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1.
Aeromagnetic (AM) and Landsat Thematic Mapper (TM) data from the south-central Zimbabwe Craton have been processed for the purpose of regional structural mapping and thereby to develop strategic models for groundwater exploration in hard-rock areas. The lineament density is greater on TM than on AM images, partly due to the resolution of the different datasets, and also because not all TM lineaments have a magnetic signature. The derived maps reveal several previously undetected lineaments corresponding to dykes, faults, shear zones and/or tectonically-related joints, striking predominantly NNE, NNW and WNW. We suggest the possible hydrogeological significance of some of these patterns as follows: the aeromagnetic data can be used to map faults and fractures of considerable depth which are likely to be open groundwater conduits at depth (typically under tension), while TM lineaments, although not necessarily open (mostly under compression), represent recharge areas.The interpreted persistent lineation and well developed fracture patterns are correlated with existing boreholes and indicate a spatial relationship between regional structures and high borehole yields (> 3 m3/h). This relationship is combined with other lithological and hydrogeological information to identify potential regional groundwater sites for detailed ground investigations. These are defined as dyke margins, faults, fractures/joints or intersections of any combination of these structures. Priority should be given to coincident AM/TM lineaments (e.g., NNW and NNE fractures) and continuous structures with large catchment areas (e.g., NNE and WNW faults). The late Archaean (2.6 Ga) granites are considered the most favourable unit because of their associated long and deep brittle fractures between numerous bornhardts (inselbergs) and kopjes. Several small-scale TM lineaments also form important local sources of groundwater for hand-dug wells. Based on measured rock susceptibilities from the area, we present a model of the typical magnetic responses from the possible groundwater exploration targets. The developed magnetic model could be applicable to similar terrains in other Archaean Cratons.  相似文献   

2.
We discuss techniques to represent groundwater flow in carbonate aquifers using the three existing modeling approaches: equivalent porous medium, conduit network, and discrete fracture network. Fractures in faulted stratigraphic successions are characterized by dominant sets of sub-vertical joints. Grid rotation is recommended using the equivalent porous medium to match higher hydraulic conductivity with the dominant orientation of the joints. Modeling carbonate faults with throws greater than approximately 100 m is more challenging. Such faults are characterized by combined conduit-barrier behavior. The barrier behavior can be modeled using the Horizontal Flow Barrier Package with a low-permeability vertical barrier inserted to represent the impediment of horizontal flow in faults characterized by sharp drops of the piezometric surface. Cavities can occur parallel to the strike of normal faults generating channels for the groundwater. In this case, flow models need to account for turbulence using a conduit network approach. Channels need to be embedded in an equivalent porous medium due to cavities a few centimeters large, which are present in carbonate aquifers even in areas characterized by low hydraulic gradients. Discrete fracture network modeling enables representation of individual rock discontinuities in three dimensions. This approach is used in non-heavily karstified aquifers at industrial sites and was recently combined with the equivalent porous medium to simulate diffusivity in the matrix. Following this review, we recommend that the future research combines three practiced modeling approaches: equivalent porous medium, discrete fracture network, and conduit network, in order to capture structural and flow aspects in the modeling of groundwater in carbonate rocks.  相似文献   

3.
Field characterization of a trichloroethene (TCE) source area in fractured mudstones produced a detailed understanding of the geology, contaminant distribution in fractures and the rock matrix, and hydraulic and transport properties. Groundwater flow and chemical transport modeling that synthesized the field characterization information proved critical for designing bioremediation of the source area. The planned bioremediation involved injecting emulsified vegetable oil and bacteria to enhance the naturally occurring biodegradation of TCE. The flow and transport modeling showed that injection will spread amendments widely over a zone of lower‐permeability fractures, with long residence times expected because of small velocities after injection and sorption of emulsified vegetable oil onto solids. Amendments transported out of this zone will be diluted by groundwater flux from other areas, limiting bioremediation effectiveness downgradient. At nearby pumping wells, further dilution is expected to make bioremediation effects undetectable in the pumped water. The results emphasize that in fracture‐dominated flow regimes, the extent of injected amendments cannot be conceptualized using simple homogeneous models of groundwater flow commonly adopted to design injections in unconsolidated porous media (e.g., radial diverging or dipole flow regimes). Instead, it is important to synthesize site characterization information using a groundwater flow model that includes discrete features representing high‐ and low‐permeability fractures. This type of model accounts for the highly heterogeneous hydraulic conductivity and groundwater fluxes in fractured‐rock aquifers, and facilitates designing injection strategies that target specific volumes of the aquifer and maximize the distribution of amendments over these volumes.  相似文献   

