共查询到20条相似文献，搜索用时 15 毫秒
1.
《Journal of African Earth Sciences》2009,53(4-5):161-166
Integrated geophysical methods involving magnetic and dipole–dipole resistivity (DDR) were conducted across a prominent zone of weakness clearly observable in Landsat MSS and SLAR images in the Precambrian basement complex of southwestern Nigeria. Up till now, the location and existence of this megascopic structure have not been confirmed using geophysical methods. With the objective of delineating this weak zone and its structural attributes, three traverses were established at 500 m intervals across it, and geophysical measurements were made at 10 m intervals along these traverses. Qualitative interpretation of the magnetic data obtained shows a diagnostic signature of a near-vertical fault, trending along a NNE–SSW direction. Also, the quantitative interpretation of the data using the non-linear least-squares regression technique indicates that the width of the magnetic anomaly ranges from 90 to 150 m, its dip angle varies between 75° and 85°; the anomaly is concealed by a regolith of approximately 15 m thickness. Furthermore, a 2D resistivity inversion of the field resistivity data reveals a three-layer model, representing thin resistive topsoil underlain by weathered bedrock, resistive bedrock with a distinct low resistivity zone located within the bedrock. The most plausible explanation for this low resistivity zone is that it was formed by shearing activities during Late Precambrian times. Conclusively, the integrated approach employed in this research confirms the existence of the supposed Ifewara shear zone (ISZ). 相似文献
2.
Integrated surface electrical resistivity and electromagnetic (EM) surveys were conducted in a hard-rock terrain of Southwestern
Nigeria in the vicinity of active oxidation sewage treatment ponds. The aim was to detect soil contamination due to the spread
of sewage effluent, locate possible leachate plumes and conductive lithologic layers, and access the risk of groundwater pollution
in the vicinity of the sewage-ponds. Dipole–dipole resistivity profiling and very low frequency (VLF) data were acquired at
10-m intervals along five 200-m long east-west geophysical traverses. Resistivity sections obtained revealed four subsurface
geologic layers comprised of lateritic clay, clayey sand/sand, weathered/fractured bedrock, and competent bedrock. A distinct
low resistivity zone corresponding to the contamination plume (labeled B) was delineated from all the resistivity sections.
This low zone extends into the weathered bedrock and possibly suggests contamination of this layer. The filtered real component
of the processed VLF data detected three distinct anomaly zones that are representative of fractured zones filled with conductive
fluids and/or lithologic boundaries that possibly serve as conduits for the movement of contaminated effluents. The results
obtained from the two methods suggest possible contamination of the subsurface soil layers and groundwater in the vicinity
of the sewage-ponds. The existence of this contaminated plume poses a serious threat to the ecosystem and health of the people
living in the vicinity of the sewage-ponds. 相似文献
3.
Animesh Mandal Arkoprovo Biswas Saurabh Mittal William K. Mohanty Shashi Prakash Sharma Debashish Sengupta Joydip Sen A. K. Bhatt 《Journal of the Geological Society of India》2013,82(6):601-606
Beldih mine at the central part of the South Purulia Shear Zone (SPSZ) has been reported with low grade uranium-bearing formation within quartz-magnetite-apatite host in kaolinized formation. Therefore, the present integrated geophysical study with gravity, magnetic, radiometric, very low frequency electromagnetic (VLF) and gradient resistivity profiling methods around the known mineralized zones aimed at identifying the exact geophysical signatures and lateral extent of these uranium mineralization bands. The closely spaced gravity-magnetic contours over the low to high anomaly transition zones of Bouguer, reduced-to-pole magnetic, and trend surface separated residual gravity-magnetic anomaly maps indicate the possibility of high altered zone(s) along NW-SE direction at the central part of the study area. High current density plots of VLF method and the low resistive zones in gradient resistivity study depict the coincidence with low gravity, moderately high magnetic and low resistivity anomalies at the same locations. Moderate high radioactive zones have also been observed over these locations. This also suggests the existence of radioactive mineralization over this region. Along profile P2, drilled borehole data revealed the presence of uranium mineralization at a depth of ～100 m. The vertical projection of this mineralization band also identified as low gravity, low resistivity and high magnetic anomaly zone. Thus, the application of integrated geophysical techniques supported by geological information successfully recognized the nature of geophysical signatures associated with the uranium mineralization of this region. This enhances the scope of further integrated geophysical investigations in the unexplored regions of SPSZ. 相似文献
4.
