反射地震技术在成矿地质背景与深部矿产勘查中的应用：现状与前景

矿床是地球演化过程中特定阶段、特定地质环境下形成的,深部结构、构造和动力过程是控制成矿作用的关键因素。成矿带或矿集区现今的地壳结构、构造保留着其演化动力学过程留下的痕迹,通过对不同深度地壳结构的探测,不仅可以提供成矿系统(成矿物质来源、迁移路径和存储空间)形成时期的构造背景和成矿地质环境信息,还可以直接追踪控矿构造的延深,甚至直接发现深部矿体。反射地震是深部探测的核心技术,应用于大陆地壳结构探测和油气勘探已经有近百年的历史,但应用于成矿学的研究和深部金属矿勘查只有二十几年。本文回顾了近年反射地震技术在区域成矿背景、矿集区深部3D结构和深部找矿勘查中取得的重要进展,讨论了该技术在应用中存在问题和下一步技术发展方向,为我国"地壳探测工程(SinaProbe)"在重点成矿带和矿集区开展立体探测提供一些借鉴。     

电离层层析重构的一种新算法

自适应联合迭代重构算法是电离层层析成像中一种收敛速度较快的反演算法.然而,在电离层电子密度重构过程中,该方法对迭代初值的精度要求较高.针对上述问题,本文提出了一种约束自适应联合迭代重构算法,该方法通过附加合理的平滑约束,减弱了没有观测信息的格网对迭代初值精度的依赖性,有效地提高了反演结果精度.数值模拟实验和实测数据的反演结果证实了该算法的可行性和在重构精度上的优越性.     

南北构造带及邻域地壳、岩石层速度结构特征研究

本文利用重力数据采用Parker-Oldenburg方法反演了南北构造带及邻域地区的地壳厚度,同时采用体波地震层析成像方法反演了研究区的地壳至上地幔的三维速度结构.根据计算结果对研究区的地壳及岩石层结构进行了探讨,力图揭示南北构造带及邻域地壳、岩石层变形特征,并且对青藏高原边缘活动带壳幔构造演化的深部成因、研究区的上地幔流变性及其动力学意义进行了相应的讨论.通过分析研究表明南北构造带地区为地壳厚度剧变区,西侧为地壳增厚区,东侧的鄂尔多斯、四川盆地为地壳稳定区,而再向东为地壳逐渐减薄区.中国岩石层减薄与增厚的边界基本被限定在大兴安岭—太行山—秦岭—大巴山—武陵山一带,这也是东部陆缘带和中部扬子、鄂尔多斯克拉通地区深部构造边界的分界线,其两侧不仅浅层地质构造存在较大的差异,上地幔深部的物性状态和热活动也明显不同,这说明研究区的岩石层和软流层结构以及深部物质的分布存在横向非均匀性.中部地区和青藏高原深部构造边界的分界线位于东经100°—102°左右.     

Special 3D electric resistivity tomography (ERT) array applied to detect buried fractures on urban areas: San Antonio Tecómitl,Milpa Alta,México

Geophysical techniques can be employed to understand the physical characteristics of the subsurface in highly populated areas, where urban settlements present structural problems. An interesting example is presented in this investigation, where three-dimensional Electric Resistivity Tomography (ERT-3D) is applied employing alternative arrays that allow investigating the subsoil beneath the affected constructions.A small neighborhood comprised by a block of houses within the town of San Antonio Tecómitl is studied. The area is found towards the outskirts of the Chichinautzin range, in the southern limit of the Mexican Basin. This settlement suffers of strong damage in their structures due to fractures and subsidence within the subsoil. ERT-3D was carried out to characterize the subsoil beneath this urban complex. A special resistivity array (‘horse-shoe’ geometry) employing a combination of ‘L’, equatorial-dipole and minimum-coupling methods was carried out to fully ‘illuminate’ the subsoil beneath the block of houses. Computed resistivity models depicted a high resistivity pattern that coincides with the affected houses. Such pattern seems to extend beyond the limits of the surveyed areas, and is probably part of a more complex fracture system. It is very likely that fractures have been produced due to the poorly consolidated soil, which is part of a transition zone; the intense water extraction, that form ‘voids’ in the subsoil causing subsidence effects and finally the existence of regional faults to greater extent that might control these shallow features.     