4.
Both “hot-spot” type and possibly island-arc volcanoes may form at the intersections of fractures whose spacing is near the thickness of the lithosphere and increases with increasing thickness. An approximate equality between layer thickness and spacing of major fractures observed in some sedimentary rocks and clay cake models may thus extend to the “mega-joints” that have fractured the lithosphere and controlled volcano spacing on the earth, and possibly on Mars. If the hot-spot fractures are interpreted as due to shear, many hot-spot fracture systems suggest roughly north-south least principal stress, or, alternatively in some instances, a 90° rotation of this pattern.  相似文献   

5.
We combine detailed mapping and microstructural analyses of small fault zones in granodiorite with numerical mechanical models to estimate the effect of mesoscopic (outcrop-scale) damage zone fractures on the effective stiffness of the fault zone rocks. The Bear Creek fault zones were active at depths between 4 and 15 km and localize mesoscopic off-fault damage into tabular zones between two subparallel boundary faults, producing a fracture-induced material contrast across the boundary faults with softer rocks between the boundary faults and intact granodiorite outside the boundary faults. Using digitized fault zone fracture maps as the modeled fault geometries, we conduct nonlinear uniaxial compression tests using a novel finite-element method code as the experimental “laboratory” apparatus. Map measurements show that the fault zones have high nondimensional facture densities (>1), and damage zone fractures anastamose and intersect, making existing analytical effective medium models inadequate for estimation of the effective elastic properties. Numerical experiments show that the damage zone is strongly anisotropic and the bulk response of the fault zone is strain-weakening. Normal strains as small as 2% can induce a reduction of the overall stiffness of up to 75%. Fracture-induced effective stiffness changes are large enough to locally be greater than intact modulus changes across the fault due to juxtaposition of rocks of different lithologies; therefore mesoscopic fracturing is as important as rock type when considering material or bimaterial effects on earthquake mechanics. These results have important implications for earthquake rupture mechanics models, because mesoscopic damage zone fractures can cause a material contrast across the faults as large as any lithology-based material contrast at seismogenic depths, and the effective moduli can be highly variable during a single rupture event.  相似文献   

6.
The geomorphology of the central Coastal Range, a north-south trending horst along the west coast of northern Calabria, is governed largely by major faults, fault scarps and the distribution of principal rock types, as well as by a variety of slope processes operative in a Mediterranean climate. Segments of the major rivers and streams have three principal orientations parallel to major faults in the study area: northwest right-oblique slip faults (oldest); E-W oblique slip faults; NE left-oblique slip faults; and north-south right oblique normal faults (youngest), all of which cut pre-Tertiary metamorphic rocks, Mesozoic limestone, Miocene molasse and calcarenite. Small, underfit alluvial fans, composed chiefly of locally derived debris flow detritus, are present at the mouths of large, west-flowing canyons, some of which reach eastward to the crest of the mountain range. Not only do the north-south normal faults displace rocks and structures of all orientations, but they also make steep scarps in the small alluvial fans and in sediments of the coastal plain. Locally, some of the scarps are buried by recent debris flow deposits. Incipient young rivers utilized the weaknesses along the major faults and cracks as avenues of erosion. Smaller streams and gullies generally flowed westward downflank of the north-trending horst and incised, thereby, deep, V-shaped canyons; some of them have captured older, SW-flowing canyons. Locally, they were guided in other directions where they encountered faults or tectonic fractures. The rocks present a varied resistance to erosion, depending upon the degree of cementation by groundwater salts, upon the orientation of the foliation, and upon the rocks themselves. Thus, mica schist with a relatively flat foliation forms nearly vertical sea cliffs, but the sea cliffs are more gentle where the foliation is steep or dips towards the sea. Therefore, downslope movements are facilitated by seaward slip on foliation, schistosity, bedding and fault surfaces, and are evinced especially by large and deep pre-Holocene landslides (Sackung) in phyllite having areal dimensions up to 2 Km2. Other downslope processes include surficial creep and soil slip, particularly of highly fractured phyllite and schist, block sliding and rock falls.  相似文献   