Geological and geophysical interpretation of TEM data has revealed changes to the subsurface from the “Kraton-3” peaceful underground nuclear explosion (PUNE). The explosion was conducted on 24 August 1978 at a depth of 577 m in Middle Cambrian limestone on the eastern periphery of the Tunguska basin (Western Yakutia). The site is located in an area of 100 to 300 m thick permafrost and pressurized aquifers with Na-Ca-Cl brines (up to 400 g/l TDS) and cryopegs. The “Kraton-3” epicenter is only 160 m away from a fault emerging along the Markha River.TEM responses collected at 22 stations along three profiles image a layered-earth background resistivity pattern. The highly resistive uppermost layer, ~ 150–200 m thick, consists of perennially frozen ice-rich rocks. Dry permafrost on watersheds of the Markha right side reaches 1200 ohm?m, while the hypsometrically lower frozen ground along the fault is 10 to 40 times less resistive. That is exactly the place of the PUNE epicenter, and the resistivity lows may record permafrost degradation and taliks (unfrozen layers).The layers below are conductive and correspond to Upper Cambrian and Middle Cambrian (I) aquifers with brines. The top of the Upper Cambrian aquifer along the central profile is highly variable in depth, especially along the fault on the river left bank. The data indicate a local groundwater anomaly above the explosion: the Middle Cambrian I brines, which show up as a conductor in the resistivity pattern, become ~ 300 m shallower, most likely rising along the rubble chimney above the UNE containment cavity; the lateral extent of the anomaly reaches 400 m. There may exist paths for mass and heat transport maintained by pressurized brines in the system “containment cavity–rubble chimney–fault zone–ground surface”. 相似文献
5.
Electromagnetic and geoelectric investigation of the Gurinai Structure, Inner Mongolia, NW China 总被引：2，自引：0，他引：2
S. Hlz D. Polag M. Becken R. Fiedler-Volmer H.C. Zhang K. Hartmann H. Burkhardt 《Tectonophysics》2007,445(1-2):26
In three field campaigns between the years 2000 and 2004 geophysical measurements were conducted in the Ejina Basin, NW China. Research work in the year 2004, which is described in this paper, was concentrated on the Gurinai Structure (101°25′E, 41°N) situated in the southeastern part of the Ejina Basin in transition to the dune fields of the Badain Jaran Shamo. On satellite images the Gurinai Structure can be identified by two almost 100 km long, subparallel, N–S-striking lineaments, which may indicate tectonic deformations of late Quaternary sediments. To get a coherent picture of the structure a geophysical survey employing three electromagnetic methods – magnetotellurics (MT), transient electromagnetics (TEM), and geoelectrics (DC) – has been conducted to map the subsurface resistivity at different depth scales.The geophysical data interpretation for shallow and intermediate depth down to a few hundred meters links the subsurface distribution of electric resistivity to geomorphological units known from field work in reference with satellite images. The westerly lineament of the Gurinai Structure coincides with a subvertical change in electric resistivity. Together with geomorphological indications from fieldwork and the analysis of elevation data (SRTM), a tectonic deformation of unconsolidated sediments along a fault with an extensional component is interpreted. In the central and eastern part of the Gurinai Structure a shallow resistive subsurface layer can be traced into the first dunes of the Badain Jaran Shamo. This resistive subsurface layer is linked to the presence of fresh water, indicating infiltration from the dune field. Also, in the eastern part of the Gurinai Structure a resistive, approximately ENE-striking feature can be seen at intermediate depth, which is interpreted as a crystalline basement ridge. Towards the southern margin of the Gurinai Structure a trough-shaped unit with low resistivities and a thickness of about 1 km is identified and can be explained by a sediment package saturated with fluids of high salinity or substantial amounts of clay. The strike direction of the structure can be connected to the regional pattern of tectonic faults and seismicity.The interpretation of electromagnetic data at various depth scales contributes to the general understanding of the Ejina Basin's buildup and tectonic setting in the vicinity of the Gurinai Structure. 相似文献
6.
