厚度在2km-5km，平均大约为3.5km左右。对近地表接收函数进行CCP叠加成像获得了研究区域内的Moho面埋深分布，CCP成像结果在渤海湾盆地内可以看到十分清晰的Moho面转换波震相，获得的Moho面埋深分布结果与人工地震宽角反射的结果较为一致，渤海湾盆地的Moho面埋深深度在28km-36km左右，平均约为33km。

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