广角地震速度结构反演中的不确定性分析

Analysis on uncertainty in velocity structure inversion in wide-angle seismic experiments

海底地震仪(ocean bottom seismometer,OBS)广角地震探测是研究海洋深部精细壳幔结构的重要地球物理手段,其主要原理是利用广角折射/反射震相信息来反演地壳深部结构,从而为深入认识海底构造特征提供最为基础的地震学信息。文章基于地震层析成像结构反演的基本原理,针对OBS广角地震速度结构反演中可能存在的一些不确定性因素,用Tomo2D软件构建了不同的理论模型和数据体,对初始模型中沉积层的变化是如何影响整个地壳速度结构的反演结果、不同台站间距所构建的数据体对下地壳高速层分辨精度的控制影响以及不同数据体在分辨深部异常体的有效性等方面进行了分析研究,以此深入了解OBS广角地震速度结构反演中的不确定性因素。研究结果显示,地壳深部较厚(大于4km)的高速层能够被很好地反演出来,而较薄(小于2km)的高速层虽能够识别,但其形态规模恢复得较差;初始模型中沉积层速度结构的失真度影响着高速层的识别,沉积层失真度越大,对高速层的恢复越差;地壳内部存在低速层对反演高速层的规模有较为明显的影响,但低速层的厚度变化对其影响不大;对于文中10km台站间距的模型,能够识别的速度异常体规模在水平方向上最小约为5km,在纵向上最小约为3km,而20km和30km台站间距的模型最小能够识别的速度异常体规模为水平向10km、纵向3km。

Ocean bottom seismometer (OBS) wide-angle seismic exploration is an important geophysical method to obtainelaborate crust-mantle structure of deep sea. The main theory is to invert the deep crustal structure by refraction and reflectionseismic phase and to provide basal seismic information for studying crust structural features. Based on the principle of seismictomography structure inversion, we aimed at the uncertainty of velocity structure inversion in OBS wide-angle seismicexperiment in this study. Several synthetic models and data were built by Tomo2D to study issues such as how differentsedimentary layers may influence the inversion result of whole crust velocity structure, how the OBS space may influence theresolving accuracy of high velocity layer (HVL) in lower crust and the effectiveness of identifying the deep crust anomalous bodies. Our results show that the HVL thicker than 4 km can be well inverted and the thinner HVL (<2 km) is poorly invertedfor its scale and shape. The deviation of sedimentary thickness in initial model can influence how to identify HVL; the largerdeviation corresponds to worse identification. The low-velocity layer within the crust has significant impact on inverting thescale of HVL in lower crust, but the thickness of the low-velocity layer is insignificant. The smallest scale of velocityanomalous body that can be identified is 5 km in the horizontal and 3 km in the vertical when the OBS space is 10 km; andthey are 10 km in the horizontal and 3 km in the vertical when the OBS space is 20 km and 30 km.