地震层析地球内部密度分布横向不均匀研究

本文使用Woodhouse和Dziewonski等分别用S波和P波层析分析提供的,以670km深度为分界的上地幔和下地幔地震波传播所反映出的横向不均匀的球谐函数系数,假定地幔内部密度与正常水平的偏离分别与P波和S波的速度V_p和V_s成比例（下地幔δρ=γδV_p,上地幔δρ=σδV_S²）,并用一组新的球谐函数系数表示全地幔密度分布模型.计算结果表明:1.比例系数γ=0.199,σ=3.13×10^-5时,该全地幔密度模型可以70％恢复Dziewonski提供的下地幔密度模型,50％恢复Woodhouse提供的上地幔密度模型.合成密度分布基本上反映了分别由P波和S波速度异常所提供的地幔密度分布特征;2.所有穿过太平洋中部的剖面均明显地显示出一个低密度异常从地表一直延伸到核-幔边界;3.所有大陆下部均存在一个高密度异常区,而且深入到1200-1300km深度处;4.由密度异常的正、负交替基本上将部分地区地幔分成为3层,其深度依次为:第1层25-1300km左右,第2...     

Seismic recognition of local inhomogeneities

A simple method to contour local inhomogeneities using seismic data is proposed. It formalizes an approximate inversion method which is based on the interpretation of local inhomogeneities as making the differences between an actual seismic data set and a previous reference model. It uses the optimal statistical criteria of parameter estimation and recognition and the ray representation of the waves spreading. Any combination of direct, reflected and/or other types of waves may be used as the database. Inhomogeneities, having a size two times above the wavelength of the seismic waves, can be resolved. Laboratory experiments, using ultrasonic waves and analysis of data from field experiments, confirmed the theoretical results. The method can be used to search for ore bodies, kimberlite cubes, oiltraps, etc.     

Three-dimensional petrographical investigations on borehole rock samples: a comparison between X-ray computed- and neutron tomography

Technical difficulties associated with excavation works in tectonized geological settings are frequent. They comprise instantaneous and/or delayed convergence, sudden collapse of gallery roof and/or walls, outpouring of fault-filling materials and water inflows. These phenomena have a negative impact on construction sites and their safety. In order to optimize project success, preliminary studies on the reliability of rock material found on site are needed. This implies in situ investigations (surface mapping, prospective drilling, waterflow survey, etc.) as well as laboratory investigations on rock samples (permeability determination, moisture and water content, mineralogy, petrography, geochemistry, mechanical deformation tests, etc.). A set of multiple parameters are then recorded which permit better insight on site conditions and probable behavior during excavation. Because rock formations are by nature heterogeneous, many uncertainties remain when extrapolating large-scale behavior of the rock mass from analyses of samples order of magnitudes smaller. Indirect large-scale field investigations (e.g. geophysical prospecting) could help to better constrain the relationships between lithologies at depth. At a much smaller scale, indirect analytical methods are becoming more widely used for material investigations. We discuss in this paper X-ray computed tomography (XRCT) and neutron tomography (NT), showing promising results for 3D petrographical investigations of the internal structure of opaque materials. Both techniques record contrasts inside a sample, which can be interpreted and quantified in terms of heterogeneity. This approach has the advantage of combining genetic parameters (physico-chemical rock composition) with geometric parameters resulting from alteration or deformation processes (texture and structure). A critical analysis of such 3D analyses together with the results of mechanical tests could improve predictions of short- and long-term behavior of a rock unit. Indirect methods have the advantage of being non-destructive. However, as it is the case with large-scale geophysical surveying, XRCT and NT are affected by several error factors inherent to the interaction of a radiation modality (X-ray or neutron beam) with the atomic structure of the investigated materials. Recorded signals are therefore in particular cases not artifact-free and need to be corrected in a subsequent stage of data processing.