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Commercial digital photogrammetric software has been applied to convergent stereoscopic photography of human tooth replicas prepared to exhibit optical texture resulting in successful generation of 3D coordinate data. Tooth replicas were imaged using a semi-metric 35 mm camera and f = 100 mm macro lens on extension bellows. Model precision was within acceptable limits of 12 μ m or better for manual target matching and 21 μ m or better for automatic image matching. Further improvement in optical texture is required to achieve automatic image matching precision comparable to that of manual target matching. Small errors in interior orientation parameters attributed to instability in the bellows as well as small errors in the relative orientation resulted in some systematic errors. The use of a fixed camera lens system is expected to reduce these errors. When combined with commercially available, moderately priced, digital SLR cameras this brings 3D model generation closer to everyday clinical dental practice. 相似文献
405.
Elizabeth R. Stanway Malcolm N. Bremer Matthew D. Lehnert 《Monthly notices of the Royal Astronomical Society》2008,385(1):493-510
A large population of z > 5 Lyman-break galaxies has been identified in recent years. However, the high-redshift galaxies selected by different surveys are subject to a variety of selection effects – some overt, others more subtle. We present an analysis of sample completeness and contamination issues in high-redshift surveys, focusing on surveys at z ≈ 5 and using a spectroscopically confirmed low-redshift sample from the DEEP2 survey in order to characterize contaminant galaxies. We find that most surveys underestimate their contamination from highly clustered galaxies at z ≈ 1 and stars. We consider the consequences of this for both the rest-frame ultraviolet luminosity function and the clustering signal from z ≈ 5 galaxies. We also find that sources with moderate strength Lyman α emission lines can be omitted from dropout surveys due to their blue colours, again effecting the derived luminosity functions. We discuss the points of comparison between different samples, and the applicability of survey-specific results to the population at z > 5 in general. 相似文献
406.
Terry V. Callaghan Malcolm C. Press John A. Lee David L. Robinson Clive W. Anderson 《Polar research》1999,18(2):191-197
This paper compares the responses of two contrasting Arctic ecosystems to climate change simulations: a polar semi-desert (in Svalbard) and a dwarf shrub heath (at Abisko, northern Sweden). These ecosystems are located close to the northern-and southernmost extremes of the Arctic region, respectively. Inmacts of simulated climatic changes were determined through factorial perturbation experiments, where growing season temperature, nutrient availability and water supply were manipulated. The results are compared with the impact of interannual variation in climate on the growth of a keystone moss species, Hylocomium splendens , from the wider circumpolar area. The perturbation studies revealed that current interannual variability in temperature and the temperate tolerance of many species may exceed predicted changes in mean summer temperature over the next century. Arctic ecosystems differed in their responses to environmental manipulations, with the structure of the dwarf shrub health being affected through shifts in competitive hierarchy, potentially leading to lower biodiversity, and the polar semi-desert being affected through invasion, potentially leading to higher diversity. H. splendens showed negative responses to perturbation at the sub-Arctic site, in contrast to the positive relationship between temperature and growth observed in the natural environment. This apparent discrepancy may result from: (i) artefacts arising from the perturbations, such as lower atmospheric relative humidity; (ii) non-equilibrium responses during the relatively short-term perturbation studies and/or (iii) ecotypic variation in the moss population. Thus, caution should be employed when extrapolating from perturbations studies to both longer time-scales and different ecosystems within the Arctic. 相似文献
407.
