三峡库区典型滑坡地质与地球物理电性特征

滑坡属于地质灾害中的重要灾种之一,查明滑坡区的地质与地球物理特征,对分析滑坡形成机制、评价滑坡稳定性具有重要意义。鉴于此,本文将高密度电法引入三峡库区腹地万州区滑坡地质调查中,在区内四方碑滑坡、塘角1号滑坡和麻柳林滑坡进行地球物理探测工作,并结合钻孔资料对实测结果进行对比验证。实测结果表明：在地形复杂地区开展地球物理探测工作,通过数据反演与分析,可获取滑坡体地层结构及滑移面;结合相关地质资料,通过对比解译,可弥补单纯依靠钻孔信息来确定滑移面形态的不足;3个典型滑坡区地球物理实测资料揭露滑坡体和滑床地球物理电性特征表现为,由崩坡积物、第四系粉质黏土以及含水碎石块组成的滑坡体电阻率低于40 Ω·m,而由砂岩与泥岩组成的滑床电阻率高于40 Ω·m,高密度电法对基岩滑坡和土质滑坡都能获取较好的结果。     

Resistivity imaging survey to delineate subsurface seepage of hydrocarbon contaminated water at Karbala Governorate,Iraq

This research is an attempt to accomplish a 3-D resistivity imaging survey, which was carried out near a water well contaminated with hydrocarbon materials in Karbala governorate. Two-dimensional resistivity imaging measurements were collected along four parallel profiles, using a Wenner array with electrode spacing of 1 m. The RES3DINV program was used to invert the apparent resistivity data. The results displayed a resistivity distribution of the subsurface in a three-dimensional volume. Thus, both the horizontal and vertical extents of the contaminated zone were displayed. This technique revealed a low resistivity zone at depth ranges from 3 to 6 m in the investigation area, but the seepage starts at depth ranges between 2 and 3 m and continues down depth (may be to the groundwater level). This low resistivity zone is the most likely location for a subsurface seepage of contaminated water. It is clear that the sufficient measurement points along 2-D lines in a small area can increase the 3-D imaging resolution, and nearly real 3-D imaging can be achieved, when the size of subsurface anomaly compared with the electrode spacing (a) of the Wenner array is taken into consideration.     

Electrical imaging of the groundwater aquifer at Banting,Selangor, Malaysia

A geophysical study was carried out in the Banting area of Malaysia to delineate groundwater aquifer and marine clay layer of the alluvial Quaternary deposits of Beruas and Gula Formations. The Beruas Formation is formed by peat and clayey materials as well as silt and sands, whereas the Gula Formation consists of clay, silt, sand and gravels. Both Formations were deposited on top of the Carboniferous shale of the Kenny Hill Formation. A 2-D geoelectrical resistivity technique was used. Resistivity measurement was carried out using an ABEM SAS 4000 Terrameter. The 2-D resistivity data of subsurface material for each survey line was calculated through inverse modelling and then compared with borehole data. The resistivity images of all the subsurface material below the survey lines show similar pattern of continuous structure of layering or layers with some lenses with resistivity ranging from 0.1 to 50 Ωm. The upper layer shows resistivity values ranging from 0.1 to 10 Ωm, representing a clay horizon with a thickness up to 45 m. The second layer with depth varies from 45 to 70 m below surface and has resistivity values ranging from 10 to 30 Ωm. Borehole data indicate coarse sand with some gravels for this layer, which is also the groundwater aquifer in the study area. The lowermost layer at a depth of 70 m below ground level shows resistivity values ranging from 30–50 Ωm and can be correlated with metasedimentary rocks consisting of shale and metaquartzite.     

Geophysical survey to estimate the 3D sliding surface and the 4D evolution of the water pressure on part of a deep seated landslide

Geophysical surveys were conducted on the very unstable front part of the La Clapière landslide in the French Alps (Alpes Maritimes). The electrical resistivity survey was carried out to obtain, for the first time on this deep-seated landslide, 3D information on the slipping surface and the vertical drained faults. Moreover, we planned to follow within time (6 months) the evolution of the saturated zones (presence of gravitational water) and their percolation into the shearing zones. Our 4D results showed the importance of the complex water channelization within the slope and relation to geological discontinuities.     