7.
Wave‐induced fluid flow plays an important role in affecting the seismic dispersion and attenuation of fractured porous rocks. While numerous theoretical models have been proposed for the seismic dispersion and attenuation in fractured porous rocks, most of them neglect the wave‐induced fluid flow resulting from the background anisotropy (e.g. the interlayer fluid flow between different layers) that can be normal in real reservoirs. Here, according to the theories of poroelasticity, we present an approach to study the frequency‐dependent seismic properties of more realistic and complicated rocks, i.e. horizontally and periodically layered porous rock with horizontal and randomly orienting fractures, respectively, distributed in one of the two periodical layers. The approach accounts for the dual effects of the wave‐induced fluid flow between the fractures and the background pores and between different layers (the interlayer fluid flow). Because C33 (i.e., the modulus of the normally incident P‐wave) is directly related to the P‐wave velocity widely measured in the seismic exploration, and its comprehensive dispersion and attenuation are found to be most significant, we study mainly the effects of fracture properties and the stiffness contrast between the different layers on the seismic dispersion and attenuation of C33. The results show that the increasing stiffness contrast enhances the interlayer fluid flow of the layered porous rocks with both horizontal and randomly orienting fractures and weakens the wave‐induced fluid flow between the fractures and the background pores, especially for the layered porous rock with horizontal fractures. The modelling results also demonstrate that for the considered rock construction, the increasing fracture density reduces the interlayer fluid flow while improves the dispersion and attenuation in the fracture‐relevant frequency band. Increasing fracture aspect ratio is found to reduce the dispersion and attenuation in the fracture‐relevant frequency band only, especially for the layered porous rock with horizontal fractures.  相似文献   

8.
In granite aquifers, fractures can provide both storage volume and conduits for groundwater. Characterization of fracture hydraulic conductivity (K) in such aquifers is important for predicting flow rate and calibrating models. Nuclear magnetic resonance (NMR) well logging is a method to quickly obtain near-borehole hydraulic conductivity (i.e., KNMR) at high-vertical resolution. On the other hand, FLUTe flexible liner technology can produce a K profile at comparable resolution but requires a fluid driving force between borehole and formation. For three boreholes completed in a fractured granite, we jointly interpreted logging NMR data and FLUTe K estimates to calibrate an empirical equation for translating borehole NMR data to K estimates. For over 90% of the depth intervals investigated from these boreholes, the estimated KNMR are within one order of magnitude of KFLUTe. The empirical parameters obtained from calibrating the NMR data suggest that “intermediate diffusion” and/or “slow diffusion” during the NMR relaxation time may occur in the flowing fractures when hydraulic aperture are sufficiently large. For each borehole, “intermediate diffusion” dominates the relaxation time, therefore assuming “fast diffusion” in the interpretation of NMR data from fractured rock may lead to inaccurate KNMR estimates. We also compare calibrations using inexpensive slug tests that suggest reliable KNMR estimates for fractured rock may be achieved using limited calibration against borehole hydraulic measurements.  相似文献   

9.
Spatial heterogeneity in the subsurface of karst environments is high, as evidenced by the multiphase porosity of carbonate rocks and complex landform features that result in marked variability of hydrological processes in space and time. This includes complex exchange of various flows (e.g., fast conduit flows and slow fracture flows) in different locations. Here, we integrate various “state‐of‐the‐art” methods to understand the structure and function of this poorly constrained critical zone environment. Geophysical, hydrometric, and tracer tools are used to characterize the hydrological functions of the cockpit karst critical zone in the small catchment of Chenqi, Guizhou Province, China. Geophysical surveys, using electrical resistivity tomography (ERT), inferred the spatial heterogeneity of permeability in the epikarst and underlying aquifer. Water tables in depression wells in valley bottom areas, as well as discharge from springs on steeper hillslopes and at the catchment outlet, showed different hydrodynamic responses to storm event rainwater recharge and hillslope flows. Tracer studies using water temperatures and stable water isotopes (δD and δ18O) could be used alongside insights into aquifer permeability from ERT surveys to explain site‐ and depth‐dependent variability in the groundwater response in terms of the degree to which “new” water from storm rainfall recharges and mixes with “old” pre‐event water in karst aquifers. This integrated approach reveals spatial structure in the karst critical zone and provides a conceptual framework of hydrological functions across spatial and temporal scales.  相似文献   

