复杂介质有限元法2.5维可控源音频大地电磁法数值模拟   总被引：38，自引：13，他引：25       下载免费PDF全文

底青云  Martyn Unsworth  王妙月 《地球物理学报》2004,47(4):723-730

利用有限元2.5维可控源音频大地电磁法（简称CSAMT）数值模拟方法,对100Ωm均匀半空间介质中有限长度的电偶极源产生的电场、磁场及视电阻率、相位特征进行了数值模拟,研究了场的空间变化规律. 在一个象限中,场的特征存在双叶现象,当收发距大于4个趋肤深度时,电阻率较接近介质真实的电阻率,这些结果为观测系统和收发距的选择提供了依据. 波数域场的特征表明,低波数对源的贡献占较大的比例. 有限元法2.5维CSAMT数值模拟的优势在于能较准确地获得复杂介质结构的波场特征. 本文结合直立异常体、倾斜异常体及断陷模型对CSAMT电阻率、相位剖面特征及频率曲线特征的可靠性进行了研究. 数值模拟结果直观地给出了异常体的剖面异常形态. 通过对比研究异常体的剖面异常形态和半空间场的特征进一步说明本文方法和软件在模拟复杂介质结构场特征时是可靠的. 这为认识观测数据,指导反演解释提供了较好的依据.  相似文献   

The Tilia decline: vegetation change in lowland Britain during the mid and late Holocene     

Michael J. Grant  Martyn P. Waller  Jon A. Groves 《Quaternary Science Reviews》2011,30(3-4):394-408

Pollen data indicate Tilia was an important component of the primary woodland cover of many lowland areas in northern and central Europe. High values recorded in mid-Holocene pollen diagrams are generally followed by well-defined declines. In this study the palynological, spatial and temporal trends associated with declines in Tilia pollen at sites from lowland Britain are assessed to evaluate the nature and relative importance of the processes responsible. Processes primarily related to the depositional environment (paludification, marine inundation, breaks in sedimentation) and percentage representation of pollen data play the dominant role in 44% of the 164 declines included in the study. Anthropogenic activity can account for the remainder. Tilia rarely recovers from low values after anthropogenic declines, suggesting such declines have the potential to elucidate patterns in the destruction of primary woodland across lowland Britain. Other than declines associated with the mid-Holocene Ulmus decline event, few anthropogenic Tilia declines have been reliably dated prior to c. 5000 cal. BP. Tilia largely disappeared from areas with calcareous and loamy soils as a result of clearance activity in the period 5000–3000 cal. BP (Late Neolithic to Late Bronze Age). Between 3000 and 2500 cal. BP few declines are recorded suggesting a link between clearance activity and climate change. Later declines are concentrated on the sandier lithologies and often associated with heathland formation. The scarcity of declines after 2000 cal. BP is likely to be related to the exhaustion of primary woodland.  相似文献   

Sulphide oxidation under partially anoxic conditions at the bed of the Haut Glacier d'Arolla,Switzerland     

Simon H. Bottrell  Martyn Tranter 《水文研究》2002,16(12):2363-2368

Oxygen isotopic data are presented for bulk glacial meltwaters draining the Haut Glacier d'Arolla, Valais, Switzerland and for the sulphate contained within them in an attempt to assess the redox status of the subglacial chemical weathering environment. The sulphate derived from subglacial chemical weathering is so depleted in ¹⁸O that it must have formed, at least partially, in an anoxic environment. Under these conditions, Fe³⁺ can act as an oxidizing agent and oxygen atoms incorporated into sulphate are derived from ¹⁸O‐depleted water molecules (by contrast, dissolved O₂ is strongly enriched in ¹⁸O). These data therefore support the hypothesis that sectors of the glacier bed are anoxic and that Fe³⁺ may act as a significant oxidizing agent under these conditions. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Holocene environmental change in the West African Sahel: sedimentological and mineral‐magnetic analyses of lake sediments from Jikariya Lake,northeastern Nigeria     

H. Wang  J. A. Holmes  F. A. Street‐Perrott  M. P. Waller  R. A. Perrott 《第四纪科学杂志》2008,23(5):449-460

Previous studies have demonstrated long‐term changes in effective moisture in sub‐Saharan Africa. Here, we reconstruct Holocene environments using a ～7 m lake‐sediment sequence recovered from the northeastern Nigerian Sahel and attempt to distinguish basin‐specific changes from regional climatic variations. The sequence was analysed for sedimentological properties, mineral magnetism and pollen, and dated by ¹³⁷Cs, ²¹⁰Pb excess and ¹⁴C. Extremely arid conditions of the terminal Pleistocene ended ca. 11 500 cal. BP (calendar years) when climate ameliorated and a lake developed until the occurrence of an arid event leading to lake desiccation at ～11 200 cal. BP. Following this, climate ameliorated and a water body re‐emerged. Very wet conditions predominated 11 200–5600 cal. BP, followed by drought between 5600 and 5500 cal. BP and a return to moderate humidity from 5500 to 4000 cal. BP. After 4000 cal. BP, a marked deterioration occurred, culminating in lake desiccation at ca. 800 cal. BP. After this time the climate remained generally dry and the re‐emerging lake was shallow. Comparison of these results with other well‐dated sequences in the region demonstrates the importance of basin‐specific influences on the palaeolimnological records in addition to regional climatic controls. Disentangling these different controls, as well as the reconstruction of Holocene climate, therefore requires a multiple‐basin approach. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

