滇西三江构造带电性结构特征——以福贡-巧家剖面为例

为查明滇西三江构造带及邻区复杂的构造特征,并揭示该区深部电性结构,沿福贡—巧家布设了一条长约410 km的大地电磁剖面.共观测到61个物理点,其中宽频大地电磁测点41个,长周期大地电磁测点20个.通过对采集到的数据进行一系列的处理、反演,得到了沿剖面的壳幔电性结构模型.并结合研究区内区域地质资料及其他地球物理资料,对剖面所经过的各个主要地质构造单元及主要断裂带进行了综合解释.电性结构模型揭示沿剖面地壳电性层次复杂,深部电性结构由西往东呈分块展布,横向变化大,壳内广泛发育低阻异常.在中甸构造带（香格里拉地块）和盐源—永胜构造带深部壳幔存在大规模低阻异常,这可能与地下局部熔融体和地热流有关;康滇构造带壳幔存在大规模高阻异常,表明地壳中曾经有地幔物质侵入;在大凉山构造带地下10~50 km深处存在一呈横向"半月形"展布的低阻体,电阻率值不满10 Ωm,结合地质资料与前人的研究成果,推测该低阻体成因应与青藏高原东南缘"地壳管道流"有一定关联.     

轻非水相液体污染过程的高密度电阻率成像法室内监测

为探讨高密度电阻率成像法监测多孔介质中轻非水相液体迁移过程的有效性,本文通过三维砂槽进行了非饱和带中轻非水相液体的污染试验,并利用高密度电阻率成像法进行了同步的动态监测.试验之后,将砂槽层层挖开,通过数码成像,获取了污染区域的实际范围与形状.结果表明,由高密度电阻率成像法圈定的污染区域在范围与形状上都与实际的结果比较接近,并可通过三维电阻率相对值的时间变化明显的看出轻非水相液体的污染过程.这说明利用高密度电阻率成像法对非饱和多孔介质中轻非水相液体的空间分布范围进行圈定并监测其迁移过程是完全可行的.     

On the use of statistical methods to interpret electrical resistivity data from the Eumsung basin (Cretaceous), Korea

Electrical resistivity mapping was conducted to delineate boundaries and architecture of the Eumsung Basin Cretaceous. Basin boundaries are effectively clarified in electrical dipole–dipole resistivity sections as high-resistivity contrast bands. High resistivities most likely originate from the basement of Jurassic granite and Precambrian gneiss, contrasting with the lower resistivities from infilled sedimentary rocks. The electrical properties of basin-margin boundaries are compatible with the results of vertical electrical soundings and very-low-frequency electromagnetic surveys. A statistical analysis of the resistivity sections is tested in terms of standard deviation and is found to be an effective scheme for the subsurface reconstruction of basin architecture as well as the surface demarcation of basin-margin faults and brittle fracture zones, characterized by much higher standard deviation. Pseudo three-dimensional architecture of the basin is delineated by integrating the composite resistivity structure information from two cross-basin E–W magnetotelluric lines and dipole–dipole resistivity lines. Based on statistical analysis, the maximum depth of the basin varies from about 1 km in the northern part to 3 km or more in the middle part. This strong variation supports the view that the basin experienced pull-apart opening with rapid subsidence of the central blocks and asymmetric cross-basinal extension.     

Challenges of using electrical resistivity method to locate karst conduits—A field case in the Inner Bluegrass Region, Kentucky

Conduits serve as major pathways for groundwater flow in karst aquifers. Locating them from the surface, however, is one of the most challenging tasks in karst research. Geophysical methods are often deployed to help locate voids by mapping variations of physical properties of the subsurface. Conduits can cause significant contrasts of some physical properties that can be detected; other subsurface features such as water-bearing fractures often yield similar contrasts, which are difficult to distinguish from the effects of the conduits. This study used electrical resistivity method to search for an unmapped karst conduit that recharges Royal Spring in the Inner Bluegrass karst region, Kentucky, USA. Three types of resistivity techniques (surface 2D survey, quasi-3D survey, and time-lapse survey) were used to map and characterize resistivity anomalies. Some of the major anomalies were selected as drilling targets to verify the existence of the conduits. Drilling near an anomaly identified by an electrical resistivity profile resulted in successful penetration of a major water-filled conduit. The drilling results also suggest that, in this study area, low resistivity anomalies in general are associated with water-bearing features. However, differences in the anomaly signals between the water-filled conduit and other water-bearing features such as water-filled fracture zones were undistinguishable. The electrical resistivity method is useful in conduit detection by providing potential drilling targets. Knowledge of geology and hydrogeology about the site and professional judgment also played important roles in locating the major conduit.     