Magnetotelluric (MT) investigations were carried out along a profile in the greenschist–granulite transition zone within the south Indian shield region (SISR). The profile runs over a length of 110 km from Kuppam in the north to Bommidi in the south. It covers the transition zone with 12 MT stations using a wide-band (1 kHz–1 ks) data acquisition system. The Mettur shear zone (MTSZ) forms the NE extension of Moyar–Bhavani shear zone that traverses along the transition zone. The regional geoelectric strike direction of N40°E identified from the present study is consistent with the strike direction of the MTSZ in the center of the profile. The 2-D conductivity model derived from the data display distinct high electrical resistivity character (10,000 Ω m) below the Archaean Dharwar craton and less resistive (< 3000 Ω m) under the southern granulite terrain located south of the MTSZ. The MTSZ separating the two regions is characterized by steep anomalous high conductive feature at lower crustal depths. The deep seismic sounding (DSS) study carried out along the profile shows dipping signatures on either side of the shear zone. The variation of deep electrical resistivity together with the dipping signature of reflectors indicate two distinct terrains, namely, the Archaean Dharwar Craton in the north and the Proterozoic granulite terrain towards south. They got accreted along the MTSZ, which could represent a possible collision boundary. 相似文献
7.
8.
Surface geophysical methods were used to determine the locations of fracture zones in part of the hard rock area in Sonebhadra District of Uttar Pradesh, India. The survey comprises three DC resistivity profile using the gradient profiling technique and ten very low frequency electromagnetic (VLF-EM) traverses profiles. The methods were used over survey lines extending between 200 and 400 m; the results were correlated to locate fracture zones for the purpose of groundwater exploration. Qualitative interpretation of the VLF-EM was carried out using Fraser and Karous–Hjelt filters. The result of the interpretation revealed a number of subsurface zones with high real component current density that defines the potential subsurface features (probably fracture zones). The subsurface feature concurred with the low resistive zones indentified from the gradient resistivity profiling. The zones where further inferred quantitatively using data obtained from DC resistivity sounding at some selected anomalous points. The result obtained proves the efficiency of integrating both methods in detecting fractures zones in hard rock area. 相似文献
9.
The use of resistivity sounding and two-dimensional (2-D) resistivity imaging was investigated with the aim of delineating and estimating the groundwater potential in Keffi area. Rock types identified are mainly gneisses and granites. Twenty-five resistivity soundings employing the Schlumberger electrode array were conducted across the area. Resistivity sounding data obtained were interpreted using partial curve matching approach and 1-D inversion algorithm, RESIST version 1.0. The 2-D resistivity imaging was also carried out along two traverses using dipole–dipole array, and the data obtained were subjected to finite element method modeling using DIPRO inversion algorithm to produce a two-dimensional subsurface geological model. Interpretation of results showed three to four geoelectrical layers. Layer thickness values were generally less than 2 m for collapsed zone, and ranged from 5 to 30 m for weathered bedrock (saprolite). Two major aquifer units, namely weathered bedrock (saprolite) aquifer and fractured bedrock (saprock) aquifer, have been delineated with the latter usually occurring beneath the former in most areas. Aquifer potentials in the area were estimated using simple schemes that involved the use of three geoelectrical parameters, namely: depth to fresh bedrock, weathered bedrock (saprolite) resistivity and fractured bedrock (saprock) resistivity. The assessment delineated the area into prospective high, medium and low groundwater potential zones. 相似文献
10.
Electrical surveys have routinely been taken to map and monitor groundwater contamination. In 1994-1996, various electrical surveys were applied to investigate contaminant distributions in the ground at the Nanjido landfill. The geophysical survey data were compared with other available information, particularly boring data. Interpretations of electrical survey data show low resistivity zones below 10 ohm-m which appear to be zones fully saturated with leachate. Annual variations of resistivity anomalies clearly indicate that resistivities and thicknesses of layers contaminated by leachate become lower and thicker in and around the Nanjido landfill during one year. In particular, mean thickness of saturated layers with leachate increased by about 3-6 m/year and the resistivity of bedrock decreased. It seems obvious that ground contamination by leachate is in progress. In the area northeast of the landfill, no evidence of bedrock contamination is indicated. Soundings made at one year intervals in this area do not show any evidence of further ground contamination by leachate. From these results, it appears that contamination of the weathered zone and bedrock is in progress mainly southwest of the Nanjido landfill. 相似文献
11.
