The Australia telescope 20 GHz survey: hardware, observing strategy, and scanning survey catalog

The Australia Telescope 20 GHz (AT20G) survey is a large area (2?? sr), sensitive (40 mJy), high frequency (20 GHz) survey of the southern sky. The survey was conducted in two parts: an initial fast scanning survey, and a series of more accurate follow-up observations. The follow-up survey catalog has been presented by Murphy et al. (MNRAS 402:2403, 2010). In this paper we discuss the hardware setup and scanning survey strategy as well as the production of the scanning survey catalog.     

Secondary Fe and Al in Antarctic paleosols: Correlation to Mars with prospect for the presence of life

Middle-Miocene age paleosols in the Antarctic Dry Valleys were studied for their compositional variation and concentrations of secondary oxides/coatings in <2 mm matrix material in tills. The paleosols date to between 10-18 Ma by ¹⁰Be, forming prior to and just following the transition from warm-based to cold-based ice, when the climate is thought to have entered a prolonged cold/dry period in which soil moisture would have been frozen most of the year. The progression of release of secondary oxides of Fe and Al shows variable percentages of individual oxihydrites relative to chemical element totals, and thus, to consume total Fe and Al would require tens of millions of additional years. The slow progression of Fe_d/Fe_t, used as a measure of relative age in other warmer alpine and Arctic climates, is shown here to amount to <15 percent of the total Fe available for weathering. Ferrihydrite variability in paleosols, often used to indicate the presence of fluctuating/perched ground water tables, suggests that liquid water may have accumulated in mid-profile in some instances, perhaps during periods when the climate was somewhat warmer for several weeks during the summer. Variable Fe and Al ratios and arithmetic functions argue for extremely slow but sustainable release of oxides in a cold, polar desert climate, in which temperatures prohibit the formation of clay minerals. The secondary mineral species present likely amount to residue from past climates that were more conducive to clay mineral genesis and aerosolic input over time. The presence of microbial life in Antarctic paleosols, with minor amounts of Fe available for physiological processes to function, argues for the potential existence of microorganisms in ice-enriched paleosols of Mars, particularly given its watery and dynamic geologic past and relatively high concentration of total Fe in subaerial paleosols. The distribution of Fe over a large part of the northern plains of Mars as determined by the GRS instrument is invoked as a comparison with the Antarctic.     

The Influence of Strong-Motion Duration on the Seismic Response of Masonry Structures

The influence of strong-motion duration on the response of saturated soils is clearly recognised and accounted for in the assessment of liquefaction potential. The degree to which duration of shaking influences damage to structures, however, remains a topic of debate, with resolution of the issue complicated by the variety of definitions of duration and the variety of structural behaviours, as well as the difficulty of decoupling the specific effect of duration from other features of the ground motion. A suite of seven structural models with strength and stiffness degrading characteristics, designed to reflect the seismic behaviour of masonry structures commonly encountered in many parts of Europe, are analysed using a suite of almost 500 strong-motion accelerograms. Correlations are explored between the damage, measured in terms of the strength degradation, and a range of strong-motion parameters, demonstrating that Arias intensity and spectral acceleration at the fundamental initial period of the structure are both reasonably good damage indicators for such structures. A significantly improved correlation is obtained by using the elastic spectral accelerations averaged over a period range from the initial period of the structure to a value approximately three times greater, reflecting the stiffness degradation as the shaking progresses. The scatter in the correlation is shown to be partially explained by differences in duration. This revised version was published online in July 2006 with corrections to the Cover Date.     

The use of a landscape simulator in the validation of the SIBERIA landscape evolution model: transient landforms

SIBERIA is a physically based model for the geomorphic evolution of landforms. It is essential that the SIBERIA model be tested or validated against controlled landform development. Previous studies have demonstrated that SIBERIA is able to simulate declining equilibrium landforms and in this paper we examine SIBERIA's ability to simulate landforms as they evolve to their declining equilibrium form. These landscapes are termed transient landforms. Landscapes generated by SIBERIA were compared to those produced by a physical model (experimental model landforms) at stages of evolution. Comparison of the experimental landscapes with the simulated landscapes using total mass, hypsometric curve, width function, cumulative area distribution and area–slope demonstrate that SIBERIA can simulate the experimental model landscape during development (i.e. transient landscapes). Copyright © 2002 John Wiley & Sons, Ltd.     

The impact of depression removal on catchment geomorphology,soil erosion and landscape evolution

The current generation of landscape evolution models use a digital elevation model for landscape representation. These programs also contain a hydrological model that defines overland flow with the drainage network routed to an outlet. One of the issues with landscape evolution modelling is the hydrological correctness of the digital elevation model used for the simulations. Despite the wide use and increased quality of digital elevation models, data pits and depressions in the elevation data are a common feature and their removal will remain a necessary step for many data sets. This study examines whether a digital elevation model can be hydrologically correct (i.e. all depressions removed so that all water can run downslope) before use in a landscape evolution model and what effect depression removal has on long‐term geomorphology and hydrology. The impact on sediment transport rates is also examined. The study was conducted using a field catchment and a proposed landform for a post‐mining landscape. The results show that there is little difference in catchment geomorphology and hydrology for the non‐depression removed and depression removed data sets. The non‐depression removed and depression removed digital elevation models were also evaluated as input to a landscape evolution model for a 50 000 year simulation period. The results show that after 1000 years there is little difference between the data sets, although sediment transport rates did vary considerably early on in the simulation. Copyright © 2007 John Wiley & Sons, Ltd.     

Numbers of scaled and matched accelerograms required for inelastic dynamic analyses

Selecting, scaling and matching accelerograms are critically important to engineering design and assessment, enabling structural response to be determined with greater confidence and through fewer analyses than if unscaled accelerograms are employed. This paper considers the response of an 8‐storey multiple‐degree‐of‐freedom reinforced concrete structure to accelerograms selected, linearly scaled or spectrally matched using five different techniques. The first method consists of selecting real records on the basis of seismological characteristics, while the remaining methods make an initial selection on the basis of magnitude and spectral shape before (1) scaling to the target spectral acceleration at the initial period; (2) scaling to the target spectrum over a range of periods; (3) using wavelet adjustments to match the target spectrum and (4) using wavelet adjustments to match multiple target spectra for multiple damping ratios. The analyses indicate that the number of records required to obtain a stable estimate of the response decreases drastically as one moves through these methods. The exact number varies among damage measures and is related to the predictability of the damage measure. For measures such as peak roof and inter‐storey drift, member end rotation and the Park and Ang damage index, as few as one or two records are required to estimate the response to within ±5% (for a 64% confidence level) if matching to multiple damping ratios is conducted. Bias checks are made using predictive equations of the expected response derived from the results of 1656 nonlinear time‐domain analyses of the structure under the action of unscaled accelerograms. Copyright © 2008 John Wiley & Sons, Ltd.