10.
Permeable reactive barriers (PRBs) are a popular technology for passive contaminant remediation in aquifers through installation of reactive materials in the pathway of a plume. Of fundamental importance are the degree of remediation inside the reactor (residence time) and the portion of groundwater intercepted by a PRB (capture width). Based on a two-dimensional conformal mapping approach (previously used in related work), the latter is studied in the present work for drain-and-gate (DG) PRBs, which may possess a collector and a distributor drain (“full” configuration) or a collector drain only (“simple” configuration). Inherent assumptions are a homogeneous unbounded aquifer with a uniform far field, in which highly permeable drains establish constant head boundaries. Solutions for aquifer flow fields in terms of the complex potential are derived, illustrated, and analyzed for doubly symmetric DG configurations and arbitrary reactor hydraulic resistance as well as ambient groundwater flow direction. A series of practitioner-friendly charts for capture width is given to assist in PRB design and optimization without requiring complex mathematics. DG PRBs are identified as more susceptible to flow divergence around the reactor than configurations using impermeable side structures (e.g., funnel-and-gate), and deployment of impermeable walls on drains is seen to mitigate this problem under certain circumstances.  相似文献   

11.
The groundwater flow in a fissured chalky environment at the northern border of the Paris Basin depends on several geological and hydrogeological parameters. Although the studied sector of the basin presents a homogeneous rock type, it is affected by a fracture network. In this type of environment, in which the permeability is low, the groundwater flow displays significant disruption, which is localized in the Fruges region (northern France). The interconnection of the discontinuities (network of fault and/or joints) is reliant on the structural control of groundwater flow through increases in the hydraulic connection between the unsaturated and the saturated zone. The methodology developed herein makes use of microstructural and regional analysis of the fracture patterns, and allowed consideration of the piezometric variations of the chalk aquifer during periods of low and high groundwater levels (April and October 2001) and a diagraphic representation of the estimated physical parameters (electrical resistivity). This enabled us to construct a ‘flow structure’ conceptual model in which we identify two types of faults: tight walls and flow paths that control the piezometric heads and the flow rate. Model validation was carried out on a similar sector. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

12.
Temperatures have been measured in nine boreholes (ranging from 400 to 900 m in depth) in the Jharia Gondwana sedimentary basin of the Indian shield. About two hundred thermal conductivity determinations have been made on core samples from these holes. Temperature profiles, Bullard plots and heat flow profiles of these holes indicate different types of disturbances in the shallow geothermal regime, attributable principally to groundwater movement. Heat flow in the region of the “anticlinal high” is about 0.4 HFU higher than the heat flow in the main synclinal region of the basin. The possible sources for this variation are regional groundwater movement and upwelling of thermal waters through a deep-seated fault/fracture system. The heat flow of 1.9 HFU characterizing the main synclinal region, taken as the regional value for the basin as a whole, has been related to the heat generation of the Precambrian basement rocks. A plot of heat flow vs. heat generation falls in line with three plots for the Precambrian complexes of the Indian shield, indicating the absence of a thermal anomaly due to deeper crustal conditions underneath this basin.  相似文献   

13.
Cooling lava commonly develop polygonal joints that form equant hexagonal columns. Such fractures are formed by thermal contraction resulting in an isotropic tensional stress regime. However, certain linear cooling fracture patterns observed at some lava–ice contacts do not appear to fit the model for formation of cooling fractures and columns because of their preferred orientations. These fracture types include sheet-like (ladder-like rectangular fracture pattern), intermediate (pseudo-aligned individual column-bounding fractures), and pseudopillow (straight to arcuate fractures with perpendicular secondary fractures caused by water infiltration) fractures that form the edges of multiple columns along a single linear fracture. Despite the relatively common occurrence of these types of fractures at lava–ice contacts, their significance and mode of formation have not been fully explored. This study investigates the stress regimes responsible for producing these unique fractures and their significance for interpreting cooling histories at lava–ice contacts.Data was collected at Kokostick Butte dacite flow at South Sister, OR, and Mazama Ridge andesite flow at Mount Rainier, WA. Both of these lava flows have been interpreted as being emplaced into contact with ice and linear fracture types have been observed on their ice-contacted margins. Two different mechanisms are proposed for the formation of linear fracture networks. One possible mechanism for the formation of linear fracture patterns is marginal bulging. Melting of confining ice walls will create voids into which flowing lava can deform resulting in margin-parallel tension causing margin-perpendicular fractures. If viewed from the ice-wall, these fractures would be steeply dipping, linear fractures. Another possible mechanism for the formation of linear fracture types is gravitational settling. Pure shear during compression and settling can result in a tensional environment with similar consequences as marginal inflation. In addition to this, horizontally propagating cooling fractures will be directly influenced by viscous strain caused by the settling of the flow. This would cause preferential opening of fractures horizontally, resulting in vertically oriented fractures.It is important to note that the proposed model for the formation of linear fractures is dependent on contact with and confinement by glacial ice. The influence of flow or movement on cooling fracture patterns has not been extensively discussed in previous modeling of cooling fractures. Rapid cooling of lava by the interaction with water and ice will increase the ability to the capture and preserve perturbations in the stress regime.  相似文献   

