Abstract:

The main hole of Chinese Continental Scientific Drilling (CCSD) is located at Donghai County which is in the eastern side of the Tanlu fault and is at the southern part of Sulu ultrahigh pressure metamorphic belt. The VSP survey is one part of the multi-scientific research. The hole has been surveyed by employing digital seismic instrument (HF300) and digital three-component geophone technique(GeoChain). Combining the VSP surveying results and the 5,000-meter lithologic characteristics of the hole, the features of lithology grouping, seismic responses of ultrahigh-pressure metamorphic rocks, and the relationship between vp/vs and lithology have been analyzed, it comes to a conclusion that: (1) it is advisable that metamorphic rock is grouped by the characteristics of distribution and association of eclogites, because the VSP velocities match well with logging density and lithology distribution in each group；(2) all the eclogite-gneiss interfaces and the shear zones can generate strong reflections, the orthogneissparagneiss interfaces can form obvious reflections, and sometimes the reflection events of lending or banded rocks are bending, while the reflection events of shear zones are continuous; and (3)the eclogites are characterized with high velocity (6.3 km/s), low vp/vs ratio(1.2), and strong and sparse wave trains, while the paragneisses with middle velocity (6.1 km/s) and high vp/vs ratio (1.8) and sparse wave trains, the orthogneisses with low velocity (5.8 km/s) and middle vp/vs ratio (1.5) and dense wave trains; and the shear zones have the characteristics of the strong reflections and the highest vp/vs ratio (2-2)，the vp/vs ratio of shear zones is high, no matter what kind of rocks they emerge in. The research results can not only discover the seismic responses of ultrahigh-pressure metamorphic rocks, but also and more significantly, provide evidences for 2D or 3D surface seismic interpretation, decrease the ambiguity and uncertainty of interpretation, enhance the reliability of geological structure interpretation in CCSD region. 

Key words: VSP survey, Chinese Continental Scientific Drilling, ultrahigh-pressure metamorphic rock