内蒙古双尖子山矿集区三维电性结构及成矿意义

双尖子山矿集区作为中国北方东部大型的银铅锌多金属矿集区,具有多期叠加成矿的特征.为了揭示双尖子山矿集区2000 m以浅成矿要素的空间展布,本文基于有限内存拟牛顿法(L-BFGS)对覆盖矿集区的6条可控源音频大地电磁测深数据进行含源三维反演,获得了双尖子山矿集区2000 m以浅三维精细电性结构.结合重磁、区域地质、钻井及地球化学资料,得到如下结论:(1)矿集区深部存在大面积的高阻区,可能为与成矿有关的早白垩纪侵入岩的反映,在兴隆山矿段西北侧及西南侧(A、B区)、双尖子山矿段西部(C区)表现为高侵位特征;(2)矿集区与成矿密切相关的断裂系统由1条北东向深断裂(F1)和北西向、北东向以及近南北向的浅层断裂组成;(3)赋矿地层大石寨组电性特征为中低阻,主要分布在1000 m以浅,在兴隆山矿段以及双尖子山矿段东部较厚,与矿集区断裂系统组成了良好的容矿空间和成矿流体上涌通道;(4)建立了矿集区2000 m以浅的成矿模式:在兴隆山矿段,早白垩世侵入岩高度侵位至上覆大石寨组下方,侵入岩就位后析出含矿热液,并在浅部北东、北西向断裂及断裂交汇处成岩成矿,而在双尖子山矿段,成矿流体主要受北东向深断裂F1的控制,在F1断裂以及其与北西向断裂的交汇处富集,导致矿体主要分布于矿段西部.     

Magnetotellurics applied to the study of the crustal structures in Senegal (West Africa)

Abstract

In Senegal, magnetotellurie (MT) method has been used in an attempt to resolve the principal structural features by their electrical response. On the basis of numerical modelling of data, an unified model of possible crustal structure is presented for the West african margin. The results are in agreement with other independent geophysical and geological information.     

云南玉溪盆地电性结构MT测深研究

利用大地电磁测深技术探测了云南玉溪盆地的电性结构及可能的断裂构造分布。在与地质构造近垂直的方向,布设了两条测线进行大地电磁观测,应用Rebocc二维反演方法获得了玉溪盆地两条测线的视电阻率剖面。结果显示,低阻体厚度的变化趋势与盆地第四系沉积层平均厚度的变化类似,在盆地西侧,厚度较深,高阻和低阻界面变化较陡。自西向东,低阻体厚度逐渐减小,高低阻界面变化趋势趋缓。总体来说,低阻体厚度比已有资料给出的沉积层厚度要深,导致这一结果的原因一方面可能是盆地沉积层厚度在不同的区域变化较大,另一方面也可能和沉积层下的岩层富含水资源有关。     

电扰动仪数据处理软件的设计和实现

使用Visual C++开发适用于高采样率电扰动仪的数据处理软件,可以对数据进行快速绘图、交互式预处理和初步分析,并具备简单的仪器运行统计功能,极大地提高了预处理和数据分析的效率。     

基于不等式约束的最小二乘法三维电阻率反演及其算法优化

基于光滑约束的最小二乘法是三维电阻率反演的主要方法,但该方法在某些情况下存在着多解性较强的问题,且普遍耗时较长,严重制约了三维反演方法的推广与发展.为改善上述问题,将表征模型参数变化范围的不等式约束作为先验信息引入最小二乘线性反演方法中,有效地改善了反演结果的精度,降低了反演的多解性问题.为了解决耗时较长的问题,基于预条件共轭梯度(PCG)算法和Cholesky分解法的特点提出了一套优化三维电阻率反演计算效率的计算方案.在该方案中,Cholesky分解法被用来求解敏感度矩阵计算中的多个点源场的正演问题,Cholesky分解法只需对总体系数矩阵进行一次分解,然后对不同的右端向量进行回代即可.将预条件共轭梯度法引入到三维电阻率反演方程的求解中,将雅可比迭代中的对角阵作为预处理矩阵,其具有求逆方便、无需内存空间的特点,有效地加快了收敛速度.对合成数据以及实测数据的反演算例表明,借助不等式约束和反演效率优化方案,最小二乘反演方法可得到较为精确的反演结果,有效地提高了反演计算效率,具有良好的推广前景.     