三峡库区典型滑坡地球物理实测及其意义:以万州区四方碑滑坡为例

滑坡是最严重的地质灾害之一,查明滑坡形态特征及滑坡形成机制对于滑坡体稳定性分析、滑坡灾害风险管理和政府治理决策等方面具有重要意义.前人研究滑坡形态及其形成机制较少结合地球物理实测方法,其工程地质剖面绘制多局限于点信息的获取方式,而结合地球物理实测方法有利于从线、面同时获取更丰富的滑坡体地质信息,更加准确量化滑移面埋深和透视滑床形态.鉴于此,以灾害频发的三峡库区万州区为研究对象,采用网格高密度电法实测区内不同深度地层的电阻率值,并以此生成二维电阻率剖面和构建滑床三维形态;同时,将剖面电性分布特征与钻孔资料及地质调查资料相结合,对滑坡区地层结构、滑移面埋深以及滑床形态等多个影响滑坡的重要因素综合分析.结果表明,在研究区开展网格高密度电法实测工作,可获取地层电性结构特征以及构建滑坡体三维形态;实测剖面显示四方碑滑坡属于古滑坡,且存在拉裂槽现象.将实测剖面获取的滑移面埋深和构建的三维滑床形态进行钻孔标定,可对滑坡体内部结构及物质组成进行“透视”,从而为滑坡形成机制分析提供依据,同时也给其他类似区域地质调查中的滑坡稳定性评价及预测预报工作提供理论指导和技术参考.通过高密度电法揭示三峡库区典型滑坡体三维形态特征的理论方法和技术路线成功引入滑坡形成机制分析,可为研究滑坡形成机制和理论提供新思路,具有推广意义.      

Study of landslides at the archaeological site of Abakainon necropolis (NE Sicily) by geomorphological and geophysical investigations

This paper reports on the investigation of the old landslide body on which the necropolis of Abakainon, an Italian archaeological site, was built about 2400 years ago. This site is located in a geologically complex area of northeastern Sicily, where active tectonics has strongly modified the morphological landscape, as testified by the meandering path of watercourses, widespread presence of paleosurfaces, and deviations of crest line. Previous studies have hypothesized that the necropolis, suddenly destroyed by a catastrophic event in historical times, could have been built on an old landslide body. This represents the starting point of this paper, focused on geomorphological analysis and geophysical surveys, which allowed discovering peculiar elements in support of the landslide theory. In particular, the elaboration of a Digital Terrain Model allowed locating the main morphostructural elements, thus highlighting how the occurrence of landslides in the study area is strongly linked to recent tectonics. Passive seismic surveys, carried out through the acquisition of ambient noise, allowed detecting the presence of impedance contrasts, which were associated to peculiar features, such as the landslide body itself. Moreover, local strong directional effects testify the presence of faults, affecting even the necropolis area. Such results, further corroborated by the outcomes of an electrical resistivity survey, lay the foundations for interesting consideration on the possible cause of the destruction of this site, probably related to the occurrence of a historical earthquake in the first century AD.     

3D behaviour of contamination in landfill sites using 2D resistivity/IP imaging: case studies in Portugal

Aiming at defining a valid spatial contamination model, resistivity and induced polarization (IP) measurements were used to investigate contamination plumes in the vicinity of two municipal landfills (Ovar and ílhavo). Previous geophysical surveys and underground water samples confirmed the contamination. However 2D resistivity/IP surveys enabled in obtaining a more accurate spatial model. The Ovar survey consisted of two profiles with nine Wenner soundings each; the ílhavo survey was carried out along two individual lines using a Wenner standard pseudo-section. In both situations, negative IP values were found associated with positive IP values, which can be explained mainly by 2D or 3D geometric effects caused by the presence of the conductive plumes. The data were modelled using a 2D inversion program (RES2DINV) and the resulting resistivity and chargeability distributions were displayed as pseudo-sections. The resistivity and chargeability pseudo-sections define the contamination plumes and the sedimentary structure. These case studies illustrate the advantages of 2D resistivity/IP surveys for the mapping of shape and dimension of contamination associated with landfills.     

