| 上地壳纵横波速度结构相关反演成像方法 |
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| 引用本文： | 王晓,周小鹏,张新彦,白志明,滕吉文.上地壳纵横波速度结构相关反演成像方法[J].地球物理学报,2015,58(10):3553-3570. |
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| 作者姓名： | 王晓 周小鹏 张新彦 白志明 滕吉文 |
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| 作者单位： | 1. 中国科学院地质与地球物理研究所, 岩石圈演化国家重点实验室, 北京 100029;
2. 中国科学院大学, 北京 100049 |
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| 基金项目： | 中国地震局公益性行业科研专项(201408023)和国家自然科学基金(41374062,41174075,41274070,41474068)联合资助. |
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| 摘 要： | 基于纵横波初至走时数据的层析成像方法越来越广泛地被应用于揭示不同构造域壳幔速度结构特征.我们从同一地质体的纵横波速度属性相关这一基本思想出发,提出一种相关反演成像的方法:纵横波速度反演交替进行,在迭代反演过程中每通过一次反演获得相应的纵波速度(或横波速度)结构后,更新相应的纵横波速度比模型以及相应的横波(或纵波)速度反演的初始模型,然后继续开展后续横波(或纵波)速度反演工作.在反演过程中依据纵横波速度的相关性信息和射线路径长度将走时残差以不同权重分配到射线路径经过的单元,依据网格节点周围平均的慢度扰动更新速度模型.正反演过程分别基于有限差分走时计算方法和反投影成像方法.两种典型模型试验表明,该技术应用于上地壳速度结构反演成像过程,可有效提高反演结果的可靠性,在很大程度上避免了常规单独反演纵波和横波速度过程容易带来的畸变和失真.该方法应用于重建青藏高原西部札达—泉水沟深地震测深(DSS)剖面下方的上地壳速度结构,揭示出与青藏高原西缘板块碰撞相关的上地壳速度结构特征.
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| 关 键 词： | 上部地壳 纵横波速度结构 相关反演 有限差分 反投影 |
| 收稿时间： | 2014-12-29 |
Tomographic imaging of velocity structure in upper crust based on correlated inversion of VP and VS |
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| WANG Xiao,ZHOU Xiao-Peng,ZHANG Xin-Yan,BAI Zhi-Ming,TENG Ji-Wen.Tomographic imaging of velocity structure in upper crust based on correlated inversion of VP and VS[J].Chinese Journal of Geophysics,2015,58(10):3553-3570. |
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| Authors: | WANG Xiao ZHOU Xiao-Peng ZHANG Xin-Yan BAI Zhi-Ming TENG Ji-Wen |
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| Institution: | 1. State Key Laboratory of Lithosphere Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China |
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| Abstract: | Presently seismic tomographic methods based on first arrivals of deep seismic sounding traveltime data have been widely used to image the upper crustal velocity structure. However, generally there are some difficulties in the inversion process using conventional methods: (1) How to construct reliable upper crust models using Pg and Sg data; (2) How to quantitatively estimate the reliability of reconstructed models; and (3) How to decrease the uncertainty and distortion validly.
Based on the idea that the VP and VS anomalies from a same geological body are closely related each other, we develop a correlated inversion method: VP and VS inversion are carried out alternately, in which the corresponding VP/VS model and initial VS (or VP) model are updated using the obtained VP (or VS) model after each inversion. The slowness perturbation resulted from the traveltime residuals is weighted and projected to the grid-points on the ray trace based on the updated correlation information, thus the velocity models are updated. Forward and inversion processes are respectively based on finite-difference calculations of travel times and the back-projection inversion method.
Correlated inversion of VP and VS models and sequential inversion technique are incorporated into the tomographic process to ensure the reliability of inversion results and avoid the distortion of the resulting VP/VS model. Comparison with the conventional inversion results using the code of Ammon and Vidale (1993), both of the derived VP and VS models by our approach have been obviously improved, which are clearly closer to the original theoretical ones. While those constructed using the code of Ammon and Vidale (1993) have been seriously distorted. Moreover, the maximum VP/VS error based on the conventional approach is up to 0.2, while the maximum VP/VS error with our approach is only 0.02~0.03.
Tests using two typical models show that the technology used in inversion tomography of the velocity structure within upper crust, which can effectively improve the reliability of the inversion results, can avoid the uncertainty and distortion resulted from the conventional single VP and VS inversion process on a large scale. This method has been successfully applied to the western Tibetan plateau to reconstruct the velocity structure of upper crust beneath the Zhada-Quanshuigou deep seismic sounding profile. The derived models reveal the remained upper crustal velocity structure features resulted from plate collision in the western Tibetan plateau. Two similar obvious low-value anomalies of VP, VS and VP/VS with the maximum depth 8~10 km are present beneath the Zhada and Shiquanhe basin, which are probably the effect of the subduction of Indian plate beneath the Eurasian continent since Cenozoic or the closure of the Mesozoic Bangong-Nujiang oceanic basin. The disclosed Gar basin has a sedimentary thickness of 3~4 km, where the sedimentation and deep processes may be due to the gradual uplift of the Ayilariju Mountains and the strike-slip movement of the Karakoram fault. |
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| Keywords: | Upper crust VP and VS structure Correlated inversion Finite-difference Back-projection |
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