We analyze the interannual monthly variability of oxygen isotope ratios in data from IAEA stations along the Atlantic coast of South America between 23° and 34° S to evaluate the influence of parameters such as temperature, rainfall amount and moisture source contribution on meteoric water recharging two karst systems in subtropical Brazil. In addition, a 2 year monitoring program performed on soil and cave drip and rimstone pool waters from sampling sites with contrasting discharge values and located at 100 and 300 m below the surface in the Santana Cave System (24°31′ S; 48°43′ W), is used to test the influence of hydrologic and geologic features on the temporal variations of seepage water δ18O.
Interannual monthly variations in δ18O of rainfall reflect primarily regional changes in moisture source contribution related to seasonal shifts in atmospheric circulation from a more monsoonal regime in summer (negative values of δ18O) to a more extratropical regime in winter (positive values of δ18O). Variations in groundwater δ18O indicate that the climatic signal of recent rainfall events is rapidly transmitted through the relatively deep karst aquifer to the cave drip waters, regardless of location of collection in the cave. In addition, the data also suggest that water replenishment in the system is triggered by the increase in hydraulic head during periods when recharge exceeds the storage capacity of the soil and epikarst reservoirs. Significant perturbations in the groundwater composition, characterized by more positive values of δ18O, are probably connected to an increased Atlantic moisture contribution associated with extratropical precipitation. This implies that the δ18O of speleothems from caves in this region may be a suitable proxy for studying tropical–extratropical interactions over South America, a feature that is intrinsically related to the global atmospheric circulation. 相似文献
We have identified numerous well-preserved elutriation and fluidization structures probably induced by liquefaction in Quaternary gravels and gravelly sediments of braided fluvial channel deposits in the Rio Grande do Norte and Ceará states, northeastern Brazil. They show evidence of upward-directed water escape after sediment deposition and before sediment compaction. Among the several types of structures observed, the most frequent are pillars, pockets and dikes. These structures range in width from a few centimeters to as much as 4 m, and in height from 60 cm to 4 m. Dikes, pillars and pockets are systematically associated. Clastic dikes vented large quantities of sand to the upper layers or the surface; pebbles and cobbles from the host rock sank into the dikes and formed pillars and pockets. Pockets form the root part; pillars form the intermediate part and dike, the upper part of the composite structure. The morphology of the structures in sectional and plan views indicates a 3D geometry composed of a tabular dike and pillar that present a downward continuous transition to a bowl-shaped pocket. This “stratigraphy” of liquefaction features is different from that usually presented in the current literature.
Field data suggest that both the location and the geometry of the features were controlled by sedimentary properties rather than joints and small faults. The size and abundance of these features suggest that they were formed by great events rather than localized mechanisms. Field evidence also indicates that these features are the product of fluidization and elutriation and may have been induced by liquefaction processes associated with seismic shaking. A nonseismic origin related to elutriation processes, however, cannot be ruled out for some of the features. 相似文献
Generating maps of small areas using conventional aerial photography is of great interest for small engineering firms. The main problem is the high cost of the sophisticated digital photogrammetric workstations usually employed. In this paper, a low-cost close range photogrammetric software package is used to measure the three-dimensional coordinates of points on the land surface from a photogrammetric flight at a scale of approximately 1:5000. Furthermore, the influence of the type of scanner used to digitise photographs (consumer-grade or photogrammetric scanner), the resolution of the digital images and the number of control points required are examined. The root mean square errors obtained at the check points, using a low-cost close range software package, scanning aerial images with a photogrammetric scanner and 24 ground control points, were around 116 mm for X and Y coordinates, and 191 mm for Z. These levels of accuracy allow the generation of planimetric maps at a scale of 1:1500 and topographic maps with a contour interval of around 1 m. When the images were scanned with a consumer-grade scanner, the root mean square errors were around 150 mm for X and Y, and 271 mm for Z. 相似文献
A detailed 2D model has been constructed and validated for Euroseistest valley, in northern Greece. We take advantage of this model to investigate what parameters, in addition to surface soil conditions (obviously the most important parameter), can be used to correctly characterize site response in a 2D structure. Through a parametric analysis using 2D numerical simulations for SH waves, we explore the differences between the computed ground motion for different simplifications of the valley's structure. We consider variations in the velocity structure within the sediments, and variations of the shape between sediments and bedrock. We also compare the results from different 1D models reflecting current approaches to the determination of site response. Our results show clearly that, in the case of Euroseistest, site response owes fundamentally to its closed basin shape because it is largely controlled by locally generated surface waves. Thus, in terms of predicting site response, a rough idea of its shape ratio and of the average mechanical properties of the sediments are better than a very detailed 1D profile