Mineshaft imaging using surface and crosshole 3D electrical resistivity tomography: A case history from the East Pennine Coalfield, UK

Hidden mineshafts located in urban areas are a significant problem across much of the industrialized world. Electrical resistivity tomography (ERT) is a technique that can detect and characterize hidden mine entries by exploiting resistivity contrasts between the shaft and surrounding materials, resulting from either compositional or structural differences. A case study is presented in which both surface and crosshole 3D ERT surveys are used to image a hidden backfilled mineshaft at a built environment site, situated on Carboniferous Lower Coal Measures strata in the UK.Backfilled shafts generally present the greatest challenge for detection using geophysical methods, as contrasts between the fill and bedrock are typically low compared to air or water-filled conditions. Nevertheless, the shaft in this case was identified by both the surface and crosshole 3D surveys. The shaft appeared as a strongly resistive anomaly relative to background materials, which we interpreted as resulting from the disturbed fabric of the fill materials rather than any significant compositional differences. The study highlighted the respective strengths and weaknesses of the surface and crosshole ERT methodologies for this type of problem. The surface survey, which covered a non-rectangular area to accommodate irregular boundaries and other physical obstructions, provided a relatively rapid means of investigating the study site. However, this method had a limited depth of investigation and was constrained in its coverage by the locations of buildings. By contrast, the 3D crosshole method was able to image the shaft to the level of the deepest borehole electrodes. Although crosshole ERT is too expensive to be used for large-scale mineshaft surveys, this study clearly demonstrates its suitability for targeted investigations where surface methods cannot be deployed, such as scanning beneath surface structures or in situations where it is essential for resolution to be maintained with depth.     

基于交叉梯度交替结构约束的二维地震走时与全通道直流电阻率联合反演

在利用不同的地球物理勘探方法对地下复杂介质成像时,因观测系统的非完备性及数据本身对某些岩石物性的不敏感性,单独成像的结果存在较大的不确定性和不一致性.对于地震体波走时成像与直流电阻率成像,均面临着成像阴影区问题.对于地震走时成像,地震射线对低速区域覆盖较差形成阴影区,造成低速区域分辨率降低.对于电阻率成像,电场线在高阻区域分布较少,造成高阻区域分辨率较低.为了提高地下介质成像的精度,Gallado和Meju（2003）提出了基于交叉梯度结构约束的联合地球物理成像方法.在要求不同的物性模型拟合各自对应的数据同时,模型之间的结构要求一致,即交叉梯度趋于零.为了更有效地实现基于交叉梯度的结构约束,我们提出了一种新的交替结构约束的联合反演流程,即交替反演不同的数据而且在反演一种数据时要求对应的模型与另一个模型结构一致.新的算法能够更容易地把单独的反演系统耦合在一起,而且也更容易建立结构约束和数据拟合之间的平衡.基于新的联合反演流程,我们测试了基于交叉梯度结构约束的二维跨孔地震走时和直流电阻率联合成像.合成数据测试表明,我们提出的交替结构约束流程能够很好地实现基于交叉梯度结构约束的联合成像.与单独成像结果相比,地震走时和全通道电阻率联合成像更可靠地确定了速度和电阻率异常.     

