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1.
This study presents an approach for delineating groundwater basins and estimating rates of recharge to fractured crystalline bedrock. It entailed the use of completion report data (boring logs) from 2500 domestic wells in bedrock from the Coventry Quadrangle, which is located in northeastern Connecticut and characterized by metamorphic gneiss and schist. Completion report data were digitized and imported into ArcGIS® for data analysis. The data were processed to delineate groundwater drainage basins for the fractured rock based on flow conditions and to estimate groundwater recharge to the bedrock. Results indicate that drainage basins derived from surface topography, in general, may not correspond with bedrock drainage basins due to scale. Estimates of recharge to the bedrock for the study area indicate that only a small fraction of the precipitation or the amount of water that enters the overburden recharges the rock. The approach presented here can be a useful method for water resource‐related assessments that involve fractured rock aquifers.  相似文献   

2.
Contaminated groundwater in fractured bedrock can expose ecosystems to undesired levels of risk for extended periods due to prolonged back-diffusion from rock matrix to permeable fractures. Therefore, it is key to characterize the diffusive mass loading (intrusion) of contaminants into the rock matrix for successful management of contaminated bedrock sites. Even the most detailed site characterization techniques often fail to delineate contamination in rock matrix. This study presents a set of analytical solutions to estimate diffusive mass intrusion into matrix blocks, it is recovered by pumping and concentration rebound when pumping ceases. The analytical models were validated by comparing the results with (1) numerical model results using the same model parameters and (2) observed chloride mass recovery, rebound concentration, and concentration in pumped groundwater at a highly fractured bedrock site in Alberta, Canada. It is also demonstrated that the analytical solutions can be used to estimate the total mass stored in the fractured bedrock prior to any remediation thereby providing insights into site contamination history. The predictive results of the analytical models clearly show that successful remediation by pumping depends largely on diffusive intrusion period. The results of initial mass from the analytical model was used to successfully calibrate a three-dimensional discrete fracture network numerical model further highlighting the utility of the simple analytical solutions in supplementing the more detailed site numerical modeling. Overall, the study shows the utility of simple analytical methods to support long-term management of a contaminated fractured bedrock site including site investigations and complex numerical modeling.  相似文献   

3.
Application scope of geostatistics has been gradually extended from original geologic field to soil science and ecological field, etc. and its successful application results have been widely demonstrated. But little information is reported as to the direct use of geostatistical method to work out the distribu- tion map of groundwater characteristics. In this paper the semivariogram of geostatistics, in combina- tion with GIS, was used to quantitatively study the spatial variation characteristics of groundwater table depth and mineralization degree and their relation to the landuse changes. F test of the used spherical model reached a very significant level, and the theoretical model can well reflect the spatial structural characteristics of groundwater table depth and mineralization degree and achieve an ideal result. This shows that the application of the method in the dynamical simulation of groundwater is feasible. And this paper also provides useful reference for the application of geostatistics in the study of the dy- namical variations of groundwater resources in the oasis.  相似文献   

4.
Fracture Density Distributions and Well Yields in Coastal Maine   总被引：1，自引：0，他引：1  
Understanding the distribution of water-bearing fractures in crystalline rock is an important component in evaluating the availability and vulnerability of water resources throughout the northeastern U.S. The State of Maine requests well drillers to report estimates of fracture depths and fracture yields for all bedrock wells drilled in the state. Using these data we analyze fracture-depth and fracture-yield data from 227 bedrock wells in coastal Maine in order to understand how fracture locations and yields are distributed with depth. Numerical simulations and statistical tests show that it is not possible to infer how fractures are distributed with depth: fracture depths are consistent with several distributions, including uniform fracture density with depth. In order to understand how fracture yield varies with depth, we group yield data into 50 foot depth intervals and compare distributions in each interval using nonparametric statistical tests. These tests show that the distribution of fracture yield in different depth intervals are statistically equivalent. These results imply that there is no empirical justification for limiting well depth when drilling for water resources in fractured bedrock in coastal Maine.  相似文献   

