中国大陆科学钻探先导孔零偏VSP资料解释

用六级三分量检波器在中国大陆科学钻探先导孔中实施了零偏VSP测量.数据处理结果表明,中国大陆科学钻探孔区超高压变质岩石的地震波速度主要介于4500～7000m/s之间,显著高于一般的沉积岩地区,而且随深度变化不明显.声波测井速度系统地稍低于VSP层速度,可能是由于井壁处岩石的完整性受到破坏而造成的.地震波速度与岩石密度和岩性存在明确的对应关系,榴辉岩的密度和地震波速度均显著高于片麻岩类岩石;由榴辉岩退变生成的斜长角闪岩类岩石,其密度和地震波速度均呈现出较大的变化,主要与其退变质程度有关;超基性岩中的裂隙系统导致其密度和地震波速度大幅度下降.由于榴辉岩与其他岩石类型之间存在较大的波阻抗差异,因此用零偏VSP资料标定该区地震波的地质层位是有效的.关于地震波反射的原因,通过综合研究地震波(包括反射纵波、上行转换横波、井筒波)的特征、岩石速度和密度分布以及井径变化,认为主要是岩性分界面、韧性剪切带和断裂(带),但还有一些因素尚待进一步研究.

Interpretation of zero-offset vsp profiles from the pilot hole of Chinese continental scietific drilling

A zero-offset VSP observation was carried out with a 6-level 3-component geophone at the pilot hole of Chinese Continental Scientific Drilling (CCSD). The results of data processing show that the seismic velocities of the ultrahigh pressure metamorphic rocks in the hole area range mainly from 4500 to 7000m/s, being apparently higher than that of common sedimentary rocks and not varying obviously with their seated depth. Sonic velocities are systematically slightly lower than the VSP interval velocities partly because of the effect of drilling damage around the hole. Seismic velocities of rocks are related evidently to their densities and lithologies. The eclogites are much higher in density and velocity than gneisses; plagioclase amphibolites vary in velocities and densities in a large range between eclogite and gneisses because they are retrograded from the eclogite and their lithology and physical properties vary synchronously with the degree of retrogressive metamorphism; and velocity and density of ultramafic rock are lowered apparently due to fractures in it. Since large impedance contrasts exist between eclogite and other rocks, it is effective that the VSP profiles are used to calibrate geological horizons of seismic reflections. With regard to the origin of seismic reflections, compositional contrasts, ductile shear zones and fractures are considered as the main cause on the basis of integrated study of the features of seismic waves (including PP reflections, PS converted reflections and tubewaves), distribution of velocities and densities and the caliper conditions, but some other elements involved need to be studied in detail.