Curie-point depth from spectral analysis of magnetic data in central-southern Europe

The basal depth of the outer layer with internal magnetic sources was calculated from magnetic data available within a roughly 500 km wide and 1200 km long area, running from central Germany to southern Italy. The dataset, deriving from different aeromagnetic surveys, is reduced to the reference altitude of 3000 m a.s.l. and a reference year of 1980.0. The adopted method, which transforms the spatial data into the frequency domain, provides a relationship between the two-dimensional spectrum of the magnetic anomalies and the top and centroid depths of the magnetic sources. The magnetic layer bottom depth (MLBD) thus obtained is 29-33 km deep in the stable areas (central Europe Variscan units, Corsica-Sardinia Variscan block) and corresponds to the Moho, having an average temperature of 560 °C. From the Alps to the Apennines, MLBD ranges between 22 and 28 km and is clearly shallower than the Moho. In these units, the wide variation of MLBD appears to be compatible with the presence of shallow magnetised bodies, consisting of lower crustal rocks (Ivrea-Verbano zone), ophiolitic units (Penninic zone and Voltri Massif) and intrasedimentary basic volcanic bodies (Po Basin). An average value of 25 km can be attributed to MLBD, which corresponds to a temperature of 550 °C. In the peri-Tyrrhenian zone and the Ligurian Sea, MLBD is below the Moho, which ranges from 17 to 20 km depth, and it has a temperature matching approximately to the Curie temperature of magnetite (580 °C).     

Postglacial topographic evolution of glaciated valleys: a stochastic landscape evolution model

The retreat of valley glaciers has a dramatic effect on the stability of glaciated valleys and exerts a prolonged influence on the subsequent fluvial sediment transport regime. We have studied the evolution of an idealized glaciated valley during the period following retreat of ice using a numerical model. The model incorporates a stochastic process to represent deep‐seated landsliding, non‐linear diffusion to represent shallow landsliding and an approximation of the Bagnold relation to represent fluvial sediment transport. It was calibrated using field data from several recent surveys within British Columbia, Canada. We present ensemble model results and compare them with results from a deterministic linear‐diffusion model to show that explicit representation of large landslides is necessary to reproduce the morphology and channel network structure of a typical postglacial valley. Our model predicts a rapid rate of fluvial sediment transport following deglaciation with a subsequent gradual decline, similar to that inferred for Holocene time. We also describe how changes in the model parameters affect the estimated magnitude and duration of the paraglacial sediment pulse. Copyright © 2005 John Wiley & Sons, Ltd.     

Developing efficient scalar and vector intensity measures for IDA capacity estimation by incorporating elastic spectral shape information

Scalar and vector intensity measures are developed for the efficient estimation of limit‐state capacities through incremental dynamic analysis (IDA) by exploiting the elastic spectral shape of individual records. IDA is a powerful analysis method that involves subjecting a structural model to several ground motion records, each scaled to multiple levels of intensity (measured by the intensity measure or IM), thus producing curves of structural response parameterized by the IM on top of which limit‐states can be defined and corresponding capacities can be calculated. When traditional IMs are used, such as the peak ground acceleration or the first‐mode spectral acceleration, the IM‐values of the capacities can display large record‐to‐record variability, forcing the use of many records to achieve reliable results. By using single optimal spectral values as well as vectors and scalar combinations of them on three multistorey buildings significant dispersion reductions are realized. Furthermore, IDA is extended to vector IMs, resulting in intricate fractile IDA surfaces. The results reveal the most influential spectral regions/periods for each limit‐state and building, illustrating the evolution of such periods as the seismic intensity and the structural response increase towards global collapse. The ordinates of the elastic spectrum and the spectral shape of each individual record are found to significantly influence the seismic performance and they are shown to provide promising candidates for highly efficient IMs. Copyright © 2005 John Wiley & Sons, Ltd.     

Evolutionary aseismic design and retrofit of structures with passive energy dissipation

A new computational framework is developed for the design and retrofit of building structures by considering aseismic design as a complex adaptive process. For the initial phase of the development within this framework, genetic algorithms are employed for the discrete optimization of passively damped structural systems. The passive elements may include metallic plate dampers, viscous fluid dampers and viscoelastic solid dampers. The primary objective is to determine robust designs, including both the non‐linearity of the structural system and the uncertainty of the seismic environment. Within the present paper, this computational design approach is applied to a series of model problems, involving sizing and placement of passive dampers for energy dissipation. In order to facilitate our investigations and provide a baseline for further study, we introduce several simplifications for these initial examples. In particular, we employ deterministic lumped parameter structural models, memoryless fitness function definitions and hypothetical seismic environments. Despite these restrictions, some interesting results are obtained from the simulations and we are able to gain an understanding of the potential for the proposed evolutionary aseismic design methodology. Copyright © 2005 John Wiley & Sons, Ltd.     