5.
The depth to soil–bedrock interface, which is one of the major parameters in the site response analysis, has been often investigated by surface-wave tests. The round-robin tests for a surface-wave method in Korea revealed that a long measurement array in surface-wave tests is not appropriate in locating soil–bedrock interface. In this paper, for the improved profiling of depth to soil–bedrock interface in 2-D image, short measurement array was introduced for the beamforming technique, which is a robust array processing technique adopted in a long-array format for stiffness profiling. Numerical simulation and field applications of the short-array beamforming technique indicate that the method is valid even for surface wave propagation with mode-related complexity. Depth to soil–bedrock interface and shear-wave velocity profiles determined by the short-array beamforming technique were in good agreement with layer stratifications of boring logs, resistivity map, shear-wave velocity profiles of downhole tests and CAP-SASW tests.  相似文献   

6.
Shear wave velocity–depth information is required for predicting the ground motion response to earthquakes in areas where significant soil cover exists over firm bedrock. Rather than estimating this critical parameter, it can be reliably measured using a suite of surface (non-invasive) and downhole (invasive) seismic methods. Shear wave velocities from surface measurements can be obtained using SH refraction techniques. Array lengths as large as 1000 m and depth of penetration to 250 m have been achieved in some areas. High resolution shear wave reflection techniques utilizing the common midpoint method can delineate the overburden-bedrock surface as well as reflecting boundaries within the overburden. Reflection data can also be used to obtain direct estimates of fundamental site periods from shear wave reflections without the requirement of measuring average shear wave velocity and total thickness of unconsolidated overburden above the bedrock surface. Accurate measurements of vertical shear wave velocities can be obtained using a seismic cone penetrometer in soft sediments, or with a well-locked geophone array in a borehole. Examples from thick soil sites in Canada demonstrate the type of shear wave velocity information that can be obtained with these geophysical techniques, and show how these data can be used to provide a first look at predicted ground motion response for thick soil sites.  相似文献   

7.
The hydrogeologic framework of fractured sedimentary bedrock at the former Naval Air Warfare Center (NAWC), 1 1 For more information on the USGS Toxics Substances Hydrology Program at the Naval Air Warfare Center visit the NAWC website at http://nj.usgs.gov/nawc/
Trenton, New Jersey, a trichloroethylene (TCE)-contaminated site in the Newark Basin, is developed using an understanding of the geologic history of the strata, gamma-ray logs, and rock cores. NAWC is the newest field research site established as part of the U.S. Geological Survey Toxic Substances Hydrology Program, Department of Defense (DoD) Strategic Environmental Research and Development Program, and DoD Environmental Security Technology Certification Program to investigate contaminant remediation in fractured rock. Sedimentary bedrock at the NAWC research site comprises the Skunk Hollow, Byram, and Ewing Creek Members of the Lockatong Formation and Raven Rock Member of the Stockton Formation. Muds of the Lockatong Formation that were deposited in Van Houten cycles during the Triassic have lithified to form the bedrock that is typical of much of the Newark Basin. Four lithotypes formed from the sediments include black, carbon-rich laminated mudstone, dark-gray laminated mudstone, light-gray massive mudstone, and red massive mudstone. Diagenesis, tectonic compression, off-loading, and weathering have altered the rocks to give some strata greater hydraulic conductivity than other strata. Each stratum in the Lockatong Formation is 0.3 to 8 m thick, strikes N65°E, and dips 25° to 70°NW. The black, carbon-rich laminated mudstone tends to fracture easily, has a relatively high hydraulic conductivity and is associated with high natural gamma-ray count rates. The dark-gray laminated mudstone is less fractured and has a lower hydraulic conductivity than the black carbon-rich laminated mudstone. The light-gray and the red massive mudstones are highly indurated and tend to have the least fractures and a low hydraulic conductivity. The differences in gamma-ray count rates for different mudstones allow gamma-ray logs to be used to correlate and delineate the lithostratigraphy from multiple wells. Gamma-ray logs and rock cores were correlated to develop a 13-layer gamma-ray stratigraphy and 41-layer lithostratigraphy throughout the fractured sedimentary rock research site. Detailed hydrogeologic framework shows that black carbon-rich laminated mudstones are the most hydraulically conductive. Water-quality and aquifer-test data indicate that groundwater flow is greatest and TCE contamination is highest in the black, carbon- and clay-rich laminated mudstones. Large-scale groundwater flow at the NAWC research site can be modeled as highly anisotropic with the highest component of permeability occurring along bedding planes.  相似文献   