The influence of the ocean on the chemistry of precipitation in Nova Scotia*     

J.K. Underwood  D.H. Waller  D. Thirumurthi 《大气与海洋》2013,51(3):467-479

Abstract

Between the fall of 1977 and December 1980, over 800 samples of precipitation were collected at 9 rural locations across Nova Scotia.

Average pH was 4.61, and an average conductivity of 25 μS cm^?1 confirmed the low ionic strength of rain and snow. Of all ions, chloride was the most abundant at 76 μeq L^?1 (2.7 mg L^?1). Oceanic origins were perceived as the source for chloride as well as for Na, Mg, 51 % of K, 21% of Ca, and 21% of SO₄, and the sea collectively accounted for 66% of all ions inNova Scotian precipitation. In contrast, the dominating ions in New York, New Hampshire, Ontario and Norway were H⁺ and SO₄.

In Nova Scotia, chloride concentrations in rain (mg L^?1) are a function of distance D (km) from the coast and may be described by the equation Cl [mg L^?1] = 4.5D^?0.42. It indicates that Cl concentrations are reduced by about 50% within 0.5 km of the coast, and 80% within 5 km. The continental North American chloride data examined are also consistent with the relationship.

The shape of the chloride curve is similar to curves others have developed from rain and lake water chemistry observations in Holland and the United Kingdom, although those, and other European data, indicate that the marine influence is less pronounced in northeastern North America.

The marine influence of precipitation chemistry on Nova Scotia has a maximum seasonal amplitude during the winter.  相似文献   

Linking places to problems: geospatial theories of neighborhoods, alcohol and crime     

Dennis M. Gorman  Paul J. Gruenewald  Lance A. Waller 《GeoJournal》2013,78(3):417-428

This paper provides a critical review of two broad categories of social ecological theories of crime, social integration and place-based theories, and their relationships to spatial assessments of crime patterns. Social integration theories emphasize the role of neighborhood disorganization on crime, while place-based theories stress the social interactions within and between places as a source of crime. We provide an analysis of the extent to which these two types of theorizing describe processes and mechanisms that are truly ecologic (identify specific interactions between individuals and their environments) and truly spatial (identify specific movement and interaction patterns of individuals and groups) as they endeavor to explain crime outcomes. We suggest that social integration theories do not provide spatial signatures of sufficient specificity to justify the application of spatial statistical techniques as quantitative arbiters of the theory. On the other hand, place based theories go some way toward addressing these issues because the emphasis is placed on understanding the exact physical and social characteristics of place and the activities that occur around locations as sources of crime. Routine activities and crime potential theories attempt to explain clustering or “hot spots” of crime in ways that give clear spatial dimension by looking at micro-spatial interactions between offenders and targets of crime. These theories have strong ecological implications as well, since they contain specific statements about how people use the space around them and how these patterns of use are related to patterns of criminal activity. We conclude by identifying a set of requirements for successful empirical tests of geospatial theories, including the development of valid measures of key theoretical constructs and the formulation of critical empirical assessments of geospatial hypotheses derived from motivating theory.  相似文献   

Exploring the spread of methamphetamine problems within California, 1980 to 2006     

William R. Ponicki  Lance A. Waller  Lillian R. Remer  Paul J. Gruenewald 《GeoJournal》2013,78(3):451-462

The introduction and spread of high potency methamphetamine has led to dramatic increases in drug-related problems in California. Prior research suggests that drug abuse rates are related to local demographic and economic characteristics, law enforcement activities, and sentencing practices. Methamphetamine abuse in particular has been shown to be reduced by laws regulating the raw materials needed for its production. This research models the regional effects of such laws on the spatio-temporal patterns of growth of methamphetamine-related problems across California from 1980 to 2006. Amphetamine-related arrests and hospital discharges related to amphetamine abuse/dependence were assembled for California counties over the years 1980–2006. Varying-parameter Bayesian space–time models were used to relate the implementation of major laws controlling the distribution of methamphetamine precursors to observed patterns of arrests and discharges and to allow such associations to vary by location. The models used conditionally autoregressive (CAR) Bayesian spatial priors to allow spatial correlation in estimation of county-specific growth in these measures over three distinct time periods: before the 1989 law, between the 1989 and 1997 laws, and after the 1997 law. Growth of arrests and discharges were related to demographic and economic indicators to determine geographic areas more or less susceptible to the spread of methamphetamine problems. Although both problem measures increased during all three periods, each of the precursor laws was associated with short-term reductions in the growth of arrests and discharges. Growth was greatest in central California counties prior to 1989 and increased in coastal counties in later years. From 1980 to 1989 growth was highest for counties with low incomes and high proportions of white residents, while 1989–1997 growth was highest in counties with fewer whites and more Hispanics. Growth after 1997 was not significantly associated with county characteristics. This research demonstrates that the precursor laws did suppress the growth of methamphetamine related arrests and hospital discharges. It also demonstrates specific geographic patterns in the growth of methamphetamine arrests and abuse across California during this time. Early patterns of growth were related to economic and demographic characteristics, while later patterns were not. This suggests that some counties were uniquely susceptible to the early spread of the methamphetamine epidemic, although problems eventually grew dramatically in all California counties.  相似文献   

