库尔勒地震台四分量钻孔应变观测资料可靠性论证

对库尔勒地震台的巴伦台、库米什、小泉沟子台3套四分量钻孔应变仪观测数据进行多角度可靠性分析,除按常规由4个元件的读数计算2个面应变数值及相关系数值（数据自检）外,做以下补充：对潮汐因子内精度进行计算;通过相对标定元件格值,视矫正幅度大小及对数据自检效果进行改进;与同一时段相邻区域陆态网络形变观测资料进行对比,进一步论证观测资料的可靠性。结果表明,3个子台的应变观测数据可靠,能够真实反映所处区域的构造活动和应变,可以用于区域地震预测和地震科学研究。     

基于3D-FEM的洞穴型地层双侧向测井数值模拟及响应特征研究

洞穴型碳酸盐岩储层非均质性强、电测井响应复杂、测井识别和表征难度大,利用数值模拟方法明确该类储层的电测井响应特征,可为洞穴识别与评价提供理论依据.本文基于三维有限元素法(3D-FEM),引入边界局部加密技术,实现对复杂球形洞穴的精细刻画与响应精确计算;对比分析井眼钻穿型洞穴和井旁洞穴的双侧向测井响应特征和敏感性,进而考察双侧向测井对两者敏感范围的差异.结果表明:洞穴的存在导致双侧向测井响应明显降低,受洞穴边界及仪器探测深度等影响,井眼钻穿型洞穴双侧向测井曲线复杂,而井旁洞穴曲线呈"抛物线"型;双侧向测井对井眼钻穿型洞穴的敏感性远大于对井旁洞穴的敏感性,深侧向和浅侧向测井最大可对洞穴边界距井壁0.5 m和0.3 m的井旁洞穴敏感.     

短偏移距电磁探测技术概述

由于人工源电磁探测效果与收发距离有一定关系,本文首先分析了收发距离为零的中心回线源瞬变电磁法（Transient Electromagnetic Method,TEM）视电阻率定义和算法特点,回溯了从圆形回线到矩形的修正式中心回线装置的发展过程;概述了从中心回线发展到接地导线源短偏移瞬变电磁（Short-Offset TEM,SOTEM）理论上的可行性;列举了将LOTEM（Long-Offset TEM）从数倍于探测深度的观测区推进到近源区后的探测实例.对于频率域人工源电磁法,介绍了起源于MT（Magneto-Telluric）的CSAMT（Controlled Source Audio-frequency Magneto-Telluric）与电磁频率测深法不同发展道路.电磁频率测深将观测区从远区推进到了中区;广域电磁法通过全区视电阻率新定义不仅提高了解释精度,而且将传统CSAMT远场观测模式推进到中区探测模式;最后指出,如能采用新的点微元假设计算方法,可有望分离自有场和辐射场,实现频域电磁近源探测.     

Illuminating and optimising a three‐dimensional model of an oil shale seam and its volume distribution using the transient electromagnetic induction method,central part of Jordan

The oil shale exploration program in Jordan is undertaking great activity in the domain of applied geophysical methods to evaluate bitumen‐bearing rock. In the study area, the bituminous marl or oil shale exhibits a rock type dominated by lithofacies layers composed of chalky limestone, marls, clayey marls, and phosphatic marls. The study aims to present enhancements for oil shale seam detection using progressive interpretation from a one‐dimensional inversion to a three‐dimensional modelling and inversion of ground‐based transient electromagnetic data at an area of stressed geological layers. The geophysical survey combined 58 transient electromagnetic sites to produce geoelectrical structures at different depth slices, and cross sections were used to characterise the horizon of the most likely sites for mining oil shale. The results show valuable information on the thickness of the oil shale seam at 3.7 Ωm, which is correlated to the geoelectrical layer between 2‐ and 4 ms transient time delays, and at depths ranging between 85 and 105 m. The 300 m penetrated depth of the transient electromagnetic soundings allows the resolution of the main geological units at narrow resistivity contrast and the distinction of the main geological structures that constrain the detection of the oil shale seam. This geoelectrical layer at different depth slices illustrates a localised oil shale setting and can be spatially correlated with an area bounded by fold and fault systems. Also, three‐dimensional modelling and inversion for synthetic and experimental data are introduced at the faulted area. The results show the limitations of oil shale imaging at a depth exceeding 130 m, which depends on the near‐surface resistivity layer, the low resistivity contrast of the main lithological units, and the degree of geological detail achieved at a suitable model's misfit value.     

