基于波动方程的地震层析成像应用研究

发布时间：2018-06-18 06:41

  本文选题：波动方程 + 地震层析成像　； 参考：《清华大学》2016年博士论文

【摘要】：基于波动方程的层析成像方法被认为是研究地球内部结构和动力学过程的新一代成像方法。首先,本文利用伴随方法、Born近似以及格林函数等发展了基于波动方程的地震层析成像和震源定位联合反演方法。针对该方法需要求解正演波场和伴随波场来计算层析成像的敏感核的问题,本研究提出了具有四阶精度的高效率、低存储需求、低数值频散、易并行计算的三维近似解析中心差分方法,分析了其稳定性和数值频散关系,并针对不同的介质模型给出了地震波传播的波场模拟结果。GPU/CPU异构并行加速试验表明该方法具有优良的并行加速效果。随后,利用推导的差分方法在一个地壳-Moho面-地幔模型中计算了不同地震波震相,包括直达波、反射波、多次反射波以及首波的波形和走时敏感核。结果表明,无论是波形敏感核还是走时敏感核,都主要分布在以地震波传播几何路径为中心的第一菲涅尔带区域内;后续震相的敏感核不仅存在于第一菲涅尔带,也包含反射点或散射点的等时线。基于波动方程层析成像敏感核包含等时线是其能天然解决多路径问题的原因之一。在应用方面,利用基于波动方程的走时层析成像对北京及其周边区域地壳结构进行层析成像研究。成像结果显示该区域地壳的强非均匀性与地震的发生存在关联性。唐山地震震中位置下方的低速异常可能表明流体的存在;唐山地震及其余震的发生与流体对孕震层的弱化作用以及扩散、运移有关。在对美国南加利福尼亚Landers地震区域的层析成像研究中,尝试了同时利用基于波动方程的层析成像方法和Moho反射震相对该区域整个地壳进行成像。检测板测试结果表明Moho反射震相能够显著提高下地壳的层析成像结果分辨率。高分辨率成像结果揭示了近地表的非均匀性与地表地质构造有关;地震多发生于高速、低泊松比异常区域或者高低速异常交界区域。成像结果揭示的中下地壳中沿圣安德烈斯断层东南端靠近索尔顿海槽存在明显的低速异常,可能是岩浆扩张对地壳入侵和驱动作用导致下地壳存在流体所致;其北端的高速异常为含流体区域的边界。在Big Bear地震震中下方存在一个高速P波,低速S波,高泊松比异常,该异常的形成可能与受上地幔中残留的古法拉隆板块下沉过程中对部分熔融的软流圈物质驱动所形成的低速异常对下地壳侵袭作用有关。
[Abstract]:The wave equation based tomography method is considered to be a new generation imaging method for studying the internal structure and dynamic processes of the earth. Firstly, the associated method born approximation and Green's function are used to develop seismic tomography and joint inversion method based on wave equation. In order to solve the problem that the forward wave field and the accompanying wave field need to be solved to calculate the sensitive core of tomography, this paper proposes a method with high efficiency, low storage requirement and low numerical dispersion with four order precision. A three-dimensional approximate analytical central difference method for parallel computation is presented, and its stability and numerical dispersion relationship are analyzed. The simulation results of seismic wave propagation in different media models are given. The parallel acceleration experiments of GPU / CPU show that this method has good parallel acceleration effect. Then, the seismic phases of different seismic waves, including direct wave, reflection wave, multiple reflection wave, first wave waveform and travel-time sensitive nucleus, are calculated in a crustal-Moho area-mantle model by using the derived difference method. The results show that both wave-sensitive nuclei and travel-time sensitive nuclei are mainly distributed in the first Fresnel zone centered on the geometric path of seismic wave propagation, and the subsequent seismic phase sensitive nuclei not only exist in the first Fresnel zone, but also in the first Fresnel zone. Isochronous lines that also contain reflection or scattering points. The inclusion of isochron in wave equation tomography is one of the reasons why it can solve multipath problem naturally. In application, the traveling time tomography based on wave equation is used to study the crustal structure in Beijing and its surrounding areas. The imaging results show that there is a correlation between the strong heterogeneity of the crust and the occurrence of earthquakes. The low velocity anomaly beneath the epicenter of the Tangshan earthquake may indicate the existence of the fluid, and the occurrence of the Tangshan earthquake and its aftershocks is related to the weakening, diffusion and migration of the fluid to the seismogenic layer. In the study of tomography in the Landers seismic region of Southern California, the method based on wave equation and the Moho reflection earthquake are used simultaneously to image the whole crust of the region. The measured results show that the Moho reflection phase can significantly improve the resolution of the tomography results of the lower crust. The high resolution imaging results reveal that the near surface heterogeneity is related to the surface geological structure, and the earthquakes mostly occur in the areas of high speed, low Poisson's ratio anomaly or the junction of high and low velocity anomalies. The imaging results reveal that there are obvious low-velocity anomalies in the middle and lower crust along the southeastern end of the San Andreas fault near the Solton trough, which may be caused by the intrusion and drive of magma to the crust and the existence of fluids in the lower crust. The high speed anomaly at the northern end is the boundary of the fluid-containing region. There is a high velocity P wave, low velocity S wave and high Poisson ratio anomaly below the Big Bear earthquake epicenter. The formation of the anomaly may be related to the invasion of the lower crust caused by the low-velocity anomaly driven by the partially melted asthenosphere material during the subsidence of the paleo-Faralon plate in the upper mantle.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O175
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本文编号：2034558