Use of fallout 137Cs for documenting the chronology of overbank sediments from the river Sava, Croatia, and interpreting their geochemical patterns

This paper attempts to determine the impact of former Pb-Zn-Ba mining activities in Slovenian drainage basin on overbank sediments of the Sava river (West Croatia). Among the four studied sequences (S7, S8, S9, S10), the most upstream profile S7 showed the most pronounced impact of ore mining. Mercury was enriched more than 100-fold; Pb and Zn had a strong positive correlation (r > 0.99). Two factor models clearly revealed the Pb-Ba-Hg association in the lower-lying carbonate (CA) population of all four profiles, but only in the overlying silicate (SI) population of profile S7. Factor analysis confirmed this association in the total sediment (90 samples) as well as separately in the CA (33 samples) and SI (57 samples) populations. Based on a comparison with an undisturbed soil profile, the vertical distribution of ¹³⁷Cs in profile S7 was interpreted to be the combination of caesium deposition caused by a flood in 1964 and by post-depositional redistribution.     

Fractally Invariant Distributions and an Application in Geochemical Exploration

Fractal modelling has been applied extensively as a means of characterizing the spatial distribution of geological phenomena that display self-similarity at differing scales of measurement. A fractal distribution exists where the number of objects exhibiting values larger than a specified magnitude displays a power-law dependence on that magnitude, and where this relationship is scale-invariant. This paper shows that a number of distributions, including power-function, Pareto, log-normal and Zipf, display fractal properties under certain conditions and that this may be used as the mathematical basis for developing fractal models for data exhibiting such distributions. Population limits, derived from fractal modelling using a summation method, are compared with those derived from more conventional probability plot modelling of stream sediment geochemical data from north-eastern New South Wales. Despite some degree of subjectivity in determining the number of populations to use in the models, both the fractal and probability plot modelling have assisted in isolating anomalous observations in the geochemical data related to the occurrence of mineralisation or lithological differences between sub-catchments. Thresholds for the main background populations determined by the fractal model are similar to those established using probability plot modelling, however the summation method displays less capacity to separate out anomalous populations, especially where such populations display extensive overlap. This suggests, in the geochemical data example provided, that subtle differences in the population parameters may not significantly alter the fractal dimension.     

BTEX异常对海域油气藏指示作用的研究

常规的油气化探方法一般是应用烷烃类作为油气勘探的基本指标,但是烷烃类易受人类活动和生物活动的干扰,使常规的化探技术在油气勘探中一直受到限制。人们开始考虑采用直接的生物标记化合物来预测油气藏,如芳烃系列中的苯系化合物苯(benzene)、甲苯(toluene)、乙苯(ethylbenzene)、二甲苯(xylem),简称BTEX。国内外研究表明,BTEX基本不受人类活动和生物活动的干扰,对油气藏具有良好的指示作用。依据南海某区的实测资料,采用自主研发的油气化探可视化评价软件进行数据处理,剥离背景异常,圈出有效的BTEX异常,并与靶区地层剖面进行了对比,结果证实了BTEX对海域油气藏具有良好的指示作用。     

唐山地区干旱演变特征分析

研究区地处干旱和湿润气候的过渡地带,这一地区导致干旱发生的直接原因是干旱的气候和水资源的短缺,主要就是降水量的偏少.从简单实用角度考虑,本文根据唐山地区1956～2005年的水文和气象资料,采用降水距平、径流系数、她表湿润度指数等指标对唐山地区的干旱演变趋势进行分析,揭示出该地区近50年的干旱演变特征.     

不规则网土壤测量在水系沉积物异常查证中的应用

在大兴安岭森林沼泽景观区,植被茂盛、土壤表层腐殖质发育,且通视条件差,运用传统的规则网土壤测量方法进行1/5万水系沉积物异常查证,工作周期长、劳动强度大;采用不规则网土壤测量方法进行异常查证,取得了快速评价异常、追踪异常源,缩短找矿周期的效果。