时域瞬变电磁法三维有限差分正演技术研究

瞬变电磁法应用广泛,三维数值模拟是研究复杂地质模型异常响应规律的重要技术手段之一,也是反演的基础.目前瞬变电磁数值模拟的不足主要有两个方面:第一,场源是在地表水平、浅层介质均匀的条件下计算的,限制了应用范围;第二,地下边界采用Dirichlet边界条件,导致计算空间很大,耗时较长.针对上述问题,在三维正演时,场源采用有限长细导线模型,在Maxwell有源差分方程中直接加入电流密度进行计算.在地表面加入空气层,避免了复杂的向上延拓计算,也可以对地形影响下的响应规律进行分析.在空气边界和地下边界均采用CPML吸收边界条件,并改进了CPML的参数分布,能够吸收空气介质和大地介质中的低频电磁波而反射误差极小,在满足计算精度的条件下可以有效减小节点数量.对循环迭代方法进行优化,将计算域、CPML区域和场源的空间循环统一转化为矩阵方式,加快了计算速度,但是空间消耗增大了约4~5倍.采用三维有限差分正演算法对均匀半空间模型、层状模型和地形模型进行了计算,并与解析解进行了对比验证.     

地面磁共振与瞬变电磁横向约束联合反演方法研究

大地电阻率分布信息是影响磁共振地下水探测反演结果准确性的重要因素.在众多电磁法勘探技术中,瞬变电磁法具有高分辨率、高效率和大探测深度等优势,能准确探测地下几百米范围内的电阻率分布信息.因此磁共振与瞬变电磁联合解释方法具有重要意义.然而,利用单一测点拼接的磁共振与瞬变电磁联合解释方法进行模拟二维反演时存在解释结果不唯一,容易出现错误异常体等问题,尤其在复杂地质情况下,同一测线上相邻测点探测结果连续性差,解释结果偏离实际.基于此,本文提出磁共振与瞬变电磁横向约束联合反演方法（Laterally Constrained Inversion,简称LCI）,重点引入外推积分法（quadrature with extrapolation,简称QWE）,解决了传统正演过程中基于直接数值积分方法引起的求解效率低的问题,保证了联合反演方法的顺利实施,进而以相邻测点地下结构应具备连续性为依据,引入横向约束反演思想,通过在联合反演目标函数中加入相邻测点间各模型参数约束矩阵,提高磁共振解释结果准确性,加强探测剖面地质结构和含水模型连续性.经过理论模型证实,本文提出的LCI方法能有效提高传统一维反演结果的稳定性和唯一性.最后,对安徽黄山野外实际探测数据进行横向约束联合反演,验证了磁共振与瞬变电磁LCI联合反演方法的实用性.本文的研究成果将为磁共振与瞬变电磁空间约束联合反演奠定基础.     

A finite-element time-domain forward-modelling algorithm for transient electromagnetics excited by grounded-wire sources

A three-dimensional finite-element time-domain forward-modelling algorithm is developed to simulate transient electromagnetics excited by grounded-wire sources. The main advantage of this finite-element time-domain algorithm is that full transmitting-current waveforms and complex-shaped sources resulting from topography can be directly dealt with in this algorithm. The models used to test this algorithm include a homogeneous half-space model, a stratified-medium model, the model of a complex conductor at a vertical contact and the Ovoid Zone massive sulfide deposit at Voisey's Bay, Canada. The homogeneous half-space model is used to determine the truncation boundary for a computational domain, and to compare with the electromagnetic responses excited by step-off, step-on and direct current waveforms. For the stratified-medium model, results demonstrate that full transmitting waveforms have strong effects on the observed electromagnetic responses. The model of a complex conductor at a vertical contact is designed for the grounded electrical source airborne transient electromagnetic method and is also used to examine the effectiveness of the broadside and inline configurations for such a vertical, thin plate embedded in the subsurface. The area of the Ovoid Zone massive sulfide deposit possesses non-negligible topography, the effects of which on the shapes of the grounded-wire sources must be taken into account when implementing the finite-element time-domain solver. The results show that both the broadside and inline electromagnetic responses are strongly affected by the massive conductive ore body.     

