基于OBS数据的南黄海沉积地层速度结构特征

南黄海盆地是在前古生代变质基底及中-古生代海相沉积基底之上发育起来的中-新生代陆相叠合盆地.本文基于南黄海深部地学探测的主动源地震数据(OBS2013测线),通过多尺度层析成像方法利用初至波走时反演得到测线下方沉积层的纵波速度结构,结合多道地震、重、磁等资料,综合分析南黄海盆地北部沉积地层的特征.结果表明,OBS2013测线下方的地层纵、横向上有多个速度分界面,纵向上以印支面为界,下部挤压与上部伸展地层速度分别呈现高、低速特征;横向上表现为众多断裂,断裂控制了盆地发育,个别断裂发生走滑.断裂将速度剖面划分为四个纵波低速区和五个高速区,6km深度以内纵波速度的低值区(<4.5km·s-1)是中-新生代沉积地层;而高值区(>5km·s-1)归属于不同的形成机制:北部高速区对应千里岩隆起区的变质岩,中部高速区是被挤压的海相沉积地层,南部高速区属于中部隆起,为埋藏较浅、但厚度较大的中、古生代海相地层,部分位置可能含有火成岩.北部坳陷的中、南部区域,在陆相中-新生代沉积盆地之下的海相地层中发育砂岩,该区域(埋深不超过6km)的砂岩沉积分布于约2km厚度的地层中.

Velocity structure of sedimentary formation in the South Yellow Sea Basin based on OBS data

After many years of geological and geophysical studies, the South Yellow Sea (SYS) is believed to be the front of South China Block when colliding with the North China Block. The SYS basin is filled by the Mesozoic-Cenozoic terrigenous sediments overlying the Pre-Paleozoic and Mesozoic-Paleozoic marine sediments. As the results of the multi-stage tectonic activity and sedimentation events, the SYS basin has very complex structure. Many geophysical surveys have been conducted to uncover the detailed basin structures. However, the connection of the shallow and deep structures remain to be studied, either due to the low resolution of crustal structure for the magnetic and gravity methods or due to the lack of deep structure information in the short-offset marine seismic data.
Here, we use the first-arrival time data of a wide-angle OBS array (OBS2013) and the multi-scale seismic tomography (MST) to invert the P-wave velocity structure of the SYS.The MST method is devised to cope with the poor determinacy caused by the uneven raypaths coverage. By decomposing the inverted area with the sub-models with different cell sizes, the MST method can invert the velocity model on the meshes with correct scale and provide a smoother velocity model that is suitable for the geologic interpretation. The accuracy of the first-arrival times is improved after adopting a five-station moving average method, which suppresses the random noise and enhances the first-arrival waveforms by averaging the nearby traces. The selection of data is based on the angle between the array and the shot-receiver line. Here, the picks with angle less than two degrees are adopted to avoid the off-line influence. To show the resolution of our MST algorithm, we conducted a checker-board test using the same inversion parameters and shot-receiver geometry with the case of field-data inversion. The resulted velocity profile shows that the velocity anomalies shallower than 6 km depth is properly recovered. By comparing the inverted velocity model with multi-channel seismic profile, the sedimentary formation is interpreted, referring to gravity and magnetic data.
The final inverted velocity model shows strong lateral and vertical velocity variations. In the vertical, the sedimentary formation is divided into two parts by the Indosinian surface. The top one is the extensional formation with low P-wave velocity, and the other is the extrusion formation carrying a high speed feature. In the transverse, six boundaries of lateral velocity (F1-F6) were interpreted as six faults, which control the development of the SYS basin. Based on the research results, we infer F1 to be the boundary of Jiaolai basin and F4 to be a strike-slip fault, which is a normal fault on its top and the reverse fault at its bottom. The faults divide the sedimentary formation beneath the survey line into low or high P-wave velocity zones. Four low velocity zones (LVZs, <4.5 km·s^-1) and five high velocity zones (HVZs, >5 km·s^-1) are identified beneath 2013OBS line with the prior gravity, magnetic and conventional seismic data. The LVZs, namely Ⅰ,Ⅱ,Ⅲ and Ⅳ, have the similar gravity and magnetic anomaly characteristics, and are demonstrated to be formed in the faulted period. In the HVZs, area Ⅴ consists of the metamorphic rocks formed during the collision orogeny in the Qianliyan Uplift. The other four HVZs show as high-value gravity anomaly and low-value magnetic anomaly in the Northern Depression, which correlate with the feature of the marine sedimentary strata. However, sandstone is confirmed located at the strata with 2 km thickness in Ⅷ and Ⅸ (buried depth <6 km) with a lower velocity (5 km·s^-1, P-wave), referring to the V_P/V_S. Based on the short wavelength magnetic anomaly, there might be igneous rocks in the HVZ Ⅸ of the Central Uplift.