at the central site. Although the details of ground motion may vary significantly between the models, the relative amount of surface waves generated in the 2D models seems to be relatively constant. Moreover, if we quantify the additional amplification caused by the lateral heterogeneity in terms of the ‘aggravation factor’ introduced by Chávez-García & Faccioli [7], a roughly constant factor between 2 and 3 seems to appropriately take into account the effects of lateral heterogeneity. Of course, a correct estimate of the overall impedance contrast is necessary to correctly predict the maximum amplification, a caveat that also applies to 1D models. In this sense, Euroseistest rings an alarm bell. In this valley the more significant impedance contrast lies at about 200 m depth, and it is missed both by consideration of the average shear wave velocity of the first 30 m (the Vs30 criterion) or using the detailed velocity profile down to a depth where a shear wave velocity larger than 750 m/s is found. Our conclusions indicate that, in order to improve current schemes to take into account site effects in building codes, the more to be gained comes from consideration of lateral heterogeneity, at least in the case of shallow alluvial valleys, where locally generated surface waves are likely to be important. 相似文献
Drying of masonry specimens was monitored by means of a two-dimensional (2D) magnetic resonance imaging (MRI) technique. The
external surfaces stayed wet for longer if NaCl was present instead of pure water only. This corroborates many practical observations
that salts aggravate dampness in masonry. A slower evaporation process and not hygroscopicity was the cause. That suggests
that salt-induced dampness may, in general, arise simply from changes in the drying process of masonry materials. That also
implies that the height and depth at which crystallization occurs in walls may depend on the relative equilibrium humidity
(RHeq) and other properties of salts that influence drying of porous materials. Evaporation rates of free surfaces of pure
water and saturated NaCl solution were measured by a gravimetric technique. The results indicate that slow drying of salt-contaminated
materials is not due only to the lower RHeq of salt solutions. The effective surface of evaporation is likely to be reduced
perhaps due to blocking of pores by salt crystals. Final salt-distribution maps of the specimens show that: (a) salts may
affect the inner materials of the masonry, even in evaporation-induced processes that lead crystallization to occur predominantly
on the external surface; (b) distinct internal distribution patterns occur if masonry composition varies.
Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users. 相似文献
The cutting operations of dimension stone at quarries produce a large amount of fines that, in turn, cause a negative environmental impact over local surface waters. This article presents a detailed analysis of the mobilized contamination associated with runoff waters in a large granite quarry (~200 ha) which is located in Porriño (Galicia, NW Spain), the most important production centre of dimension stone in Spain. There, an intensive monitoring survey was developed in order to characterize the hydrology of the system as well as the release and transport of pollutants. This was accomplished by means of different control sections for the flow and by the collection of 52 random water samples. Automatic samplers were also used to collect 192 water samples associated with runoff generated by significant rain events. For each of the samples collected, a wide battery of analytic determinations was performed, including solid loads, turbidity, organic pollution, nutrient and selected metals. Upon careful examination it has been possible to obtain different parameters related to the contamination control, like the “event mean concentration” (EMC), maximum concentrations and specific sediment loads. Moreover, a comprehensive statistical study including parameter correlation and cumulative probability analyses helped to understand the pollutant mobilization behaviour within the quarry system. It can be concluded that mobilized contamination shows the anomalous presence of some metals, whose concentrations are in excess when compared to the natural rock. This excess metal amount must be attributed to an anthropogenic source associated to mining and cutting operations. 相似文献
The Mw 7.4 earthquake of 21 January 2003 occurred within the Mexican subduction zone and produced many damages of masonry
constructions in the towns of Colima state, México. The macroseismic investigation of damages produced by the earthquake in
Colima city was realized for 3,332 constructions within the area of study representing about 20% of the total city area and
covered with the different type of constructions. The 7-grade scale of damage was used to describe the damage distribution.
The damage matrix, constructed for the area, showed that the damage distribution varied from 63% of constructions with relatively
slight damages (grades 1–3) to 29% of constructions that had significative damages (grades 4–5) and 8% of completely destructed
or demolished masonry. The damage matrices, constructed for 12 subzones of the area of study, reflected two tendencies in
the damage distributions: the predominance of slight damages of the recent constructions situated in the northern and eastern
parts of the area and the predominance of significant damages of the older constructions in the southern and western parts
of the area. It was observed a significant dependence of damage index upon the age of constructions and the type of masonry.
The comparison of the observed damage matrix with the damage probability matrix calculated for Colima masonry in 1999 gives
MM intensity VII in Colima. 相似文献
Natural Resources Research - Adequate management of water resources in aquifer systems implies knowledge of the different parameters of hydrological balance. However, only limited data exist for... 相似文献