STUDY ON QUATERNARY MAGNETIC STRATIGRAPHY IN WUHAN AREA,CHINA

With the lateral and vertical expansion of cities, urban geology becomes critical for urban construction. Wuhan City, as one of the largest cities in China, was chosen by China Geological Survey as pilot city to study multiple elements of urban geological survey. 90% of Wuhan area is covered by Quaternary strata, which means that most of the city is built on Quaternary sediments. The study of Quaternary stratigraphic structure of Wuhan area is a crucial groundwork for the urban geological survey of Wuhan. Due to the badly lagging of research on Quaternary stratigraphy of Wuhan area, this study selects four boreholes from hundreds of cores in this area in the project of Wuhan Urban Geological Survey for magnetic stratigraphic study and paleomagnetic analysis. This work mainly focuses on the borehole SK2 which possesses well-developed strata and is representative for magnetic strata division. Wuhan is located in the eastern Jianghan Basin where the Quaternary sediment has a fluctuant bottom edge, forming a half-graben shape boundary in large scale, as revealed by a large amount of boreholes. The borehole SK2 is located in the western Dongxi Lake depression. SK2 reveals continuous deep Quaternary sediment except for a short gap of late Pleistocen-early Holcene. Moreover, the grain size of drill core is generally smaller than other cores nearby, which is more suitable for paleomagnetic study. In this study, we collected 117 samples with an interval of 0.25~0.30m from relative fine grain layers in the borehole for paleomagnetic study. Demagnetization and sample measurement were conducted in State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS. By comparing with other well dated cores nearby, Wuhan's Quaternary magnetic stratigraphic chronology framework is initially established. Our data show that:1)the Brunhes-Matsuyama boundary(B/M)of borehole SK2 is clear, locating at 29.6m, and sediment at 1.2~29.6m belongs to Bmnhes normal polarity chron; 2)Two polarity drifts at 9.3~9.6m and 14.1~15.0m are observed in Brunhes. These samples collected from the Xiashu Loess yield two polarity drifts, which can be compared with the Xiashu Loess at Nanjing. The comparison between Xiashu Loess from borehole SK2 and that at Nanjing suggests that negative polarity drift at 14.1~15.0m is the Blake reverse polarity subchron(0.104~0.128Ma BP)and the polarity drifts at 9.3~9.6m(0.07~0.08Ma BP)is Norwegian-Greenland Sea reversal-polarity subchron; 3)Three polarity drifts were observed in Matsuyama, which is comparable with the borehole at Zhoulao and the borehole R25 in the center of the Jianghan Basin. They are at 38.2~39.8m(Jaramillo:0.99~1.07Ma BP), 66.5~71.9m(Olduvai:1.77~1.95Ma BP)and 75.8m(Reunion:2.01Ma BP); 4)Based on the forementioned paleomagnetic data, the depositional age structure of this borehole is:1.2~14.1m(late Pleistocene), 15~29.6m(middle Pleistocene)and 29.6~78.8m(early Pleistocene); 5)Our data show that deposition rate increased during 1.47~1.57Ma BP(54.9~61.3m)and 1.07~1.21Ma BP(39.8~47.3m)because they are fluvial sediments. This study builds the Quaternary stratigraphy and timescale for Wuhan area for the first time. However, because of the complex Quaternary sediment type in Wuhan area and the coarse sediment in the Matuyama, more comprehensive study is needed in the future to test whether our Quaternary stratigraphy and timescale is the best.     

武汉地区第四纪磁性地层

武汉地区90%为第四系所覆盖,但目前针对第四纪年代地层的研究严重滞后。文中在武汉市城市地质调查施工的数百个钻孔中选择了地层发育齐全、适于进行磁性地层研究的SK2钻孔进行了古地磁极性测试和分析,并重点对SK2钻孔进行了磁性地层的划分,初步建立了武汉市第四纪磁性地层的年代序列。结果表明,SK2钻孔的布容与松山(B/M)界线位于孔深约29. 6m处,其古地磁年龄约0. 78Ma;松山反向带中的正极性亚时贾拉米洛(Jaramillo)、奥杜威(Olduvai)、留尼汪(Reunion)Ⅱ分别位于孔深38. 2～39. 8m、66. 5～71. 9m和75. 8～78. 4m处。依据磁性地层结果对研究钻孔进行了第四纪地层划分,SK2钻孔的1. 2～14. 1m为上更新统,15～29. 6m为中更新统,29. 6～78. 8m为下更新统。计算SK2钻孔地层的沉积速率后发现,古地磁年代1. 47～1. 57Ma BP和1. 07～1. 21Ma BP为2个沉积速率相对增大的阶段。     

江宁台井下地电阻率电极接地电阻分析

电极是否稳定可靠是地电阻率观测系统的重要指标之一,主要通过测量电极的接地电阻大小来评价其状态。首先,分析现有测量方法,认为线路加电测量法更合理,提出考虑电缆线电阻的线路加电测量法;其次,采用该方法和接地电阻测试仪测量方法,测量江宁台多套地电观测系统电极的接地电阻,比较分析后者的结果误差;第三,讨论其接地电阻超出标准、同深度不同电极接地电阻差异及同井孔不同深度电极接地电阻差异,其原因可能与地下低电阻率区域及电极周围的地层湿度、介质的孔隙度有关,并分析了加电测量法存在的不足;最后,当接地电阻超标时,应该依据其变化幅度来判定电极是否稳定可靠。