Spatial-temporal disease mapping of illicit drug abuse or dependence in the presence of misaligned ZIP codes     

Li Zhu  Lance A. Waller  Juan Ma 《GeoJournal》2013,78(3):463-474

Geo-referenced data often are collected in small, administrative units such as census enumeration districts or postal code areas. Such areas vary in geographic area and population size and may change over time. In research into drug-related health issues within the United States, U.S. Postal Service ZIP codes represent a commonly used unit for data collection, storage, and spatial analysis because of their widespread availability in health databases through patient contact and billing information. However, the ZIP code was developed for the specific purpose of delivering mail and may be changed at any time, and its design and development does not take into consideration problems that may arise in data collection, analysis, and presentation in health studies. In this paper, we propose a spatial hierarchical modeling approach to quantify trends within ZIP-code based counts when some fraction of ZIP codes change over the study period, that is, when the data are spatially misaligned across time. We propose a data vector approach and adjust the spatial auto-correlation structure within our Bayesian hierarchical model to provide inference for our misaligned data. We motivate and illustrate our approach to explore spatio-temporal patterns of amphetamine abuse and/or dependence in Tracy, California over the years 1995–2005. Uncertainty associated with misaligned data is modeled, quantified, and visualized. The approach offers a framework for further investigation into other risk factors in order to more fully understand the dynamics of illicit drug abuse or dependence across time and space in imperfectly measured data.  相似文献   

An 11 000-year record of vegetation and environment from Lago di Martignano,Latium, Italy     

Martyn G. Kelly  Brian Huntley 《第四纪科学杂志》1991,6(3):209-224

A pollen diagram from Lago di Martignano, a maar lake in central Italy, provides an 11000-year record of vegetation and environment change. The earliest pollen spectra are dominated by Artemisia and Gramineae, representing late glacial steppe vegetation typical of the Mediterranean region. Broad-leaved forests were established by ca. 11 000 yr BP. Although Quercus initially dominated their canopy, a wide range of other mesophyllous trees were also present. Pollen values for sclerophyllous tree and shrub taxa characteristic of Mediterranean woodlands and scrub are initially low (<10%). After ca. 7000 yr BP, however, they begin to increase and rise to a peak of >40% of total land pollen at ca. 6700 yr BP, with Olea europaea the single most abundant taxon. Human influence upon the vegetation only becomes significant somewhat after this peak, with progressive clearance of woodland and expansion of herbaceous communities. Castanea sativa and luglans regia pollen is recorded consistently from the beginning of the rise in pollen values for taxa characteristic of Mediterranean scrub communities. Pollen values for arable crops increase progressively after ca. 5500 yr BP, following the peak pollen values for taxa characteristic of Mediterranean scrub vegetation. Late glacial and Holocene climate changes have been complex in this region, with the present character of the climate developing only during the last millennium. Rates of change of pollen spectra peak during this period.  相似文献   

Contributions from glacially derived sediment to the global iron (oxyhydr)oxide cycle: Implications for iron delivery to the oceans     

Rob Raiswell  Martyn Tranter  Martin Siegert  Philippe Huybrechts 《Geochimica et cosmochimica acta》2006,70(11):2765-2780

Estimates of glacial sediment delivery to the oceans have been derived from fluxes of meltwater runoff and iceberg calving, and their sediment loads. The combined total (2900 Tg yr⁻¹) of the suspended sediment load in meltwaters (1400 Tg yr⁻¹) and the sediment delivered by icebergs (1500 Tg yr⁻¹) are within the range of earlier estimates. High-resolution microscopic observations show that suspended sediments from glacial meltwaters, supraglacial, and proglacial sediments, and sediments in basal ice, from Arctic, Alpine, and Antarctic locations all contain iron (oxyhydr)oxide nanoparticles, which are poorly crystalline, typically ∼5 nm in diameter, and which occur as single grains or aggregates that may be isolated or attached to sediment grains. Nanoparticles with these characteristics are potentially bioavailable. A global model comparing the sources and sinks of iron present as (oxyhydr)oxides indicates that sediment delivered by icebergs is a significant source of iron to the open oceans, beyond the continental shelf. Iceberg delivery of sediment containing iron as (oxyhydr)oxides during the Last Glacial Maximum may have been sufficient to fertilise the increase in oceanic productivity required to drawdown atmospheric CO₂ to the levels observed in ice cores.  相似文献