Imaging of wastewater percolation in heterogeneous soil using electrical resistivity tomography (ERT): a case study at east of Tenth of Ramadan City,Egypt

The objective of this study is to evaluate the effectiveness of DC resistivity surveys for imaging the wastewater percolation around the stabilization ponds in the Tenth of Ramadan City, the desert fringes of East Nile Delta, Egypt. Detailed resistivity surveys, including DC soundings and electrical resistivity tomography (ERT), were carried out along several profiles. Furthermore, synthetic modeling of ERT was designed to optimize the survey configurations and interpretation of the results. A 2D modeling of smoothness-constrained least-squares inversion scheme was applied to delineate the possible wastewater infiltration zones from oxidation ponds. Because the geoelectrical interpretation has a degree of non-uniqueness, the resistivity inversion was constrained using borehole lithological information and soil sample laboratory measurements. The DC inversion results indicate decreasing resistivity down to a depth of 15 m around waste disposal sites. The inferred soil zone close to the oxidation ponds was a mixture of sand, silt and clay. Moreover, the clay minerals were characterized by moderate swelling that could have reduced the vertical infiltration speed, causing wastewater seepage, especially around unlined disposal sites and open surface drains. Accordingly, the medium-to-low resistivity values can be attributed to wastewater leakage in clayey sand soil. Because the area slopes generally toward the northeast, the surface seepage was dominant in the shallow impermeable sandy clay subsoil. Therefore, measuring soil parameters is a complementary method to optimize resistivity interpretation, with potential for mitigating environmental hazards from wastewater leakage around disposal ponds.     

Mapping lithological variations in a river basin of West Bengal,India using electrical resistivity survey: implications for artificial recharge

Groundwater is a treasured earth’s resource and plays an important role in addressing water and environmental sustainability. However, its overexploitation and wide spatial variability within a basin and/or across regions are posing a serious challenge for groundwater sustainability. Some parts of southern West Bengal of India are problematic for groundwater occurrence despite of high rainfall in this region. Characterization of an aquifer in this area is very important for sustainable development of water supply and artificial recharge. Electrical resistivity surveys using 1-D and 2-D arrays were performed at a regular interval from Subarnarekha River at Bhasraghat (south) to Kharagpur (north) to map the lithological variations in this area. Resistivity sounding surveys were carried out at an interval of 2–3 km. Subsurface resistivity variation has been interpreted using very fast simulated annealing (VFSA) global optimization technique. The analysis of the field data indicated that the resistivity variation with depth is suitable in the southern part of the area and corresponds to clayey sand. Interpreted resistivity in the northern part of the area is relatively high and reveals impervious laterite layer. In the southern part of the area resistivity varies between 15 and 40 Ωm at a depth below 30 m. A 2-D resistivity imaging conducted at the most important location in the area is correlated well with the 1-D results. Based on the interpreted resistivity variation with depth at different locations different types of geologic units (laterite, clay, sand, etc.) are classified, and the zone of interests for aquifer has been demarcated. Study reveals that southern part of the area is better for artificial recharge than the northern part. The presence of laterite cover in the northern part of the area restricts the percolation of rainwater to recharge the aquifer at depth. To recharge the aquifer at depth in the northern part of the area, rainwater must be sent artificially at depth by puncturing laterite layers on the top. Such studies in challenging areas will help in understanding the problems and finding its solution.     

Electrical resistivity tomography (ERT) of a coastal carbonate aquifer (Port-Miou, SE France)

As part of a larger regional research program “KarstEAU”, the authors have applied electrical resistivity tomography (ERT) techniques to characterize heterogeneities in the Port-Miou coastal karst aquifer (Cassis, SE France). Field surveys were carried out on intensely fractured and karstified Urgonian carbonates. Extensive research has characterized macro- and micro-scale geology of the Port-Miou area and particularly underground water-filled conduits and fault/fracture and karst systems within a former quarry. The authors applied 2D ERT along two surface profiles of length 420 and 595 m to test capability for delineating subsurface conduits and possibly relationship between conduit and fault/fracture/karst orientation; and 3D ERT with a dense 120 electrode array at 1 m spacing (11 × 10 m) was applied over an area of the quarry that had been profiled using 3D georadar and which has had intensive nearby structural geological interpretation. The 2D profiling imaged several underground conduits at depths to >50 m below ground surface and below sea level, including possibly the main Port Miou submarine spring and smaller springs. The 2D profiling within the quarry provided a better understanding of the connectivity between major fractures and faults on the quarry walls and secondary springs along the coast supporting flow of the secondary springs along interpreted fracture orientations. In addition, 2D inversion-derived conductivity models indicate that high resistivity zones above sea-level are associated with non-saturated zones and low resistivity anomalies in the non-saturated zone are associated with residual clays in paleokarsts. A partitioned lower resistivity zone below sea-level can be associated with a higher porosity/permeability zone with fractures and karstic features. Inversion models of the dense 3D ERT data indicate a higher resistivity volume within the larger surveyed block. The survey characterized the non-saturated zone and the ERT resistivities are correlated with karst features interpreted by 3D georadar and visible in the inferior wall of the quarry.     