A combined geophysical investigation consisting of vertical electrical sounding (VES) and multielectrode system was carried
out to map the subsurface resistivity in all major lakes which are highly polluted by the discharge of sewage and other chemical
effluents in greater Hyderabad, India. The structural features identified in the study area play a major role in groundwater
flow and storage. The interpretation of geophysical data and lithologs indicates that a silt/clay zone (predominantly silt)
has a thickness of 5–10 m all along the drainage from Patelcheruvu to the Musi River. The silt/clay zone inferred close to
the lakes is a mixture of clay, silt and sand with more silt content as indicated from the lithologs during drilling. The
low resistivity values obtained can be attributed to the pollutant accumulated in the silt which can reduce the resistivity
values. Further, the TDS of the water samples in these wells are more than 1,000 mg/l which further confirms the above scenario.
The pollution spread is less in the upstream areas whereas it is more in the downstream which can be attributed to the shallow
water table conditions and also due to the interaction of surface water and groundwater. 相似文献
12.
Lower crustal intrusions beneath the southern Baikal Rift Zone: Evidence from full-waveform modelling of wide-angle seismic data 总被引：1，自引：0，他引：1
The Cenozoic Baikal Rift Zone (BRZ) is situated in south-central Siberia in the suture between the Precambrian Siberian Platform and the Amurian plate. This more than 2000-km long rift zone is composed of several individual basement depressions and half-grabens with the deep Lake Baikal at its centre. The BEST (Baikal Explosion Seismic Transect) project acquired a 360-km long, deep seismic, refraction/wide-angle reflection profile in 2002 across southern Lake Baikal. The data from this project is used for identification of large-scale crustal structures and modelling of the seismic velocities of the crust and uppermost mantle. Previous interpretation and velocity modelling of P-wave arrivals in the BEST data has revealed a multi layered crust with smooth variation in Moho depth between the Siberian Platform (41 km) and the Sayan-Baikal fold belt (46 km). The lower crust exhibits normal seismic velocities around the rift structure, except for beneath the rift axis where a distinct 50–80-km wide high-velocity anomaly (7.4–7.6 ± 0.2 km/s) is observed. Reverberant or “ringing” reflections with strong amplitude and low frequency originate from this zone, whereas the lower crust is non-reflective outside the rift zone. Synthetic full-waveform reflectivity modelling of the high-velocity anomaly suggests the presence of a layered sequence with a typical layer thickness of 300–500 m coinciding with the velocity anomaly. The P-wave velocity of the individual layers is modelled to range between 7.4 km/s and 7.9 km/s. We interpret this feature as resulting from mafic to ultra-mafic intrusions in the form of sills. Petrological interpretation of the velocity values suggests that the intrusions are sorted by fractional crystallization into plagioclase-rich low-velocity layers and pyroxene- and olivine-rich high-velocity layers. The mafic intrusions were probably intruded into the ductile lower crust during the main rift phase in the Late Pliocene. As such, the intrusive material has thickened the lower crust during rifting, which may explain the lack of Moho uplift across southern BRZ. 相似文献
13.
四川阿坝——秀山地学断面 总被引：1，自引：2，他引：1
四川省阿坝—秀山地学断面长约1000km,横跨上扬子地台和松潘-甘孜地槽褶皱系。在综合研究现有地质、地球物理资料的基础上,对断面及邻区划分出不同性质的三大岩石圈块体;结合表壳变形特征又区分出以四川地块为中心的东、西对冲构造体系;并进一步划分出8个次级构造带(块)。在垂向上划分出地壳、岩石圈厚度及形态,讨论了地壳次级分层及壳、幔低速层、低阻层和高阻层异常的特征,提出了初步解释。指出龙门山断裂带西部地壳缩短、增厚的主要因素。概述了地壳演化。 相似文献
14.