8.
Wetlands are vulnerable to groundwater extraction, which has proven detrimental to aquatic ecosystems around the planet. As wetlands rank among the world’s most endangered ecosystems, versatile strategies are required to protect them. This paper provides a modelling-based method to delineate protection buffers in wetlands subject to groundwater extraction. The technique is sufficiently flexible to cater to a wide variety of conditions, and simple enough to underpin management decisions on a daily basis. A numerical model is used to obtain a map of the critical rate of groundwater abstraction, based on the distance between wetlands and suitable discharge thresholds. The outcomes determine the allowed pumping rate at any point under steady and transient-state conditions. A new iteration is developed every time a new pumping allowance is made. This procedure is demonstrated by means of hypothetical scenarios, as well as by a case study application in the Valle del Cauca region, Colombia.  相似文献   

9.
Over the past few decades, groundwater has become an essential commodity owing to increased demand as a result of growing population, industrialization, urbanization and so on. The water supply situation is expected to become more severe in the future because of continued unsustainable water use and projected change in hydrometeorological parameters due to climate change. This study is based on the integrated approach of remote sensing, geographical information system and multicriteria decision‐making techniques to determine the most important contributing factors that affect the groundwater resources and to delineate the groundwater potential zones. Ten thematic layers, namely, geomorphology, geology, soil, topographic elevation (digital elevation model), land use/land cover, drainage density, lineament density, proximity of surface water bodies, surface temperature and post‐monsoon groundwater depth, were considered for the present study. These thematic layers were selected for groundwater prospecting based on the literature; discussion with the experts of the Central Ground Water Board, Government of India; field observations; geophysical investigation; and multivariate techniques. The thematic layers and their features were assigned suitable weights on Saaty's scale according to their relative significance for groundwater occurrence. The assigned weights of the layers and their features were normalized by using the analytic hierarchy process and eigenvector method. Finally, the selected thematic maps were integrated using a weighted linear combination method to create the final groundwater potential zone map. The final output map shows different zones of groundwater potential, namely, very good (16%), good (35%), moderate (28%) low (17%) and very low (2.1%). The groundwater potential zone map was finally validated using the discharge and groundwater depth data from 28 and 98 pumping wells, respectively, which showed good correlation. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

10.
Tillage on hillslopes may not only induce severe soil erosion, but may also cause bedrock erosion under certain conditions. Yet, little is known about bedrock erosion by tillage in a hilly agricultural landscape, southwest China. The aim of this study is to quantify the translocation of rock fragments derived from bedrock fragmentation by hoeing under different conditions, including slope gradient, hoeing depth and soil-covered thickness using a gravel tracing method. The reliability of the gravel tracing method was confirmed by the bedrock dyeing tracing method. Hoeing depth is a significant factor affecting the translocation rate of rock fragments (Qr ). Meanwhile, under the condition of overlying soil layers (0.06−0.10 m thick), the values of Qr were significantly smaller with a reduction of 20.7−25.6%, compared with rock fragmentation by hoeing for bare bedrock. However, slope gradient was found to have insignificant effects on Qr . Fractured bedrock moved as individual small fragments, which was mainly controlled by the hitting force of the hoe, while soil moved in the shape of lumps, which was dominated by both drag force of the hoe and gravity. This study suggests that hoeing into soil-covered bedrock can diminish bedrock erosion while providing soil matrix for shallow soil layers. Our work presents a quantitative assessment of bedrock erosion by hoeing and an underlying insight into characteristics of bedrock erosion by tillage operations in hilly agricultural regions with mudstone and shale, southwest China. © 2020 John Wiley & Sons, Ltd.  相似文献   