Centrifuge modeling of the seismic responses of sand deposits with an intra-silt layer

Three dynamic centrifuge model tests were conducted at an acceleration of 80g to simulate the seismic responses of level sand deposits: an intra-silt layer was embedded in two of these sand deposits at different depths. The effects of a low-permeability intra-silt layer on the build-up and dissipation of excess pore-water pressure, surface settlement, and the related liquefaction mechanism were investigated. An intra-silt layer modifies the seismic response of the sand deposit, reduces the extent of liquefaction, and thus decreases surface settlement. The depth of the intra-silt layer is one of the factors influencing the seismic responses of the sand deposits. The magnitude of the surface settlement is proportional to the degree of liquefaction in the sand deposit. The high positive hydraulic gradients appearing in both the intra-silt layer and in the sand deposit lying on the intra-silt layer can break a thinner or weaker top layer and result in sand boiling. Our visual animation of the ratio of the excess pore-water pressure and the lateral displacement revealed that the liquefaction front travels upward during shaking and the solidification front travels upward after shaking.     

Determining drag coefficients and their application in modelling of turbulent flow with submerged vegetation

Vegetation is a key aspect of water resources and ecology in natural rivers, floodplains and irrigation channels. The hydraulic resistance of the water flow is greatly changed when submerged vegetation is present. Three kinds of drag coefficients, i.e., the drag coefficient for an isolated cylinder, the bulk drag coefficient of an array of cylinders and the vertically distributed or local drag coefficient, have been commonly used as parameters to represent the vegetation drag force. In this paper, a comprehensive experimental study of submerged stems in an open channel flow is presented. Empirical formulae for the three drag coefficients were obtained based on our experimental results and on data from previous studies. A two-layer model was developed to solve the mean momentum equation, which was used to evaluate the vertical mean velocity profile with each of the drag coefficients. By comparing the velocity distribution model predictions and the measurement results, we found that the model with the drag coefficient for an isolated cylinder and the local drag coefficient was good fit. In addition, the model with the bulk drag coefficient gave much larger velocity values than measurements, but it could be improved by adding the bed friction effect and making choice of the depth-averaged velocity within the canopy layer.     

地震层析成像方法综述

本文从地震层析成像的模型参数化、正演、反演、解评估四个方面回顾了地震层析成像方法近期发展历程以及各时段地震层析成像方法的优、缺点。     

2003年大姚6.2和6.1级地震前三维波速结构的演化

尝试在年度尺度上对云南地区地壳速度结构进行成像,以确定2003年大姚地震前三维地壳速度结构的演化,并对其发震成因的介质物性变化进行探讨.检测板分辨试验显示,在年度尺度上,大姚附近区域15km深度上节点解的分辨率在0.6左右;误差分析显示,在震源附近的误差(约0.02km/s)远小于速度变化的幅度(约0.15km/s).研究结果表明,大姚地震前震源区附近形成一条北北西向的高、低波速交界带,其走向与两次大姚地震震源机制解的走向以及通过余震精定位确定的断层走向基本吻合,且震源位于交界带的高波速一侧.此外,大姚地震震后在震源下方形成一低速体.本文得到的大姚地震前的波速结构演化可为探讨其孕震、发震条件提供约束.      

门源地区地壳三维体波速度结构及地震重定位研究

本文使用甘肃、青海数字地震台网及中国地震科学探测台阵记录到的门源地区地震的P波和S波到时资料,应用双差层析成像方法联合反演了该地区的地壳三维速度结构和震源位置参数.结合地质构造背景,研究了门源M_S6.4地震孕育发生的深部介质环境及该地区速度结构与地震活动性之间的关系.结果表明：反演之后地震的走时残差均方根显著降低,重定位后的地震在垂直方向上呈现出与断层位置有关的条带状分布.门源地区地壳速度结构存在明显的不均匀性,浅层P波和S波速度结构与地表地质构造及地形特征密切相关.研究区内地震活动性与地壳速度结构具有很强的对应关系,地震主要分布在高速异常区域及其边缘.门源M_S6.4地震震中附近的P波和S波速度结构表现出明显的高速异常,且在震源区下方存在P波低速层,这种特殊的构造条件可能是导致此次地震发生的重要原因.

     

井间地震层析成像初始速度模型

地震直达波走时层析成像可归结为求解一个大型的、稀疏的、常常是病态的线性方程组.求解方程组常用的迭代法,需要一个比较合理的初始猜测解,也即是初始速度模型.初始模型关系到反演的效率甚至成像的正确性.本文在前人研究基础上提出一种生成模型网格节点初始速度方法,假定震源到检波点路径为直线,记录每条射线穿过的单元和统计每个网格单元穿过的射线数目、自动拾取网格节点所在单元的数目等.实例中,由原始数学模型的正演旅行时资料生成节点初始速度模型,效果可以.最后,分别采用均匀模型和本文方法生成的初始模型进行迭代反演,通过比较,证实该自动生成节点初始模型的可行性和可靠性,并对存在的问题进行讨论和解释.