玄武岩内非均质性对地震资料的影响:印度德干圈闭的一个研究实例

Hydrocarbon exploration interests have renewed the need for developing new sub basalt imaging techniques. One of the most important problems encountered today is seismic imaging below basalt. In recent years, this problem appears to have been overcome partly by using long offset seismic data. However near offset data are yet to be fully utilised due to the complex waveform caused by the surface as well as internal heterogeneity of the basalts. The near normal incidence data, which influence the sub-basalt imaging, are highly useful to understand the internal structure within a basalt layer. The use of converted waves for such targets has been proposed as an alternative in a rather homogeneous basalt layer. With a few synthetic modelling exercises here we highlight the practical difficulties in dealing with more realistic and heterogeneous basalt flow. Full waveform seismograms are computed to understand the effects of intra-trappean sediments on the seismic data. A case study from the Deccan Traps of India is presented in this paper. First, we discuss the effects of intercalated sediments on the overall seismic image. Later, the sonic log data from the field are used to compute the full wave-field response using the reflectivity method and compared with the field data. The feasibility of using mode converted waves （P to S and vice-versa at the top and bottom basalt interfaces） for sub-basalt imaging in Kutch region is discussed through a series of velocity-depth profiles. By comparing with the field data we demonstrate that the effects of multiple thin layering within the basalt can strongly deteriorate the image we seek to interpret and exploit.     

Magma ascent beneath Unzen Volcano, SW Japan, deduced from the electrical resistivity structure

The resistivity structure of Unzen Volcano has been revealed by extensive magnetotelluric surveys since the first eruption on November 17, 1990. This structure comprises a highly resistive surface layer, a low-resistive second layer at several hundred meters depth, interpreted as a water-saturated layer, a resistive third layer, and a low-resistive fourth layer at 10 km depth, possibly related to the deep magmatic activity. The structure has influenced the volcanic activity of Unzen. This activity was characterized by a series of dramatic changes in eruption type: a minor phreatic eruption on November 17, 1990; phreatic eruptions after February 12, 1991, preceded by several weeks of volcanic tremor; phreatomagmatic eruptions after April 9, and dome effusion beginning May 19, 1991. This paper presents a hypothesis in which the top of the magma column rose about 20 m/day, reached the base of the water-saturated layer at the end of January, 1991, and approached the upper boundary of this layer on April 9. Thus, the temporal change of eruption type and associated phenomena are systematically explained by an interaction between magma and groundwater contained in the saturated layer.     

玄武岩内非均质性对地震资料的影响:印度德干圈闭的一个研究实例(英文)

油气勘探需要重新开发玄武岩下目的层的成像技术。我们今天遇到的最重要的问题之一是玄武岩下地震成像。近年来,由于利用长炮检距这个问题部分已经得到解决。然而,由于地表以及玄武岩的内部非均质性引起的复杂波形,近炮检距的资料仍然不能充分地被利用。影响玄武岩下成像的近垂直入射资料对于了解玄武岩层内部结构是十分有用的。对比较均匀的玄武岩目标层可选用转换波。这里我们利用几个合成模型例子重点描述了更接近现实的非均质玄武岩流引起的实际困难。模拟计算了全波地震记录以有助于了解玄武岩内沉积物对地震资料的影响。本文介绍了印度德干圈闭的一个研究实例。首先探讨了夹层沉积物对整个地震成像的影响。其次利用该区声测井资料以反射系数法计算全波场响应与实际地震资料对比, 通过一系列速度-深度剖面探讨了利用模式转换波(顶底玄武岩界面上的P波转化为S波或相反)对印度库奇地区玄武岩下成像的可行性。通过与野外资料相比较证明玄武岩中多个薄层的效应严重恶化我们所要解释和利用的图象的质量。     

Volcano-sedimentary stratigraphy in the Valsequillo Basin,Central Mexico inferred from electrical resistivity soundings