11.
Acceleration time histories of earthquake events are typically measured in seismic stations that are placed close to the soil top surface. These acceleration records are often used as input data for seismic analysis. It may be used for base excitation in seismic analysis of above ground structures with shallow foundations.. However it may not be used for seismic analysis of underground structures, or even for above ground buildings with deep foundations and several underground stories. The required base excitation data of the latter should have been measured below the top surface, at a level that may be determined according to the specific analyzed building geometry or at the bedrock below. If the acceleration time history at the bedrock would have been known, the seismic wave propagation through the soil medium, from the bedrock towards the top surface, could have been carried out and the base excitation of the buried structure could be determined. Since there is no data on the acceleration time history at the bedrock, and the only given data is the acceleration records at the top surface, the goal of this paper is to provide an exact reverse analysis procedure to determine the unknown acceleration time history at the bedrock that would exactly produce the measured acceleration time history at the top surface. Once this goal is achieved, seismic analysis of buried structures may be carried out with the determined acceleration record at the bedrock as input. This paper presents an analytical exact solution of the inverse problem for determination of the acceleration, velocity and displacement time histories at the bedrock base of a layered geological medium that are compatible with the given acceleration record at the soil top surface. This new proposed method is based on analytical solutions of the initial-boundary value problems of the linear wave equation in the case of a layered medium. The relationship between waves in one layer and waves in another adjacent layer is derived considering the continuity of stresses and displacements at the common interface between the layers. The efficiency and accuracy of the proposed method is demonstrated through several examples involving the nonstationary response of the free surface. The case of the San Fernando Earthquake is studied. Excellent agreement is achieved between the recorded free surface time history and the reconstructed signal. This excellent agreement is obtained due to the exact analytical method used in deriving the inverse problem solution. This exact analytical method allows one to obtain an acceleration (velocity/displacement) distribution along all the layers at any time.  相似文献   

12.
Water levels and water quality of open borehole wells in fractured bedrock are flow-weighted averages that are a function of the hydraulic heads and transmissivities of water contributing fractures, properties that are rarely known. Without such knowledge using water levels and water quality data from fractured bedrock wells to assess groundwater flow and contaminant conditions can be highly misleading. This study demonstrates a cost-effective single packer method to determine the hydraulic heads and transmissivities of water contributing fracture zones in crystalline bedrock wells. The method entails inflating a pipe plug to isolate sections of an open borehole at different depths and monitoring changes in the water level with time. At each depth, the change in water level with time was used to determine the sum of fracture transmissivities above the packer and then to solve for individual fracture transmissivity. Steady-state wellbore heads along with the transmissivities were used to determine individual fracture heads using the weighted average head equation. The method was tested in five wells in crystalline bedrock located at the University of Connecticut in Storrs. The single packer head and transmissivity results were found to agree closely with those determined using conventional logging methods and the dissolved oxygen alteration method. The method appears to be a simple and cost-effective alternative in obtaining important information on flow conditions in fractured crystalline bedrock wells.  相似文献   