Edmundo Placencia-Gómez Annika Parviainen Tero Hokkanen Kirsti Loukola-Ruskeeniemi 《Environmental Earth Sciences》2010,61(7):1435-1447
The Haveri tailings area contains 1.5 Mt of sulfide-bearing waste from the Au–Cu mine that operated during 1942–1961. Geophysical
and geochemical methods were used to evaluate and characterize the generation of acid mine drainage (AMD). Correlations were
examined among the electrical resistivity tomography (ERT) data, the total sulfide content and concentrations of sulfide-bound
metals (Cu, Co, Fe, Mn, Ni, Pb and Zn) of tailings samples, and the resistivity and geochemistry of surface water. The resulting
geophysical–geochemical model defines an area in the vadose tailings, where a low resistivity anomaly (<10 Ohm m) is correlated
with the highest sulfide content, extensive sulfide oxidation and low pH (average 3.1). The physical and geochemical conditions,
resulting from the oxidation of the sulfide minerals, suggest that the low resistivity anomaly is associated with acidic and
metal-rich porewater (i.e., AMD). The lower resistivity values in the saturated zone of the central impoundment suggest the
formation of a plume of AMD. The natural subsoil layer (silt and clay) and the bedrock surface below the tailings area were
well mapped from the ERT data. The detected fracture zones of the bedrock that could work as leakage pathways for AMD were
consistent with previous geological studies. The integrated methodology of the study offers a promising approach to fast and
reliable monitoring of areas of potential AMD generation and its subsurface movement over large areas (ca. 9 ha). This methodology
could be helpful in planning drill core sampling locations for geochemical and mineralogical analysis, groundwater sampling,
and choosing and monitoring remedial programs. 相似文献
15.
东坡矿区内矿产资源丰富,但已探明的铅锌资源经多年开采已出现严重危机。为配合地质找矿,在研究区内采用以地面高精度磁测为主,激电测深为辅的物探工作方法,圈出22个磁测ΔT异常,磁异常主要沿千里山岩体接触带分布。结合地质、物化探资料分析发现野鸡尾—牛角垄、妹子垄、蛇形坪—塘渣水三个有重要意义的异常区。通过钻探验证,在妹子垅预测区的M2磁异常区见到厚度10.86~32 m的矽卡岩型钨锡多金属矿体,矿床规模可达大型;在牛角垅预测区M9磁异常区见到了6.4 m的铅锌矿化破碎带。初步建立起该区物化探找矿模式为"二低三高",即低重力、低视电阻率,高强度的ΔT异常、高视极化率ηs异常,并有地球化学Pb、Zn、Ag、As等异常带。东坡铅锌矿与岩体密切相关,因此在选区上,必须选在反映岩体或隐伏岩体的重力低异常区,视电阻率低异常地段往往是构造破碎带或矽卡岩带发育地段。 相似文献
16.
为了研究东洋地区深部地质结构,在福建东洋地区开展了综合地球物理勘查,对福建东洋地区地球物理场进行了分析,结合已有的地质资料,分析研究了区域地球物理场特征及区域深部地质结构特征.研究认为:福建东洋地区位于巨型环形构造外环带西南部,研究区处在东部沿海磁场剧烈变化带和西部内陆磁场相对平缓带的过渡区域,区域航磁ΔT异常以北东向条带状异常带为主,区内分布有2处剩余重力异常高;地球物理综合剖面范围被两条北西向深大断裂分割为3部分,结合区域地质特征,推断区域南部的浅层是一套推覆无根的变质岩系高阻体,深部为一套以中生代沉积岩石为主的低阻体,区域北部浅部主要为中生代沉积岩夹薄层火山岩组成的低阻体,深部为下古生代和元古界基底岩系共同反应的高阻体,区域中部是火山岩主要发育区. 相似文献
17.