Investigation of the nature of slip surface using geochemical analyses and 2-D electrical resistivity tomography: a case study from Lapseki area, NW Turkey

The nature and subsurface structure of the slip surface of a landslide was studied on the basis of geochemical analyses and 2-D electrical resistivity tomography (ERT) survey. Head scarp and lateral slip surfaces of the landslide marked by clear slickensided shear planes were composed of the average amounts of clayey silt (32.5%) and sand (67.5%). Energy dispersive X-ray spectroscopy (EDX) data revealed the enrichment of Si (23.24%), Fe (12.2%), Al (9.51%) and C (8.34%) in the elemental composition of the disturbed slip surface. From X-ray diffractometry (XRD) data, six main clay types were determined, such as Volkonskoite, Halloysite, Ferrosilite, Saponite, Illite and Nontronite. The ERT survey displayed that the landslide developed as a reactivation on the upper part of an old landslide body.     

Monitoring of wastewater percolation in unsaturated sandy soil using geoelectrical measurements at Gabal el Asfar farm,northeast Cairo,Egypt

The use of wastewater for irrigation in sandy soil increases the pollution risk of the soil and may infiltrate to the shallow groundwater aquifer. In such environment, some important parameters need to be obtained for monitoring the wastewater in the unsaturated zone over the aquifer. These parameters include clay content, heterogeneities of the upper soils, depth to the aquifer and the variations of groundwater quality. In the present work, the efficiency of DC resistivity method in forms of 1-D and 2-D measurements was studied for wastewater monitoring in the Gabal el Asfar farm, northeast of Cairo, Egypt. Forty-one Schlumberger soundings (VES) were performed then followed by three pole-dipole 2-D profiles along some considered regions within the area. The resistivity measurements were integrated with the boreholes, hydrogeological and hydrochemical (surface and groundwater samples) information to draw a clear picture for the subsurface conditions. The obtained results were presented as cross sections and 3-D visualization to trace the clay intercalations within the unsaturated zone. In addition, a vulnerability map was created using the obtained results from 1-D Schlumberger survey and confirmed with the 2-D resistivity profiling. The obtained results have shown that the 2-D resistivity imaging technique is a powerful tool for mapping the small-scale variability within the unsaturated zone and the wastewater infiltration. However, limitations of resistivity techniques were observed in the area with limited resistivity contrast such as thin clay layers with brackish water background. Under that condition, the measured pattern of resistivity distributions depends on the applied electrode array, electrode spacing and using the available geological information during the inversion process.     

高密度电法在三峡库区滑坡勘察中的应用

高密度电阻率法是一种基于岩土体的电性差异而进行勘探的物探方法。在介绍了高密度电法基本原理和系统结构的基础上,通过在三峡库区某滑坡勘察中应用高密度电法确定滑体厚度和基岩面形态的实例,探讨了在滑坡勘察中高密度电法的适用条件和野外工作方法,并结合钻探成果和工程地质测绘资料对其影像成果进行了有效解释。实践表明该技术在与常规勘察方法结合起来进行滑坡勘察时,勘探效率高。应用配套软件处理后,勘测数据生成的地电断面图直观清晰,定性、定量解释可靠,能得到符合实际的滑坡体参数,可以较好地应用于多种勘察方法相结合的滑坡勘察中。     

The Eaux-Bonnes landslide (Western Pyrenees,France): overview of possible triggering factors with emphasis on the role of groundwater

