海底广角地震剖面反演方法对比——以南海礼乐滩OBS剖面为例

Comparision of inversion method of wide angle Ocean Bottom Seismometer profile:A case study of profile OBS973-2 across Liyue bank in the South China Sea

广角地震测线（OBS973-2）位于南海南部陆缘,其地壳深部构造是研究南海共轭扩张及形成演化的直接证据之一.本文采用2D射线追踪技术,结合与之重合的多道地震测线（NH973-2）时深转换结果,对OBS973-2测线重新进行了正、反演研究,得到了礼乐滩及邻近海区的精细地壳结构.与前人结果相比,本文基于正反演速度模型,把测线分为陆壳区（0-200 km）、洋陆过渡区（200-280 km）和洋盆区（280-370 km）.地壳结构在不同区域差异明显,陆壳区沉积层厚度横向差异大,且速度横向不均匀,地壳整体厚度大（约20 km）,有横向速度差;洋陆过渡区速度和厚度横向均匀,地壳减薄（约8 km）;洋壳区地壳厚度减薄至6 km.与以往研究相比,新的认识集中在两个方面,（1）在方法上,综合广角地震和多道地震数据,借助正演和反演方法,能够得到更多更可靠的地壳结构信息.（2）在地壳结构上,结合广角地震与多道地震,得到洋陆过渡区莫霍面向海减薄的形态及其埋深（约12-18 km,海平面为0 km）;进一步验证礼乐滩区域在洋陆过渡区没有明显的高速层,为非火山型陆缘,其共轭扩张点为中沙地块;陆壳区上地壳强烈的拉张作用在速度模型表现出横向速度异常和低速区,在多道地震剖面上表现为大量10-20 km的正断层.

Deep crust structure of the wide angle seismic profile OBS973-2, which crosses the central sea basin and Liyue bank in the southern of South China Sea （SCS）, represents one of the direct evidences for conjugate spreading and evolution of SCS. For this reason,we remodeled the data by using both forward and inversion methods for a detailed crustal velocity structure, and also compared results with depth profile of multi-channel seismic （MCS） close to OBS973-2 for more reliable crust thickness. Comparing to former works, and according to the new velocitymodels, we divided the profile to three parts of continental crust （0-200 km in the profile）, continent and ocean transition （COT, 200-280 km in the profile） and ocean crust （280-370 km in the profile）. And their crust structures are obviously different. Sediment layer in the continental crust shows a feature of horizontal variations in both thickness and velocity, and total crust thickness in continental part is thick （about 20 kin）. In the COT, both thickness and velocity are horizontally smooth, and the total thickness of crust is thinner （about 8 km）. And in the ocean crust, the crust thickness thins to 6 km. There are two main conclusions, （1） the method of using data collected by wide angle seismic and multi-channel seismic and using both forward and inversion software in modeling will obtain more reliable information of crust structure, （2） the new insights on the crust structure are as follows. （a） We get the seaward thinning of Moho and its depth of about 12-18 km below the sea level in COT from both wide angle and multi-channel seismic profiles. （b） No high velocity layer was found beneath COT, which represents non volcanic passive continental margin and its coniugate spreading point is Zhongsha massif. （c） Heavy extension occurred in upper crust of continental part, which shows horizontal velocity anomalies and low velocity zone in the velocity model and a large number of normal faults in the multi-channel seismic profile.