13.
The quantitative evaluation of the effects of bedrock groundwater discharge on spatial variability of stream dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) concentrations has still been insufficient. We examined the relationships between stream DOC, DIN and DIP concentrations and bedrock groundwater contribution to stream water in forest headwater catchments in warm-humid climate zones. We sampled stream water and bedrock springs at multiple points in September and December 2013 in a 5 km2 forest headwater catchment in Japan and sampled groundwater in soil layer in small hillslopes. We assumed that stream water consisted of four end members, groundwater in soil layer and three types of bedrock groundwater, and calculated the contributions of each end member to stream water from mineral-derived solute concentrations. DOC, DIN and DIP concentrations in stream water were compared with the calculated bedrock groundwater contribution. The bedrock groundwater contribution had significant negative linear correlation with stream DOC concentration, no significant correlation with stream DIN concentration, and significant positive linear correlation with stream DIP concentration. These results highlighted the importance of bedrock groundwater discharge in establishing stream DOC and DIP concentrations. In addition, stream DOC and DIP concentrations were higher and lower, respectively, than those expected from end member mixing of groundwater in soil layer and bedrock springs. Spatial heterogeneity of DOC and DIP concentrations in groundwater and/or in-stream DOC production and DIP uptake were the probable reasons for these discrepancies. Our results indicate that the relationships between spatial variability of stream DOC, DIN and DIP concentrations and bedrock groundwater contribution are useful for comparing the processes that affect stream DOC, DIN and DIP concentrations among catchments beyond the spatial heterogeneity of hydrological and biogeochemical processes within a catchment.  相似文献   

14.
Here we use Richards Equation models of variably saturated soil and bedrock groundwater flow to investigate first-order patterns of the coupling between soil and bedrock flow systems. We utilize a Monte Carlo sensitivity analysis to identify important hillslope parameters controlling bedrock recharge and then model the transient response of bedrock and soil flow to seasonal precipitation. Our results suggest that hillslopes can be divided into three conceptual zones of groundwater interaction, (a) the zone of lateral unsaturated soil moisture accumulation (upper portion of hillslope), (b) the zone of soil saturation and bedrock recharge (middle of hillslope) and (c) the zone of saturated-soil lateral flow and bedrock groundwater exfiltration (bottom of hillslope). Zones of groundwater interaction expand upslope during periods of precipitation and drain downslope during dry periods. The amount of water partitioned to the bedrock groundwater system a can be predicted by the ratio of bedrock to soil saturated hydraulic conductivity across a variety of hillslope configurations. Our modelled processes are qualitatively consistent with observations of shallow subsurface saturation and groundwater fluctuation on hillslopes studied in our two experimental watersheds and support a conceptual model of tightly coupled shallow and deep subsurface circulation where groundwater recharge and discharge continuously stores and releases water from longer residence time storage.  相似文献   

15.
Remote sensing and geoelectrical methods were used to find water-bearing fractures in the Scituate granite under the Central Landfill of Rhode Island. These studies were necessary to evaluate the integrity of the sanitary landfill and for planning safe landfill extensions. The most useful results were obtained with fracture trace analysis using Landsat and SLAR imagery in combination with ground-based resistivity measurements using Schlumberger vertical electrical soundings based on the assumption of horizontally layered strata. Test borings and packer tests confirmed, in the presence of a lineament and low bedrock resistivity, the probable existence of high bedrock fracture density and high average hydraulic conductivity. However, not every lineament was found to be associated with high fracture density and high hydraulic conductivity. Lineaments alone are not a reliable basis for characterising a landfill site as being affected by fractured bedrock. Horizontal fractures were found in borings located away from lineaments. High values of hydraulic conductivity were correlated with low bedrock resistivities. Bedrock resistivities between 60 and 700 Ω m were associated with average hydraulic conductivities between 4 and 60 cm/day. In some cases very low resistivities were confined to the upper part of the bedrock where the hydraulic conductivity was very large. These types of fractures apparently become narrower in aperture with depth. Bedrock zones having resistivities greater than 1000 Ω m showed, without exception, no flow to the test wells. Plots of bedrock resistivity versus the average hydraulic conductivity indicate that the resistivity decreases with increasing hydraulic conductivity. This relationship is inverse to that found in most unconsolidated sediments and is useful for estimating the hydraulic conductivity in groundwater surveys in fractured bedrock. In appropriate settings such as the Central Landfill site in New England, this electric-hydraulic correlation relationship, supplemented by lineament trace analysis, can be used effectively to estimate the hydraulic conductivity in bedrock from only a limited number of resistivity depth soundings and test wells.  相似文献   