Bhadra B. K. Gor Naresh Jain Ashish K. Meena Hansraj Rao S. Srinivasa 《Hydrogeology Journal》2021,29(8):2705-2724
The Great Rann of Kachchh (GRK) in Gujarat, India, is the largest salt desert in the world, which is usually filled with seawater ingression during high tide from the Arabian Sea. As a result, the soil gets saturated with saline water that has percolated down for several meters. Groundwater exploration in Rann area is a challenging task due to the prevailing hostile environment. For this purpose, multisensor satellite data have been used to delineate the palaeochannels in search of an alternate source of drinking water. In GRK, palaeochannels represent the zone of elevated fluvial sediments with respect to the surroundings. Evolutionary history of the palaeochannels indicates upliftment of GRK area during Allah Bund faulting. For assessing the groundwater potential of the palaeochannels, high-resolution electrical resistivity tomography (HERT) surveys have been carried out with the pole-dipole method. Electrical resistivity tomograms along 710 m traverses to a depth of 250 m in Dharmsala and Gainda area show higher-resistivity zones (medium to coarse sand with brackish water) below a thick low-resistivity layer (clay with saline water). A few exploratory drillings in the area confirm the existence of the palaeochannels, which act as a confined aquifer below 100 m depth. The artesian condition of the two drilled wells at Gainda and Khardoi along the northern boundary of GRK may be attributed to hydraulic gradient along the confined layers from the Tharparkar region in Pakistan. Thus, HERT is found to be a faster and more cost-effective geophysical survey technique for study of the deep aquifer.
相似文献18.
M. Mohammadzadeh Moghaddam S. Mirzaei J. Nouraliee S. Porkhial 《Arabian Journal of Geosciences》2016,9(7):506
The Delijan region of Central Iran is a popular tourist spot due to the occurrence of hot springs and having the greatest geothermal fields in Iran. In the years 2011 and 2012, an integrated geophysical investigation, using magnetic and gravity methods, was conducted over the hot springs in order to characterize geophysical anomaly sources corresponding to the geothermal resources. The results of the geophysical investigations revealed the heat source and the reservoir of the Delijan geothermal system (DGS). Based on results of Euler depth estimation and 3D inversion of magnetic and gravity data, the depths and extension of the discovered structures were determined with a good correlation with the geological information. The results of magnetic interpretation show that the main source (heat source) of the geothermal system is located NE of the Delijan-Abgarm fault (DAf) zone at depths of 2500 to 5000 m, and the results of gravity interpretation show that the reservoir of the geothermal system is located along the DAf zone at depths of 1000 to 4000 m. Also, the horizontal gradients of gravity data reveal complex fault systems which are acting as the preferential paths to circulate the hydrothermal fluids. 相似文献
19.
Localization of water bearing fractured zones in a hard rock area using integrated geophysical techniques in Andhra Pradesh,India 总被引：1，自引：0，他引：1
Knowledge of the existence of fracture zones, their extent, intensity and direction is very useful for assessing groundwater in hardrock regions and in this context geophysical methods are widely accepted as a powerful means of study. In the modern era of exploration, application of the Resistivity Imaging technique gives a new opportunity for groundwater study in hardrock regions. Exploration surveys were conducted at one of the important sites in Maheshwaram watershed, Andhra Pradesh, India with a multielectrode resistivity imaging system. To reduce the ambiguity of geophysical interpretation some complementary geophysical studies like ground Magnetic and VLF were also carried out.A number of 2D resistivity images were prepared in a grid pattern, which clearly show the weathered and fractured zones in different parts of the study area. With the help of all 2D profiles a quasi-3D image has been created, which indicates the orientation and extension of the fracture zone in a horizontal as well as vertical direction in the study area. Strong agreement exists among the anomalies identified using the ground magnetic, VLF and resistivity imaging methods. The litholog data available in the study area also helps to interpret geophysical results to find a potential groundwater bearing zone in that area. 相似文献
20.
Groundwater in eastern Uganda mostly occurs in fractures in the crystalline basement rocks and at the interface between the overburden and bedrock. The study was aimed at improving the success rate of boreholes through the use of complimentary geophysical siting procedures in 16 locations in Kamuli District, eastern Uganda. Boreholes that were sited after applying appropriate geophysical techniques yielded adequate quantities of water, whereas those sited where such procedures were not applied were out of service sooner than expected. Techniques to determine the precise location of resistivity anomaly and vertical electrical sounding (VES) models were used to locate water-bearing zones. VESs were undertaken to provide an overview of the geology. The apparent resistivities of the water-bearing zones both from VES and resistivity profiling data, had a relationship with the success rates of the boreholes. Electrical resistivities were correlated with hydrogeological parameters. The majority of successful boreholes had, within water-bearing zones, minimum apparent resistivity values less than 200 and 100 Ohmm, from the resistivity profiling anomalies and VES, respectively. The depth to bedrock was generally greater than 20 m below ground level, which indicates potential for medium yielding boreholes. 相似文献