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传统的折射与广角反射地震和重力数据的联合解译是基于大量地壳岩石弹性性质实验室研究证实的地震P波速度和密度之间众所周知的相关性。与这一方法相矛盾的一个问题是富含钙长石的岩石具较高的P波速度,而不满足常规密度—VP关系。这说明基于常规密度—VP关系的联合反演不适用于横跨大的非造山带环斑-辉长-斜长岩地块的广角地震调查,因为其组成成分富含斜长石。同时使用VP和VS计算密度模型就可解决这一问题。岩石属性的实验室结果表明密度与VS相关性很强。此外,各向同性VS与密度的相关性较VP更强,且受高斜长石含量的影响更小。即便如此,利用这一已知的密度与VS或者密度与VP及VS的相关性进行地震和重力数据的联合反演仍然十分少见。其重要原因在于暴破地震学中S波波至的质量低,因而难以得到可靠的S波速度模型。本文阐述了在乌克兰地盾EUROBRIDGE’97广角反射和折射剖面所做的地震和重力数据联合反演的结果。该区缺失厚沉积体,使我们可以得到P波和S波速度模型。通过重力反演得到EUROBRIDGE’97剖面密度模型,我们利用密度和VP、VS模型的关系,将密度模型参数化。这样参数化后,经重力数据转换,可以获得密度和地震波速的关系。结果显示EUROBRIDGE’97剖面的密度与地震波速之间呈非线性和离散的关系。对相关性的分析表明,造成离散是缘于横跨该剖面的大的地质构造单元的差异。为解释这一差异,我们对比EUROBRIDGE’97剖面上3个主要地质单元中地震速度和密度的关系和来自乌克兰地盾及另一个前寒武纪地区的岩石学资料。可见与平均密度—速度关系的偏移量是岩石的特殊矿物组合的反映,包含了不同的岩石时代和地壳形成环境。论述了如何通过密度—速度图分析,来限定地壳成分,尤其是下地壳的成分和变质程度。
The joint interpretation of conventional refraction and wide-angle reflection seismic and gravimetric data is based on the well-known correlation between seismic P-wave velocities and densities that have been validated by laboratory studies of a large number of crustal rock elastic properties. A problem that is contradictory to this method is that the rock rich in anorthite has a higher P-wave velocity than the conventional density-VP relationship. This shows that the joint inversion based on the conventional density-VP relationship is not suitable for the wide-angle seismic survey across the large non-orogenic gabbro-gabbro-plagioclase block because of its compositionally rich plagioclase. Using both VP and VS to calculate the density model can solve this problem. Laboratory results of rock properties indicate that density is highly correlated with VS. In addition, isotropic VS is more strongly densified than VP and less affected by higher plagioclase content. Even so, joint retrieval of seismic and gravitational data using this known density and correlation of VS or density with VP and VS is still rare. The important reason is that the quality of the S-wave in the seismology is violently broken so that it is difficult to obtain a reliable S-wave velocity model. This article presents the results of the joint inversion of seismic and gravity data from the EUROBRIDGE’97 wide-angle reflection and refraction profile of the Earth’s Shield in Ukraine. The absence of thick deposits in this area allowed us to obtain P-wave and S-wave velocity models. By gravity inversion, the EUROBRIDGE’97 profile density model is obtained. We use the relationship between density and VP, VS model to parameterize the density model. After such parameterization, the relationship between density and seismic velocity can be obtained through gravity data conversion. The results show a non-linear and discrete relationship between the density of the EUROBRIDGE’97 profile and the seismic velocity. The analysis of the correlation shows that the discrepancy is caused by the large geologic structural units across the section. To explain this difference, we compared the relationship between the seismic velocity and density in the three major geological units on the EUROBRIDGE’97 profile and the petrologic data from the Ukrainian shield and another Precambrian region. It can be seen that the offset from the average density-velocity relationship is a reflection of the peculiar mineral assemblage of rocks, encompassing different rock ages and crustal environments. Discusses how to define the composition and the degree of deterioration of the crustal components, especially the lower crust, through density-velocity diagram analysis.