14.
--In a sand-covered granite terrain of northwestern Rajasthan, India, a five-frequency HLEM survey along a 10-km traverse line generated several clear and strong anomalies. Subjected to a joint constrained 1-D layered earth inversion, the IP and OP frequency soundings at each point of observation yields either no solution or one which is inconsistent with the adjacent point. On the other hand, a 3-D model fitting of these anomalies with a tabular body in a layered host leads to a more meaningful interpretation, suggesting the presence of (1) pockets of weathered conductive material and (2) resistive intruding dykes, embedded in a semi-weathered layer, as the cause of the observed anomalies. The locations of weathered pockets are probably determined by pre-existing weak structures such as joints, fractures and faults, which facilitated movement of groundwater and hence weathering. Thus covered features deep in the bedrock such as intrusives, joints, fractures and faults, which are not directly detectable by the HLEM method, being under a more conductive overburden, are indirectly detected through their imprints left in the overlying weathering profile.  相似文献   

15.
The fractured granite basement is the primary oil and gas reservoir in the Cuu Long Basin, Vietnam. Due to the complexity of this nonlayered unconventional target, combined with complicated fault and fracture systems, the seismic data quality near and within the basement section is very low. For this reason, it is important to apply improved seismic data processing workflows, filtering and migration techniques, as wells as attribute processing methods to enhance the imaging quality.Our studies show that applying different types of filters, including the f-k, Radon transform and Tau-P, improves signal to noise ratio, removing multiples, revealing basement’s top and its related fractured and fault zones. In addition, the application of multi-arrival-solution migration algorithms, such as Kirchhoff Migration and Controlled Beam Migration, provides improved imaging for identifying basement top and faults and fractures within the basement. Furthermore, the application of seismic attributes such as curvature, apparent dip, or energy gradient, is important in locating faults and fractures, whereas mapping of intensity and orientation of such structures assists the delineation of “sweet spots” and assists the planning of exploration.  相似文献   

16.
Geophysical well logging has been applied for fracture characterization in crystalline terrains by physical properties measurements and borehole wall imaging. Some of these methods can be applied to monitor pumping tests to identify fractures contributing to groundwater flow and, with this, determine hydraulic conductivity and transmissivity along the well. We present a procedure to identify fractures contributing to groundwater flow using spontaneous potential measurements generated by electrokinetic processes when the borehole water head is lowered and then monitored while recovering. The electrokinetic model for flow through a tabular gap is used to interpret the measured data and determine the water head difference that drives the flow through the fracture. We present preliminary results at a test site in crystalline rocks on the campus of the University of São Paulo.  相似文献   

17.
Over 180 springs emerge in the Panamint Range near Death Valley National Park, CA, yet, these springs have received very little hydrogeological attention despite their cultural, historical, and ecological importance. Here, we address the following questions: (1) which rock units support groundwater flow to springs in the Panamint Range, (2) what are the geochemical kinetics of these aquifers, and (3) and what are the residence times of these springs? All springs are at least partly supported by recharge in and flow through dolomitic units, namely, the Noonday Dolomite, Kingston Peak Formation, and Johnnie Formation. Thus, the geochemical composition of springs can largely be explained by dedolomitization: the dissolution of dolomite and gypsum with concurrent precipitation of calcite. However, interactions with hydrothermal deposits have likely influenced the geochemical composition of Thorndike Spring, Uppermost Spring, Hanaupah Canyon springs, and Trail Canyon springs. Faults are important controls on spring emergence. Seventeen of twenty-one sampled springs emerge at faults (13 emerge at low-angle detachment faults). On the eastern side of the Panamint Range, springs emerge where low-angle faults intersect nearly vertical Late Proterozoic, Cambrian, and Ordovician sedimentary units. These geologic units are not present on the western side of the Panamint Range. Instead, springs on the west side emerge where low-angle faults intersect Cenozoic breccias and fanglomerates. Mean residence times of springs range from 33 (±30) to 1,829 (±613) years. A total of 11 springs have relatively short mean residence times less than 500 years, whereas seven springs have mean residence times greater than 1,000 years. We infer that the Panamint Range springs are extremely vulnerable to climate change due to their dependence on local recharge, disconnection from regional groundwater flow (Death Valley Regional Flow System - DVRFS), and relatively short mean residence times as compared with springs that are supported by the DVRFS (e.g., springs in Ash Meadows National Wildlife Refuge). In fact, four springs were not flowing during this campaign, yet they were flowing in the 1990s and 2000s.  相似文献   