16.
The advance of a chemical weathering front into the bedrock of a hillslope is often limited by the rate weathering products that can be carried away, maintaining chemical disequilibrium. If the weathering front is within the saturated zone, groundwater flow downslope may affect the rate of transport and weathering—however, weathering also modifies the rock permeability and the subsurface potential gradient that drives lateral groundwater flow. This feedback may help explain why there tends to be neither “runaway weathering” to great depth nor exposed bedrock covering much of the earth and may provide a mechanism for weathering front advance to keep pace with incision of adjacent streams into bedrock. This is the second of a two‐part paper exploring the coevolution of bedrock weathering and lateral flow in hillslopes using a simple low‐dimensional model based on hydraulic groundwater theory. Here, we show how a simplified kinetic model of 1‐D rock weathering can be extended to consider lateral flow in a 2‐D hillslope. Exact and approximate analytical solutions for the location and thickness of weathering within the hillslope are obtained for a number of cases. A location for the weathering front can be found such that lateral flow is able to export weathering products at the rate required to keep pace with stream incision at steady state. Three pathways of solute export are identified: “diffusing up,” where solutes diffuse up and away from the weathering front into the laterally flowing aquifer; “draining down,” where solutes are advected primarily downward into the unweathered bedrock; and “draining along,” where solutes travel laterally within the weathering zone. For each pathway, a different subsurface topography and overall relief of unweathered bedrock within the hillslope is needed to remove solutes at steady state. The relief each pathway requires depends on the rate of stream incision raised to a different power, such that at a given incision rate, one pathway requires minimal relief and, therefore, likely determines the steady‐state hillslope profile.  相似文献   

17.
以人工爆炸波作为震源,通过现场试验获得基岩和土层场地爆炸波地震动时程,分析场地覆盖层厚度对基岩地震动峰值加速度和地震动持时的放大效应。试验结果表明:土层对基岩地震动有放大作用,基岩峰值加速度放大系数受覆盖层厚度和土层结构的共同作用影响;地震动持续时间随覆盖层厚度的增加而显著增加,受覆盖层土层结构影响不显著。  相似文献   

18.
The role of bedrock groundwater in rainfall–runoff processes is poorly understood. Hydrometric, tracer and subsurface water potential observations were conducted to study the role of bedrock groundwater and subsurface flow in the rainfall–runoff process in a small headwater catchment in Shiranui, Kumamoto prefecture, south‐west Japan. The catchment bedrock consists of a strongly weathered, fractured andesite layer and a relatively fresh continuous layer. Major chemical constituents and stable isotopic ratios of δ18O and δD were analysed for spring water, rainwater, soil water and bedrock groundwater. Temporal and spatial variation in SiO2 showed that stream flow under the base flow condition was maintained by bedrock groundwater. Time series of three components of the rainstorm hydrograph (rainwater, soil water and bedrock groundwater) separated by end member mixing analysis showed that each component fluctuated during rainstorm, and their patterns and magnitudes differed between events. During a typical mid‐magnitude storm event, a delayed secondary runoff peak with 1·0 l s−1 was caused by increase in the bedrock groundwater component, whereas during a large rainstorm event the bedrock groundwater component increased to ≈ 2·5 l s−1. This research shows that the contribution of bedrock groundwater and soil water depends strongly on the location of the groundwater table, i.e. whether or not it rises above the soil–bedrock interface. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

19.
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   

20.
Ground penetrating radar (GPR) has been used as a tool to access information about ground subsurface features. Such information is very important for different types of studies, varying from those related to archeological research to those studying geological elements of bedrock. More recently, however, GPR has been increasingly applied to environmental studies, especially for soil research. This paper presents the results of an application of GPR for the study of weathered profiles. GPR was used to discover the degree of trustworthiness of the information on the ground subsurface through the interpretation of the results of the radar sections as well as the data collected from boreholes, which reached until 21 m. The results show a relatively high degree of details obtained by GPR, indicating the possibility of speeding up ground subsurface surveys related to geomorphological, geological, and pedological studies.  相似文献   

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