Inherent microcrack populations have a significant effect on the fracture behaviour of natural rocks. The present study addresses this topic in numerical simulations of uniaxial tension and three-point bending tests. For this end, a rock fracture model based on multiple intersecting embedded discontinuity finite elements is developed. The inherent (pre-existing) microcrack populations are represented by pre-embedded randomly oriented discontinuity populations. Crack shielding (through spurious locking) is prevented by allowing a new crack to be introduced, upon violation of the Rankine criterion, in an element with an initial crack unfavourably oriented to the loading direction. Rock heterogeneity is accounted for by random clusters of triangular finite elements representing different minerals of granitic numerical rock. Numerical simulations demonstrate the strength lowering effect of initial microcrack populations. This effect is substantially stronger under uniaxial tension, due to the uniform stress state, than in semicircular three-point bending having a non-uniform stress state with a clear local maximum of tensile stress.
Forest stand structure is an important concept for ecology and planning in sustainable forest management. In this article, we consider that the incorporation of complementary multispectral information from optical sensors to Light Detection and Ranging (LiDAR) may be advantageous, especially through data fusion by back-projecting the LiDAR points onto the multispectral image. A multivariate data set of both LiDAR and multispectral metrics was related with a multivariate data set of stand structural variables measured in a Scots pine forest through canonical correlation analysis (CCA). Four statistically significant pairs of canonical variables were found, which explained 83.0% accumulated variance. The first pair of canonical variables related indicators of stand development, i.e. height and volume, with LiDAR height metrics. CCA also found attributes describing stand density to be related to LiDAR and spectral variables determining canopy coverage. Other canonical variables pertained to Lorenz curve-derived attributes, which are measures of within-stand tree size variability and heterogeneity, able to discriminate even-sized from uneven-sized stands. The most relevant result was to find that metrics derived from the multispectral sensor showed significant explanatory potential for the prediction of these structural attributes. Therefore, we concluded that metrics derived from the optical sensor have potential for complementing the information from the LiDAR sensor in describing structural properties of forest stands. We recommend the use of back-projecting for jointly exploiting the synergies of both sensors using similar types of metrics as they are customary in forestry applications of LiDAR. 相似文献
An advanced along-track scanning radiometer (AATSR) global multi-year aerosol retrieval algorithm is described. Over land, the AATSR dual-view (ADV) algorithm utilizes the measured top of the atmosphere (TOA) reflectance in both the nadir and forward views to decouple the contributions of the atmosphere and the surface to retrieve aerosol properties. Over ocean, the AATSR single-view (ASV) algorithm minimizes the discrepancy between the measured and modelled TOA reflectances in one of the views to retrieve the aerosol information using an ocean reflectance model. Necessary steps to process global, multi-year aerosol information are presented. These include cloud screening, the averaging of measured reflectance, and post-processing. Limitations of the algorithms are also discussed. The main product of the aerosol retrieval is the aerosol optical depth (AOD) at visible/near-infrared wavelengths. The retrieved AOD is validated using data from the surface-based AERONET and maritime aerosol network (MAN) sun photometer networks as references. The validation shows good agreement with the reference (r?=?0.85, RMSE?=?0.09 over land; r?=?0.83, RMSE?=?0.09 at coasts and r?=?0.96, RMSE?=?0.06 over open ocean). The results of the aerosol retrievals are presented for the full AATSR mission years 2002–2012 with seasonally averaged time series for selected regions. 相似文献
The study illustrates the critical role of accurate geological structural mapping to delineate crystalline aquifer zones more prone to high health risk due to elevated dissolved As in drinking wells.The analysis revisits the results from more than 1200 groundwater samples collected over ten years from domestic wells across the Tampere region(Finland).It is demonstrated that the highest dissolved As concentrations in the region(up to 2230 μg/L)are exclusively found near major faults and deformation zones(FDZs)detected via geophysical and geological surveys,and that a clear correlation exists between dissolved concentrations and the distance from the FDZs(r).Almost all values exceeding the drinking water limit(10 μg/L)occur at r8 km,while concentrations above 100 μg/L occur at r4 km.Solidphase As concentrations in bedrock show less dependency on FDZ than aqueous concentrations.This behavior is explained considering different mechanisms,which include enhanced sulfide oxidation and fracture connectivity,promoting preferential transport of dissolved As to FDZs and mixing of waters from different redox zones,mobilizing preferentially As(Ⅲ)or As(V).Fe hydro-oxides may also precipitate/dissolve preferentially because of FDZs,while residence time may influence the contact time between water and As-bearing minerals.It is concluded that the accurate mapping of FDZs,and in general of structural geology,provides an important preliminary information to identify where localized,sitespecific characterization of hydrogeology and geochemistry is more urgent to reduce As-related health risk from groundwater intake. 相似文献