Initial results of an electrical resistivity survey of the volcano-sedimentary sequence of the Valsequillo basin in central Mexico are presented. The volcano-sedimentary deposits preserve rich paleontological, paleoclimatic and paleoenvironmental records, which include extinct megafauna remains associated with human artifacts. The report of possible 38 ka old human footprint tracks in the Xalnene tuff attracted renewed interest in the basin stratigraphy. We examine the shallow stratigraphic sequence in the Xalnene tuff outcrop plain northwest of Cerro Toluquilla volcano using vertical resistivity soundings (VES). Inversion models of VES soundings show a layered structure of high and low resistivity units, which characterize the Xalnene tuff, lacustrine and fluvial sediments and volcanic rocks. 2-D resistivity cross sections document three major units corresponding to the Xalnene tuff and sediments filling a <30 m deep basin lying on volcanic rocks. Resistivity models provide further support for the association of Xalnene tuff with the Toluquilla volcano and emplacement of the pyroclastic deposits on a shallow lacustrine environment. The resistivity cross sections constrain the thickness of the tuff layers and underlying lacustrine sediments. Observations during the data acquisition field work provide insight on the possible origin of the apparent tracks, which seem to develop from erosion processes acting on quarrying marks. Further analysis and experimental evidence is required to understand the morphology and weathered patterns. The tuff layers are being removed by quarrying operations and the outcrops significantly altered. Adequate conservation measurements should be implemented to preserve the deposits for scientific research.     

Subsurface architecture of the Boulder Creek Critical Zone Observatory from electrical resistivity tomography

The architecture of the critical zone includes the distribution, thickness, and contacts of various types of slope deposits and weathering products such as saprolite and weathered bedrock resting on solid bedrock. A quantitative analysis of architecture is necessary for many model‐driven approaches used by pedologic, geomorphic, hydrologic or biologic studies. We have used electrical resistivity tomography, a well‐established geophysical technique causing minimum surficial disturbance, to portray the subsurface electrical resistivity differences at three study sites (Green Lakes Valley; Gordon Gulch; Betasso) at the Boulder Creek Critical Zone Observatory (BcCZO). Possible limitations of the technique are discussed. Interpretation of the specific resistivity values using natural outcrops, pits, roadcuts and drilling data as ground truth information allows us to image the critical zone architecture of each site. Green Lakes Valley (3700 MASL), a glacially eroded alpine basin, shows a rather simple, split configuration with coarse blockfields and sediments, partly containing permafrost above bedrock. The critical zone in Gordon Gulch (2650 MASL), a montane basin with rolling hills, and Betasso (1925 MASL), a lower montane basin with v‐shaped valleys, is more variable due to a complex Quaternary geomorphic history. Boundaries between overlying stratified slope deposits and saprolite were identified at mean depths of 3.0 ± 2.2 m and 4.1 ± 3.6 m in the respective sites. The boundary between saprolite and weathered bedrock is deeper in Betasso at 5.8 ± 3.7 m, compared with 4.3 ± 3.0 m in Gordon Gulch. In general, the data are consistent with results from seismic studies, but electrical resistivity tomography documents a 0.5–1.5 m shallower critical zone above the weathered bedrock on average. Additionally, we document high lateral variability, which results from the weathering and sedimentation history and seems to be a consistent aspect of critical zone architecture within the BcCZO. Copyright © 2013 John Wiley & Sons, Ltd.     

Morphology and emplacement of flows from the Deccan Volcanic Province,India

The present study is probably the first of its kind in the Deccan Volcanic Province (DVP) that deals in detail with the morphology and emplacement of the Deccan Trap flows, and employs modern terminology and concepts of flow emplacement. We describe in detail the two major types of flows that occur in this province. Compound pahoehoe flows, similar to those in Hawaii and the Columbia River Basalts (CRB) constitute the older stratigraphic Formations. These are thick flows, displaying the entire range of pahoehoe morphology including inflated sheets, hummocky flows, and tumuli. In general, they show the same three-part structure associated with pahoehoe flows from other provinces. However, in contrast to the CRB, pahoehoe lobes in the DVP are smaller, and hummocky flows are quite common. 'Simple' flows occur in the younger Formations and form extensive sheets capped by highly vesicular, weathered crusts, or flow-top breccias. These flows have few analogues in other provinces. Although considered to be a'a flows by previous workers, the present study clearly reveals that the simple flows differ considerably from typical a'a flows, especially those of the proximal variety. This is very significant in the context of models of flood basalt emplacement. At the same time, they do not display direct evidence of endogenous growth. Understanding the emplacement of these flows will go a long way in determining whether all extensive flows are indeed inflated flows, as has recently been postulated.Most of the studies relating to the emplacement of Continental Flood Basalt (CFB) lavas have relied on observations of flows from the CRB. Much of the current controversy surrounding the emplacement of CFB flows centers around the comparison of Hawaiian lava flows to those from the CRB. We demonstrate that the DVP displays a variety of lava features that are similar to those from the CRB as well as those from Hawaii. This suggests that there may have been more than one mechanism or style for the emplacement of CFB flows. These need to be taken into account before arriving at any general model for flood basalt emplacement.Editorial responsibility: T. Druitt     

