2D numerical modelling of impact cratering has been utilized to quantify an important depth-diameter relationship for different crater morphologies, simple and complex. It is generally accepted that the final crater shape is the result of a gravity-driven collapse of the transient crater, which is formed immediately after the impact. Numerical models allow a quantification of the formation of simple craters, which are bowl-shaped depressions with a lens of rock debris inside, and complex craters, which are characterized by a structural uplift. The computation of the cratering process starts with the first contact of the impactor and the planetary surface and ends with the morphology of the final crater. Using different rheological models for the sub-crater rocks, we quantify the influence on crater mechanics. To explain the formation of complex craters in accordance to the threshold diameter between simple and complex craters, we utilize the Acoustic Fluidization model. We carried out a series of simulations over a broad parameter range with the goal to fit the observed depth/diameter relationships as well as the observed threshold diameters on the Moon, Earth and Venus. 相似文献
Many cities around the world are developed at alluvial fans. With economic and industrial development and increase in population,
quality and quantity of groundwater are often damaged by over-exploitation in these areas. In order to realistically assess
these groundwater resources and their sustainability, it is vital to understand the recharge sources and hydrogeochemical
evolution of groundwater in alluvial fans. In March 2006, groundwater and surface water were sampled for major element analysis
and stable isotope (oxygen-18 and deuterium) compositions in Xinxiang, which is located at a complex alluvial fan system composed
of a mountainous area, Taihang Mt. alluvial fan and Yellow River alluvial fan. In the Taihang mountainous area, the groundwater
was recharged by precipitation and was characterized by Ca–HCO3 type water with depleted δ18O and δD (mean value of −8.8‰ δ18O). Along the flow path from the mountainous area to Taihang Mt. alluvial fan, the groundwater became geochemically complex
(Ca–Na–Mg–HCO3–Cl–SO4 type), and heavier δ18O and δD were observed (around −8‰ δ18O). Before the surface water with mean δ18O of −8.7‰ recharged to groundwater, it underwent isotopic enrichment in Taihang Mt. alluvial fan. Chemical mixture and ion
exchange are expected to be responsible for the chemical evolution of groundwater in Yellow River alluvial fan. Transferred
water from the Yellow River is the main source of the groundwater in the Yellow River alluvial fan in the south of the study
area, and stable isotopic compositions of the groundwater (mean value of −8.8‰ δ18O) were similar to those of transferred water (−8.9‰), increasing from the southern boundary of the study area to the distal
end of the fan. The groundwater underwent chemical evolution from Ca–HCO3, Na–HCO3, to Na–SO4. A conceptual model, integrating stiff diagrams, is used to describe the spatial variation of recharge sources, chemical
evolution, and groundwater flow paths in the complex alluvial fan aquifer system. 相似文献
Recently, multi-layered targets have become commonplace in both military and civilian applications, such as marine hulls, armored vehicle bodies, outside structures of bulletproof cars, and aerospace vessels. This paper studies the resistant performance of perforation in multi-layered targets. An estimation procedure is established based on the concepts of the conservation of momentum, impulse-momentum law, and conservation of energy. Experimental results of the test of Almohandes et al. are adopted to check the residual velocity of multi-layered targets. The results of the verification are good in terms of agreement for impact velocities ranging from 700 to 800 m/s, when the ratio of the projectile length to the projectile diameter (i.e. L/D) is 4.2, and the average residual velocity error of single, double- or triple-layered targets range approximately from 4.42 to 8.40%. The ballistic performance is best for the double target when the ratio of the first layer thickness to the total thickness (i.e. t1/(t1+t2)) is about 0.75, and the worse performance occurs when the ratio t1/(t1+t2) is 0.5. An air gap slightly influences the resistant performance of perforation in multi-layered targets. These results may serve as a useful reference for designers. 相似文献
Only since several decades has impact cratering been recognized as an important surface process on all planetary bodies in the Solar System. However, as the process has not yet been effectively introduced into geological curricula, it is necessary to inform a wider public about its importance for (i) planetary formation and (ii) evolution, (iii) the understanding of this process as a geological process, (iv) the terrestrial impact crater record and its limitations, and (v) the recognition criteria for terrestrial impact structures, as well as (vi) the need of improvement of the impact cratering record in the light of the potential danger of an impact catastrophe on this planet. It is, particularly for developing countries, of interest to examine the economic and educational-environmental potential of impact structures. That it is possible to carry out an effective, low-budget geological investigation of impact structures within a Second World environment is demonstrated by the discussion of the progress that has been made in recent years with regard to the Southern African impact crater record. Several recommendations on how to improve, on the one hand, the terrestrial impact crater record and, on the other, their general working situation by activation of workers in Developing Countries are discussed. 相似文献
Precambrian fluvial systems, lacking the influence of rooted vegetation, probably were characterised by flashy surface runoff, low bank stability, broad channels with abundant bedload, and faster rates of channel migration; consequently, a braided fluvial style is generally accepted. Pre-vegetational braided river systems, active under highly variable palaeoclimatic conditions, may have been more widespread than are modern, ephemeral dry-land braided systems. Aeolian deflation of fine fluvial detritus does not appear to have been prevalent. With the onset of large cratons by the Neoarchaean–Palaeoproterozoic, very large, perennial braided river systems became typical. The c. 2.06–1.88 Ga Waterberg Group, preserved within a Main and a smaller Middelburg basin on the Kaapvaal craton, was deposited largely by alluvial/braided-fluvial and subordinate palaeo-desert environments, within fault-bounded, possibly pull-apart type depositories.
Palaeohydrological data obtained from earlier work in the Middelburg basin (Wilgerivier Formation) are compared to such data derived from the correlated Blouberg Formation, situated along the NE margin of the Main basin. Within the preserved Blouberg depository, palaeohydrological parameters estimated from clast size and cross-bed set thickness data, exhibit rational changes in their values, either in a down-palaeocurrent direction, or from inferred basin margin to palaeo-basin centre. In both the Wilgerivier and Blouberg Formations, calculated palaeoslope values (derived from two separate formulae) plot within the gap separating typical alluvial fan gradients from those which characterise rivers (cf. [Blair, T.C., McPherson, J.G., 1994. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages. J. Sediment. Res. A64, 450–489.]). Although it may be argued that such data support possibly unique fluvial styles within the Precambrian, perhaps related to a combination of major global-scale tectono-thermal and atmospheric–palaeoclimatic events, a simpler explanation of these apparently enigmatic palaeoslope values may be pertinent. Of the two possible palaeohydrological formulae for calculating palaeoslope, one provides results close to typical fluvial gradients; the other formula relies on preserved channel-width data. We suggest that the latter will not be reliable due to problematic preservation of original channel-widths within an active braided fluvial system. We thus find no unequivocal support for a unique fluvial style for the Precambrian, beyond that generally accepted for that period and discussed briefly in the first paragraph. 相似文献