We have studied the lateral velocity variations along a partly buried inverted paleo–rift in Central Lapland, Northern Europe with a 2D wide-angle reflection and refraction experiment, HUKKA 2007. The experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy. The shots were recorded by 102 stations with an average spacing of 3.45 km. Two-dimensional crustal models of variations in P-wave velocity and Vp/Vs-ratio were calculated using the ray tracing forward modeling technique. The HUKKA 2007 experiment comprises a 455 km long profile that runs NNW–SSE parallel to the Kittilä Shear Zone, a major deformation zone hosting gold deposits in the area. The profile crosses Paleoproterozoic and reactivated Archean terranes of Central Lapland. The velocity model shows a significant difference in crustal velocity structure between the northern (distances 0–120 km) and southern parts of the profile. The difference in P-wave velocities and Vp/Vs ratio can be followed through the whole crust down to the Moho boundary indicating major tectonic boundaries. Upper crustal velocities seem to vary with the terranes/compositional differences mapped at the surface. The lower layer of the upper crust displays velocities of 6.0–6.1 km/s. Both Paleoproterozoic and Archean terranes are associated with high velocity bodies (6.30–6.35 km/s) at 100 and 200–350 km distances. The Central Lapland greenstone belt and Central Lapland Granitoid complex are associated with a 4 km-thick zone of unusually low velocities (<6.0 km/s) at distances between 120 and 220 km. We interpret the HUKKA 2007 profile to image an old, partly buried, inverted continental rift zone that has been closed and modified by younger tectonic events. It has structural features typical of rifts: inward dipping rift shoulders, undulating thickness of the middle crust, high velocity lower crust and a rather uniform crustal thickness of 48 km. 相似文献
Atmospheric measurement of radioactive xenon isotopes (radioxenon) plays a key role in remote monitoring of nuclear explosions, since it has a high capability to capture radioactive debris for a wide range of explosion scenarios. It is therefore a powerful tool in providing evidence for nuclear testing, and is one of the key components of the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The reliability of this method is largely based on a well-developed measurement technology. In the 1990s, with the prospect of the build-up of a monitoring network for the CTBT, new development of radioxenon equipment started. This article summarizes the physical and technical principles upon which the radioxenon technology is based and the advances the technology has undergone during the last 10 years. In contrast to previously used equipment, which was manually operated, the new generation of radioxenon monitoring equipment is designed for automated and continuous operation in remote field locations. Also the analytical capabilities of the equipment were strongly enhanced. Minimum detectable concentrations of the recently developed systems are well below 1 mBq/m3 for the key nuclide 133Xe for sampling periods between 8 and 24 h. All the systems described here are also able to separately measure with low detection limits the radioxenon isotopes 131mXe, 133mXe and 135Xe, which are also relevant for the detection of nuclear tests. The equipment has been extensively tested during recent years by operation in a laboratory environment and in field locations, by performing comparison measurements with laboratory type equipment and by parallel operation. These tests demonstrate that the equipment has reached a sufficiently high technical standard for deployment in the global CTBT verification regime. 相似文献
Three methods for enhanced delivery of in situ remediation amendments in low-permeability deposits have been tested at a site in Denmark: pneumatic fracturing, direct-push delivery, and hydraulic fracturing. The testing was carried out at an uncontaminated part of a farm site, previously used for storage of chlorinated solvents, underlain by basal clay till with hydraulic conductivity ranging from 7.1× 10–11 to 3.5 × 10–7 m/s at testing depths 2.5 to 9.5 m b.s. Fluorescent tracers fluorescein and rhodamine WT were delivered. Tests of all three delivery methods have not been carried out at a single site before, and thus, this study provides unique data for comparison of enhanced delivery methods in both the vadose and saturated zone. Results show that pneumatic fracturing with nitrogen gas and propagation pressures of 1 to 9 bar had a distribution radius of less than 2 m, and produced dense networks of tracer-filled natural fractures above the redox boundary (0 to 3 m b.s.) and widely spaced, discrete, induced, tracer-filled subhorizontal fractures at depth (>3 m b.s.). Direct-push delivery at pressures of 8 to 30 bar had a distribution radius of approximately 1 m, distributed tracer primarily in natural fractures above the redox boundary and in discrete, closely spaced (but not merging) induced fractures below the redox boundary. Hydraulic fracturing with a sand-guar mixture at pressures of 0 to 6 bar produced an elliptical, asymmetrical, bowl-shaped fracture with a physical radius of approximately 3.5 m at 3 m b.s. The geometry of hydraulic fractures attempted emplaced at 6.5 and 9.5 m b.s. is uncertain, but clearly not horizontal as desired. The direct-push delivery method is robust and efficient for enhanced delivery at the clay till site in question, which based on thorough geological characterization is deemed a geologically representative basal clay till site. 相似文献
Northern plants have to cope with a wide range of overwintering conditions, as the depth and physical properties of snow show high spatial variation in the Arctic. The overwintering of lingonberry ( Vaccinium vitis-idaea ) was studied in a reciprocal transplantation experiment between two sub-Arctic microhabitats in northern Finland. The experiment was set up in the autumn, and physiological traits related to overwintering were measured at the time of snowmelt in the following spring. The origin of the plants was not a significant source of variation for most of the traits measured, whereas major differences were observed between the two sites. Plants that overwintered at an exposed site above the treeline showed high relative winter damage, assessed by the electrolyte leakage of the leaves. No severe winter damage was observed in the plants that overwintered under a moderate snowpack at a sheltered birch forest site. These plants were able to maintain their photosynthetic capacity through the winter. A low capacity of photosystem II and a very low capacity of CO2 uptake were characteristic of the exposed site, where low temperatures and high irradiation predominate during late winter. However, photosynthetic capacity was recovered within a few days when the plants were kept under favourable conditions after the field experiment. The content of nonstructural carbohydrates was low, probably because of high respiratory losses under the snow. This short-term study suggests that lingonberry, which occupies a wide range of microhabitats in the present climate, may thrive even if the overwintering conditions change as a result of climatic warming. 相似文献