Ichthyofauna (Chondrichthyes,Osteichthyes) from the Upper Cretaceous intertrappean beds of Piplanarayanwar,Chhindwara District,Madhya Pradesh,India

A new fossiliferous intertrappean section is found 92 km southwest of the extensively studied intertrappean beds of Mohgaonkalan and Jhilmili in Chhindwara District, Central India. Application of the bulk screen‐washing method led to the recovery of a rich microvertebrate fauna represented by fish, amphibians, crocodiles, snakes and dinosaurs. In this paper, the ichthyofauna recovered from the new intertrappean section consisting of Igdabatis indicus Prasad and Cappetta, 1993, Lepisosteus indicus Woodward, 1908, Osteoglossidae gen. et sp. indet., Pycnodontidae gen. et sp. indet. and Siluriformes indet. is described. The ichthyofauna dominantly represented by the teeth of Igdabatis indicus is suggestive of a nearshore, deltaic or estuarine palaeoenvironment and Late Cretaceous (Maastrichtian) age for the intertrappean beds. The remarkable similarity of the new intertrappean fauna to that of Asifabad and the infratrappean beds of Marepalli also is suggestive of coastal‐plain conditions all along the Godavari lineament in the Late Cretaceous. Although the presence of planktonic foraminifera in the intertrappean beds of Jhilmili in the Mandla Lobe of Deccan volcanic province has been inferred in terms of a short term marine incursion from the west coast along the Narmada lineament in the Early Palaeocene, currently there is no definitive evidence for the prevalence of marine or coastal‐plain conditions along the Narmada lineament at least in the latest Cretaceous.     

Application of low-field hysteresis and memory-phenomenon studies in determining the stability of magnetization of Deccan Trap basalts from Mount Girnar and Mount Pavagarh regions,India

Two techniques, namely alternating-field demagnetization and thermal demagnetization, are widely being used for determining the stability of magnetization of a rock specimen. Recently a faster and simpler technique known as low-field hysteresis loop and memory-phenomenon test has been developed for determining the stability of magnetization directions of igneous rocks. In this paper the results of this new technique, after applying it to about 1000 specimens obtained from 250 oriented rock samples collected from 42 sites of Deccan Trap basalts from Mount Girnar (21°30′N; 70°30′E) and Mount Pavagarh (22°30′N; 73°30′E), India, are presented.The agreement between the mean natural remanent magnetization directions determined by this procedure and those computed after alternating-field demagnetization has been found to be very good. All the specimens from Mount Girnar and 94% of Mount Pavagarh specimens showed a stable line without any memory in a field ≈ 10 Oe. This indicates that the rocks from these two localities are highly stable and are most suitable ones for the determination of a precise palaeomagnetic direction.     

Joint interpretation of hydrological and geophysical data: electrical resistivity tomography results from a process hydrological research site in the Black Forest Mountains,Germany

The use of electrical resistivity tomography (ERT; non‐intrusive geophysical technique) was assessed to identify the hydrogeological conditions at a surface water/groundwater test site in the southern Black Forest, Germany. A total of 111 ERT transects were measured, which adopted electrode spacings from 0·5 to 5 m as well as using either Wenner or dipole‐dipole electrode arrays. The resulting two‐dimensional (2D) electrical resistivity distributions are related to the structure and water content of the subsurface. The images were interpreted with respect to previous classical hillslope hydrological investigations within the same research basin using both tracer methods and groundwater level observations. A raster‐grid survey provided a quasi 3D resistivity pattern of the floodplain. Strong structural heterogeneity of the subsurface could be demonstrated, and (non)connectivities between surface and subsurface bodies were mapped. Through the spatial identification of likely flow pathways and source areas of runoff, the deep groundwater within the steeper valley slope seems to be much more connected to runoff generation processes within the valley floodplain than commonly credited in such environmental circumstances. Further, there appears to be no direct link between subsurface water‐bodies adjacent to the stream channel. Deep groundwater sources are also able to contribute towards streamflow from exfiltration at the edge of the floodplain as well as through the saturated areas overlying the floodplain itself. Such exfiltrated water then moves towards the stream as channelized surface flow. These findings support previous tracer investigations which showed that groundwater largely dominates the storm hydrograph of the stream, but the source areas of this component were unclear without geophysical measurements. The work highlighted the importance of using information from previous, complementary hydrochemical and hydrometric research campaigns to better interpret the ERT measurements. On the other hand, the ERT can provide a better spatial understanding of existing hydrochemical and hydrometric data. Copyright © 2009 John Wiley & Sons, Ltd.     