The purpose of this study is to achieve an understanding of the failure mechanisms which caused the Eaux-Bonnes landslide. The geological investigations carried out on the slope of the landslide showed that the sliding mass was cut by numerous faults. The factors controlling the landslide failure were complex, and it is known that neither earthquakes nor heavy precipitation could have triggered the disruption. The groundwater within the solid rock mass has been surveyed, because significant precipitation events during the 2 years preceding the beginning of the paroxysmal phase of the landslide could have led to an increase in pore water pressure along these fractures, thereby triggering the landslide. In order to achieve a full understanding of the failure mechanism, and to identify the origin of the groundwater, a hydrogeochemical survey was carried out over a period of 1 year. The results reveal the existence of high sulphate concentrations in the groundwater originating in springs located at the bottom of the landslide. The sulphate concentrations are correlated with high calcium concentrations, and clearly indicate the presence of gypsum in the vicinity of the lower reaches of the landslide. The presence of gypsum in this area of the Pyrenees suggests that deep groundwater played a role in triggering the landslide.     

Geophysical characterization of the lithological control on the kinematic pattern in a large clayey landslide (Avignonet,French Alps)

Lithology variation is known to have a major control on landslide kinematics, but this effect may remain unnoticed due to low spatial coverage during investigation. The large clayey Avignonet landslide (French Alps) has been widely studied for more than 35 years. Displacement measurements at 38 geodetic stations over the landslide showed that the slide surface velocity dramatically increases below an elevation of about 700 m and that the more active zones are located at the bottom and the south of the landslide. Most of the geotechnical investigation was carried out in the southern part of the landslide where housing development occurred on lacustrine clay layers. In this study, new electrical prospecting all across the unstable area revealed the unexpected presence of a thick resistive layer covering the more elevated area and overlying the laminated clays, which is interpreted as the lower part of moraine deposits. The downslope lithological boundary of this layer was found at around 700 m asl. This boundary coincides with the observed changes in slide velocity and in surface roughness values computed from a LiDAR DTM acquired in 2006. This thick permeable upper layer constitutes a water reservoir, which is likely to influence the hydromechanical mechanism of the landslide. The study suggests a major control of vertical lithological variations on the landslide kinematics, which is highlighted by the relation between slide velocity and electrical resistivity.     

Magnetotelluric Investigation of the Hydrothermal System and Heat Source in the Muine-Toyoha Geothermal Area, Hokkaido, Japan

Abstract. Magnetotelluric (MT) surveys were carried out around the Muine volcano, Hokkaido, Japan, where it is expected that the heat and metal source forming the polymetallic Ag-Pb-Zn-Cu-In Toyoha deposit is present at depth. Measurements were performed at 20 sites, 18 of which were located along a WSW-ENE profile traversing the north ridge of Mt. Muine. A resistivity model obtained from 2D inversion of the MT data shows subsurface specific conductive and resistive features. Conductive layers are present at the surface of Mt. Muine. The low resistivity is probably due to the clay-rich rocks associated with the hydrothermal alteration. A high resistivity layer, which corresponds to the pre-Tertiary Usubetsu Formation, crops out east of Mt. Muine and dips westward. At the west foot of Mt. Muine, relatively high resistive layers are widely exposed. The resistivity increases with depth and exceeds 1000 ohm-m. This fact indicates that this region is not influenced by the recent hydrothermal activity. An extremely conductive zone about 3–6 km wide and 6–9 km thick exists at a depth of 2 km below Mt. Muine. This zone mostly corresponds to an elastic wave attenuation zone detected by a seismic survey. It is interpreted as a large hydrothermal reservoir or melted magma, which is a heat source of the hydrothermal system in this area.     