18.
We study the mechanical deformation of fractures under normal stress, via tangent and specific fracture stiffnesses, for different length scales using numerical simulations and analytical insights. First, we revisit an equivalent elastic layer model that leads to two expressions: the tangent stiffness is the sum of an “intrinsic” stiffness and the normal stress, and the specific stiffness is the tangent stiffness divided by the fracture aperture at current stress. Second, we simulate the deformation of rough fractures using a boundary element method where fracture surfaces represented by elastic asperities on an elastic half‐space follow a self‐affine distribution. A large number of statistically identical “parent” fractures are generated, from which sub‐fractures of smaller dimensions are extracted. The self‐affine distribution implies that the stress‐free fracture aperture increases with fracture length with a power law in agreement with the chosen Hurst exponent. All simulated fractures exhibit an increase in the specific stiffness with stress and an average decrease with increase in length consistent with field observations. The simulated specific and tangent stiffnesses are well described by the equivalent layer model provided the “intrinsic” stiffness slightly decreases with fracture length following a power law. By combining numerical simulations and the analytical model, the effect of scale and stress on fracture stiffness measures can be easily separated using the concept of “intrinsic” stiffness. We learn that the primary reason for the variability in specific stiffness with length comes from the fact that the typical aperture of the self‐affine fractures itself scales with the length of the fractures.  相似文献   

19.
The role of faults in controlling groundwater flow in the Sahara and most of the hyper-arid deserts is poorly understood due to scarcity of hydrological data. The Wadi Araba Basin (WAB), in the Eastern Sahara, is highly affected by folds and faults associated with Senonian tectonics and Paleogene rifting. Using the WAB as a test site, satellite imagery, aeromagnetic maps, field observations, isotopic and geochemical data were examined to unravel the structural control on groundwater flow dynamics in the Sahara. Analysis of satellite imagery indicated that springs occur along structurally controlled scarps. Isotopic data suggested that cold springs in the WAB showed a striking similarity with the Sinai Nubian aquifer system (NAS) water and the thermal springs along the Gulf of Suez (e.g., δ18O = −8.01‰ to −5.24‰ and δD = −53.09‰ to −31.12‰) demonstrating similar recharge sources. The findings advocated that cold springs in the WAB represent a natural discharge from a previously undefined aquifer in the Eastern Desert of Egypt rather than infiltrated precipitation over the plateaus surrounding the WAB or through hydrologic windows from deep crystalline basement flow. A complex role of the geological structures was inferred including: (1) channelling of the groundwater flow along low-angle faults, (2) compartmentalization of the groundwater flow upslope from high-angle faults, and (3) reduction of the depth to the main aquifer in a breached anticline setting, which resulted in cold spring discharge temperatures (13–22°C). Our findings emphasize on the complex role of faults and folds in controlling groundwater flow, which should be taken into consideration in future examination of aquifer response to climate variability in the Sahara and similar deserts worldwide.  相似文献   

20.
Fractures are deformations in rocks with discontinuity. They are important in a number of ways. Their presence significantly influences the strength and engineering properties as well as the hydraulic characteristics of rocks. Fractures may extend to the surface where they are observed and studied at outcrops. On the other hand, they may terminate in the subsurface or may be covered by overburden which makes them impossible to be studied and characterized at the ground surface. There has been an increasing interest in the location and characterization of fractures by earth scientists, engineers and other scientists, both at the surface and the subsurface. However, the unavailability or inaccessibility of good outcrops makes it imperative to develop methods and tools for studying fractures in the subsurface. Geophysical methods such as the resistivity methods have been very useful in this regard. The Azimuthal Square Array Resistivity Survey was used in this project to locate and characterize subsurface fractures in the crystalline rocks at Igarra. Results from the analysis and interpretation of the field data showed that the dominant fracture strike orientation is in the NNW–SSE direction. This compares well with the results of surface geologic mapping data which gave the general fracture strike orientation as N–S; however, the major large and extensive fractures are striking NNW–SSE. This information is very useful in modeling groundwater flow and contaminant transport; planning proper waste management programs as well as the Environmental Impact Assessment analysis for the study area. This study once more illustrates the satisfactory use of non-invasive geophysical methods in characterizing fractures in the subsurface especially where quality outcrops are not available or inaccessible.  相似文献   

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