Late Cenozoic uplift of the Amanos Mountains and incision of the Middle Ceyhan river gorge, southern Turkey; Ar–Ar dating of the Düziçi basalt 总被引:4,自引:0,他引:4
Ali Seyrek Tuncer Demir Malcolm Pringle Sema Yurtmen Rob Westaway David Bridgland Anthony Beck George Rowbotham 《Geomorphology》2008,97(3-4):321-355
Using the Ar–Ar technique, we have obtained the first numerical dates for the Pleistocene volcanism along the valley of the River Ceyhan in the Düziçi area of southern Turkey, in the western foothills of the Amanos Mountains. Our six dates indicate a single abrupt episode of volcanism at 270 ka. We have identified a staircase of 7 fluvial terraces, at altitudes of up to 230 m above the present level of the Ceyhan. Using the disposition of the basalt as an age constraint, we assign these terraces to cold-climate stages between marine oxygen isotope stages 16 and 2, indicating rates of fluvial incision, equated to surface uplift, that increase upstream through the western foothills of this mountain range at between 0.25 and 0.4 mm a− 1. Extrapolation of these uplift rates into the axis of the range suggests that the entire 2300 m of present-day relief has developed since the Mid-Pliocene, a view that we confirm using numerical modelling. Since 3.7 Ma the Amanos Mountains have formed a transpressive stepover along the northern part of the Dead Sea Fault Zone, where crustal shortening is required by the geometry. Using a physics-based technique, we have modelled the overall isostatic response to the combination of processes occurring, including crustal thickening caused by the shortening, erosion caused by orographic precipitation, and the resulting outward flow of mobile lower-crustal material, in order to predict the resulting history of surface uplift. This modelling suggests that the effective viscosity of the lower crust in this region is in the range 1–2 × 1019 Pa s, consistent with a Moho temperature of 590 ± 10 °C, the latter value being in agreement with heat flow data. This modelling shows that the nature of the active crustal deformation is now understood, to first order at least, in this key locality within the boundary zone between the African and Arabian plates, the structure and geomorphology of which have been fundamentally misunderstood in the past. 相似文献
408.
409.
Malcolm A. Grant 《Geochimica et cosmochimica acta》1982,46(12):2677-2680
Giggenbach (1981) has proposed a relation of van't Hoff form, log10PCO2 = A ? B/(t + 273), between temperature and CO2 partial pressure in deep liquid in geothermal systems. The proposed values for the coefficients are A = 15.26, B = 7850.The proposal is not supported by pressure-temperature data used by Giggenbach, and a survey of different fields indicates that the proposal is not valid. 相似文献
410.
Despite their direct links to human use, reservoirs are not widely utilised, relative to natural lakes, for deriving sediment
histories. One explanation is the complex sedimentation patterns observed in water storages. Here a highly unusual combination
of sedimentary records is used to determine the sedimentation history of Kangaroo Creek Reservoir, South Australia. We compare
contiguous high resolution (0.5 cm sampling interval) diatom records from an almost 1.3 m core extracted from the bottom of
the reservoir and from a 0.4 m monolith of sediment perched 15 m above the reservoir bottom on a disused bridge that was submerged
following initial reservoir filling in 1970. The diatom histories are supplemented by evidence provided by other indicators,
most notably radionuclide concentrations and ratios. Interestingly, despite the fact that the reservoir has been >20 m deep
for more than 70% of its recorded history, distinct sections of the reservoir bottom core, but not the bridge monolith, are
dominated by non-planktonic diatoms. We attribute the occurrences of these phases to inflows that occur following heavy catchment
rains at times when the reservoir is drawn down. These characteristic sections have, in turn, been used to refine the site’s
chronology. Despite having a length of almost 1.3 m, a variety of data suggests that the core has not recovered pre-reservoir
sediment, but rather spans the period from 1981 (11 years after first filling) to 2001, when the core was extracted. It is
clear, therefore, that sediments in the bottom of the reservoir are accumulating rapidly (>7 cm year−1), although more than 40% of this deposition occurs in less than 5% of the time. It appears that in the period 1996–2001,
quiescent sedimentation rates, both in the perched bridge locality and on the reservoir bottom, slowed in response to reduced
stream flow. Our findings indicate that, with caution, complex patterns of sedimentation in water storages can be disentangled.
However, it was difficult to precisely correlate diatom sequences from the two records even in periods of quiescent sedimentation,
suggesting that reservoir bottom diatom sequences should be interpreted with considerable caution. Furthermore, while storm-derived
inflows such as those identified may deliver a substantial proportion of sediment and phosphorus load to storages, the ensuing
deposition patterns may render much of the phosphorus unavailable to the overlying waters. 相似文献