二连盆地“三明治”电性结构特征及其对古生界的研究意义

由于长期以来东北地区晚古生代的构造属性一直被认为是海西期褶皱带或碰撞造山带,而一直被作为中、新生代盆地的变质结晶基底成为油气勘查的禁区,限制了古生界的油气勘探和开发."佳木斯—兴蒙地块"新认识及二连盆地赛51井取得的石炭纪碳酸盐岩油藏的发现,开辟了二连盆地古生界油气勘探的新领域.本文对二连盆地3条典型的CEMP电测量资料进行了二维电阻率反演,结合研究区地震反射资料、钻孔电测井资料、钻井岩芯资料、岩石露头资料,揭示了二连盆地具有"三明治"的电性结构特征,同时指出二连盆地古生界广泛分布,在古生界与中生界之间广泛分布一套以玄武岩、安山岩、凝灰岩等为主的火山岩.这种"三明治"电性结构特征不但给古生界的研究提供了线索和依据,而且火山岩(高阻层)与其下伏古生界地层(低阻层)之间可形成良好的储盖组合(或生储盖组合),其对于古生界油气勘探的研究具有重要的意义.同时,认为二连盆地中、新生界和古生界的构造沉积演化主要经历了4个重要阶段,即:泥盆纪的南北差异性升降阶段、石炭-二叠纪的地层稳定沉积阶段、三叠纪-晚侏罗世的古生代盆地被改造和中新生代盆地构造格架形成的重要演化阶段以及中新生代的地层沉积阶段.     

The application of spatial derivatives to non‐potential field data interpretation

Source/body edge detection is a common feature in the processing and interpretation of potential field data sets. A wide range of spatial derivatives is available to enhance the information contained in the basic data. Here the ability of these procedures to assist with the mapping interpretation of non‐potential field data is considered. The study uses airborne electromagnetic (conductivity) data but also provides a general context for other conductivity/resistivity data, provided the non‐potential field nature of active and thus spatially‐focused, measurements is acknowledged. The study discusses and demonstrates the application of a range of common spatial derivative procedures, including the analytic signal and upward continuation, to both magnetic and conductivity data. The ability of the tilt derivative to provide enhanced mapping of conductivity data is considered in detail. Tilt and its associated functions are formed by taking combinations of vertical and horizontal derivatives of the data set. Theoretical forward modelling studies are first carried out to assess the performance of the tilt derivative in relation to the detection and definition of concealed conductivity structure. The tilt derivative embodies automatic gain control that normalizes the detection and definition of both weak and strong conductivity gradients across an appropriate subsurface depth range. The use of high‐order spatial derivatives inevitably results in a degree of noise (cultural perturbation) amplification that is survey and technique specific. Both of these aspects are considered using practical case studies of jointly obtained magnetic and conductivity data at a variety of spatial scales.     

Extreme sensitivity of crosshole electrical resistivity tomography measurements to geometric errors

The effects of geometric errors on crosshole resistivity data are investigated using analytical methods. Geometric errors are systematic and can occur due to uncertainties in the individual electrode positions, the vertical spacing between electrodes in the same borehole, or the vertical offset between electrodes in opposite boreholes. An estimate of the sensitivity to geometric error is calculated for each of two generic types of four-electrode crosshole configuration: current flow and potential difference crosshole (XH) and in-hole (IH). It is found that XH configurations are not particularly sensitive to geometric error unless the boreholes are closely spaced on the scale of the vertical separation of the current and potential electrodes. However, extremely sensitive IH configurations are shown to exist for any borehole separation. Therefore, it is recommended that XH configurations be used in preference to IH schemes. The effects of geometric error are demonstrated using real XH data from a closely spaced line of boreholes designed to monitor bioremediation of chlorinated solvents at an industrial site. A small fraction of the data had physically unrealistic apparent resistivities, which were either negative or unexpectedly large. However by filtering out configurations with high sensitivities to geometric error, all of the suspect data were removed. This filtering also significantly improved the convergence between the predicted and the measured resistivities when the data were inverted. In addition to systematic geometric errors, the measured data also exhibit a high level of random noise. Despite this, the resulting inverted images correspond reasonably closely with the known geology and nearby cone penetrometer resistivity profiles.     

Determination of the 3D structure of an earthflow by geophysical methods: The case of Super Sauze, in the French southern Alps

In order to evaluate the risk associated by an earthflow to abruptly evolve into a torrential flow, the knowledge of its internal structure is necessary. Geotechnical methods are important to reach this goal. However, because of the rough topography associated with earthflows, their surface heterogeneities, and the spatial variations of the thickness of the potentially moving mass, non-intrusive geophysical methods offer a very useful tool that complements traditional geotechnical methods. We report the results of a comprehensive study covering a 150 m by 200 m area of the Super Sauze earthflow. This earthflow developed in black marls in the southern French Alps. Shallow electrical conductivity investigations, derived using low frequency domain electromagnetics, maps hidden gullies and crests and lateral variations of the clay and the water content within the first 5 m below the ground surface. Electrical resistivity tomography allows to extrapolate this information down to 10 m below the ground surface along selected transects. The vertical structure of the earthflow, down to the substratum, is defined precisely thanks to joint inversion of DC and TDEM vertical soundings along one profile: the flowing upper layer and the position of the substratum are clearly evidenced. Combining this geophysical datasets with geotechnical tests and drill holes, we provide an estimate of both the location and the volume of the potentially most dangerous areas of the earthflow.