河北昌黎地震台地电阻率异常分析

河北昌黎台地电阻率EW测道2017年6-8月出现快速下降变化,经检测,观测系统工作正常。通过环境调研和辅助资料对比分析,发现该变化与降雨和粉丝厂干扰有一定关系。采用褶积滤波法去掉降雨干扰,EW测道地电阻率仍为下降趋势,说明该下降变化不能完全用降雨来解释,不排除是地震前异常变化的可能。     

河北昌黎地震台地电阻率异常分析

河北昌黎台地电阻率EW测道2017年6—8月出现快速下降变化,经检测,观测系统工作正常。通过环境调研和辅助资料对比分析,发现该变化与降雨和粉丝厂干扰有一定关系。采用褶积滤波法去掉降雨干扰,EW测道地电阻率仍为下降趋势,说明该下降变化不能完全用降雨来解释,不排除是地震前异常变化的可能。     

水合物赋存形式对石英砂电性特征影响的数值模拟研究

天然气水合物储层精确评价关系到储层的生产与开发过程.由于储层微观结构具有非均质性, 且水合物合成分解过程中分布形式复杂, 水合物储层物性变化规律尚不明晰, 从而难以对储层水合物饱和度进行准确预测.为明确水合物赋存模式对电性特征的影响规律, 本文基于石英砂样品采用随机模拟的方法实现了颗粒包裹型、孔隙填充型和团簇状分布三类赋存模式的水合物数字岩心构建, 并进一步提取孔径分布分析不同水合物分布模式下的微观结构变化特征, 利用有限元方法对含水合物数字岩心进行电性模拟, 结合阿尔奇公式得到适用于含水合物石英砂介质的经验参数取值, 并采用实验室内水合物合成实验电阻率测量数据与实际储层电阻率测井数据对比验证了阿尔奇经验参数取值的正确性及有效性.结果表明, 随着水合物饱和度逐渐增加, 在相同水合物饱和度下, 颗粒包裹型生长引起的微观结构变化对电传导过程的影响最为显著; 孔隙填充型水合物生长引起的孔隙结构变化对电传导过程影响最小; 不同水合物赋存形式下, 沉积物孔径分布变化规律从微观角度解释了三类分布模式的电性差异.研究结果有助于研究储层内含水合物沉积物合成分解过程中的电性特征演化规律.

     

A comparison of peak and plate electrodes in electrical resistivity tomography: application to the chalky groundwater of the Beauvais aquifer (northern part of the Paris basin,France)

Electrical resistivity tomography was used in order to explore an experimental site of the LaSalle Beauvais Polytechnic Institute (France). The test was conducted along a profile line of 315 m length, using 64 electrodes deployed at an inter‐electrode spacing of 5 m, and the data were recorded using gradient, Wenner and pole–dipole arrays. The performance of plate electrodes (non‐conventional flat‐based) is compared with the performance of peak electrodes (conventional spike). The hydrogeophysical investigation of the chalk aquifer system of Beauvais shows that the performance of plate electrodes is satisfactory, leading to inversions of small root‐mean‐square errors. Peak and the plate electrodes were tested before and after injection of a salt tracer in the piezometer of the experimental site. The study demonstrates the usefulness of plate electrodes (efficient, less time consuming) and the possibility of aquifer characterization by a salt tracer. Copyright © 2010 John Wiley & Sons, Ltd.