The geometry of the active strike-slip El Tigre Fault,Precordillera of San Juan,Central–Western Argentina: integrating resistivity surveys with structural and geomorphological data

The geometry and related geomorphological features of the right-lateral strike-slip El Tigre Fault, one of the main morphostructural discontinuities in the Central–Western Precordillera of Argentina, were investigated. Achievements of this survey include: recognition of structural and geometrical discontinuities along the fault trace, identification and classification of landforms associated with local transpressional and transtensional sectors, observation of significant changes in the fault strike and detection of right and left bends of different wavelength. In the Central Segment of the El Tigre Fault, 2D electrical resistivity tomography surveys were carried out across the fault zone. The resistivity imaging permitted to infer the orientation of the main fault surface, the presence of blind fault branches along the fault zone, tectonic tilting of the Quaternary sedimentary cover, subsurface structure of pressure ridges and depth to the water table. Based on this information, it is possible to characterize the El Tigre Fault also as an important hydro-geological barrier. Our survey shows that the main fault surface changes along different segments from a high-angle to a subvertical setting whilst the vertical-slip component is either reverse or normal, depending on the local transpressive or transtensive regime induced by major bends along the trace. These local variations are expressed as sections of a few kilometres in length with relatively homogeneous behaviour and frequently separated by oblique or transversal structures.     

电阻率法探测黄土滑坡滑动面(带)的试验研究

根据滑体与滑床土的结构差异特性,其中电阻率特性也有显著不同,利用WDJD-1型多功能数字直流激电仪和改进的探测电极装置,分别在滑坡后壁原状黄土地层、探槽及钻孔中测试黄土滑坡滑面上下各层土的电阻率值。试验结果显示电阻率测试曲线在滑动带位置出现异常跃变现象,而曲线在未滑动过的黄土地层之间变化差异不明显。这一异常突变特性,在实践中可作为鉴别滑动面位置的特征标志。该探测方法充分利用现有的勘探技术条件,操作简便,试验结果直观,可在黄土滑坡勘察实践中推广使用。     

空间三维滑坡敏感性分区工具及其应用

对于滑坡敏感性分区目前有三种方法:定性法、统计法和基于岩土定量模型的确定性方法。定性法基于对滑坡敏感性或灾害评估的人为判断;统计法用一个来源于结合了权重因子的预测函数或指标;而确定性法,或者说是物理定量模型法以质量、能量和动量守恒定律为基础。二维确定性模型广泛用于土木工程设计,而无限边坡模型(一维)也用于滑坡灾害分区的确定性模型。文中提出了一个新的基于GIS(地理信息系统)的滑坡敏感性分区系统,这个系统可用于从复杂地形中确认可能的危险三维(3-D)滑坡体。所有与滑坡相关的空间数据(矢量或栅格数据)都被集成到这个系统中。通过把研究区域划分为边坡单元并假定初始滑动面是椭球的下半部分,并使用Monte Carlo随机搜索法,三维滑坡稳定性分析中的三维最危险滑面是三维安全系数最小的地方。使用近似方法假定有效凝聚力、有效摩擦角和三维安全系数服从正态分布,可以计算出滑坡失稳概率。3DSlopeGIS是一个计算机程序,它内嵌了GIS Developer kit(ArcObjects of ESRI)来实现GIS空间分析功能和有效的数据管理。应用此工具可以解决所有的三维边坡空间数据解问题。通过使用空间分析、数据管理和GIS的可视化功能来处理复杂的边坡数据,三维边坡稳定性问题很容易用一个友好的可视化图形界面来解决。将3DSlopeGIS系统应用到3个滑坡敏感性分区的实例中:第一个是一个城市规划项目,第二个是预测以往滑坡灾害对临近区域可能的影响,第三个则是沿着国家主干道的滑坡分区。基于足够次数的Monte Carlo模拟法,可以确认可能的最危险滑坡体。这在以往的传统边坡稳定性分析中是不可能的。     

Subsurface structure exploration of wide landslide area by Aerial electromagnetic exploration

Aerial electromagnetic exploration obtains the ground resistivity of a wide area by measuring the intensity of a secondary magnetic field induced in the ground by a primary magnetic field. We conducted the AEM exploration in the Yamakoshi Village where was dameged by the Mid Niigata Prefecture Earthquake in 2004, for the landslide hazard assessment. The apparent resistivity distribution in the survey area differ between the Imogawa River Basin and the Asahigawa River Basin, and the former shows high resistivity and the later shows low resistivity with 30 -m as the boundary. In the high resistivity zone that corresponds to the sandy rock distribution area, the large landslide blocks show lower resisitivity than in the surrounding ground. In sandy rock distribution areas, it is possible for the relatively low resistivity parts to be an index of the instability of slopes, even in landslide blocks where deformation has not yet appeared.