Acquiring, Processing And Interpretation Of Seismic Data From Chinese Continental Scientific Drilling Site

Chinese Continental Scientific Drilling (CCSD) is a grand scientific project and received a worldwide attention. Maobei region of Donghai, Jiangsu, is selected as the CCSD site due to its specific tectonic position, large amount of ultra-high pressure metamorphic (UHPM) rocks and abundant information deep in the crust. Seismic response of the drilling site is discussed thoroughly in this paper on the basis of two seismic explorations, aiming at revealing the internal structure of Su-Lu UHPM belt and modeling returning process of UHPM rocks.Since various rocks, complex structure and steep strata in the site, integrated well and ground seismic explorations were adopted on the two pilot holes, and 3D reverse VSP layout was used in microlog data acquiring. VSP velocity analysis shows that seismic velocities of the UHPM rocks, varying mainly from 4500 m/s to 7000m/s, are much higher than that of common sedimentary rocks and do not vary with their seated depth. It's considered that seismic velocity of metamorphic rock is related to metamorphic type and strength, density, Hthology and geologic structure, at the same time, intense foliation structure and interlayer of rocks different in velocity make seismic velocity present obvious anisotropy and transverse imhomogeneity. On the basis of microlog data, crustal surface layer in the site is divided from above to below into four belts: low-velocity belt, rapidly lowering belt, slowly lowerign belt and fresh bedrock, which embody both weathering strength and Hthology. With regard to the origin of seismic reflections, compositional contrasts, ductile shear zones and fractures are considered as the main cause. Comprehensive interpretation to seismic sections finds out geological implication of different wave groups and clearly sketch occurrence and distribution of ductile shear belt, eclogite body and fault. Moreover, dip information included in seismic files is used to analyze strata and structure dip and its variation in space. At the end, the returning mechanism of Su-Lu UHPM belt is discussed on the basis of geologic characteristics and its deforming-metamorphic history as well as abundant information deep in the crust provided by seismic files.