Stochastic ground water flow simulation with a fracture zone continuum model   总被引：3，自引：0，他引：3  

Langevin CD 《Ground water》2003,41(5):587-601

A method is presented for incorporating the hydraulic effects of vertical fracture zones into two-dimensional cell-based continuum models of ground water flow and particle tracking. High hydraulic conductivity features are used in the model to represent fracture zones. For fracture zones that are not coincident with model rows or columns, an adjustment is required for the hydraulic conductivity value entered into the model cells to compensate for the longer flowpath through the model grid. A similar adjustment is also required for simulated travel times through model cells. A travel time error of less than 8% can occur for particles moving through fractures with certain orientations. The fracture zone continuum model uses stochastically generated fracture zone networks and Monte Carlo analysis to quantify uncertainties with simulated advective travel times. An approach is also presented for converting an equivalent continuum model into a fracture zone continuum model by establishing the contribution of matrix block transmissivity to the bulk transmissivity of the aquifer. The methods are used for a case study in west-central Florida to quantify advective travel times from a potential wetland rehydration site to a municipal supply wellfield. Uncertainties in advective travel times are assumed to result from the presence of vertical fracture zones, commonly observed on aerial photographs as photolineaments.  相似文献   

Holocene environmental change at Lake Njulla (999 m a.s.l.), northern Sweden: a comparison with four small nearby lakes alongan altitudinal gradient     

Christian Bigler  Evastina Grahn  Isabelle Larocque  Adam Jeziorski  Roland Hall 《Journal of Paleolimnology》2003,29(1):13-29

We assess Holocene environmental change at alpine Lake Njulla(68°22N, 18°42E, 999 m a.s.l.) innorthernmost Sweden using sedimentary remains of chironomid head capsules anddiatoms. We apply regional calibration sets to quantitatively reconstruct meanJuly air temperature (using chironomids and diatoms) and lake-water pH(using diatoms). Both chironomids and diatoms infer highest temperatures(1.7–2.3°C above present-day estimates, includinga correction for glacio-isostatic land up-lift by0.6°C) during the early Holocene (c.9,500–8,500 cal. yrs BP). Diatoms suggest a decreasing lake-waterpH trend (c. 0.6 pH units) since the early Holocene. Usingdetrended canonical correspondence analysis (DCCA), we compare the Holocenedevelopment of diatom communities in Lake Njulla with four other nearby lakes(Lake 850, Lake Tibetanus, Vuoskkujávri, Vuolep Njakajaure) locatedalong an altitudinal gradient. All five lakes show similar initial DCCA scoresafter deglaciation, suggesting that similar environmental processes such ashigh erosion rates and low light availability associated with high summertemperature appear to have regulated the diatom community, favouring highabundances of Fragilaria species. Subsequently, the diatomassemblages develop in a directional manner, but timing and scale ofdevelopment differ substantially between lakes. This is attributed primarily todifferences in the local geology, which is controlling the lake-waterpH. Imposed on the basic geological setting, site-specific processessuch as vegetation development, climate, hydrological setting andin-lake processes appear to control lake development in northernSweden.  相似文献   

Sub-Cm Subsidence Measurements With The Wide-Angle Airborne Laser Ranging System     

Oliver Bock  Christian Thom 《Surveys in Geophysics》2001,22(5-6):537-548

The Wide-Angle Airborne Laser Ranging System (WA-ALRS) is aninstrument developed at IGN with the aim of measuring landsubsidence with sub-cm accuracy. The instrument provides rangemeasurements that are used to estimate the co-ordinates of aground-based network of cube-corner retroreflectors (CCRs).Millimeter accuracy on relative radial co-ordinates has beendemonstrated in a ground-based experiment and centimeter accuracyhas been obtained in a recent airborne experiment. The mainlimitation in the latter experiment came from the low link budgetat longer distances. In this paper we analyze the performanceexpected with two different detector modules (avalanchephotodiodes with amplifiers). Laboratory measurements are used tocharacterize detector parameters and estimate ranging accuracy.Numerical simulations predict a 2–3 mm accuracy in the verticalco-ordinate with these detectors combined to a 2-km aircraftaltitude, a 20 laser beam divergence, a 250-m CCR separation and 10³ measurements per CCR.  相似文献   

An audiomagnetotelluric study of the Meugueur-Meugueur ring structure, Aïr, Niger: ring dyke or cone sheet?     

Michel Ritz  William L. Brown  Christian Moreau  Daniel Ohnenstetter 《Journal of Applied Geophysics》1996,34(4):229-236

Three radial audiomagnetotelluric (AMT) sounding profiles were carried out across the narrow, 65-km diameter troctolitic Meugueur-Meugueur ring structure, central Aïr, Niger, to study its electrical configuration; one profile extended across the bedrock into the large Ofoud complex situated slightly off geographical centre within the ring. Apparent resistivity data from 27 sites ranged from isotropic to strongly anisotropic. In nearly all soundings, one- and two-dimensional modelling indicated the presence of a major zone of low resistivity (60–600 Ωm), about 200 m thick, dipping steeply inwards at an angle of 65–80° and extending to a depth of at least 2–5 km. This layer, overlain and underlain by rocks of higher resistivities in excess of 5000 Ωm, is taken to be the outer contact. A highly resistive body, about 200 m in width, dipping inwards to a depth of at least 4 km is taken to be the Meugueur-Meugueur intrusion, which is thus interpreted to be a cone sheet.  相似文献   

Adsorption of polar aromatic hydrocarbons on synthetic calcite   总被引：1，自引：0，他引：1  

Lene Madsen  Laila Grahl-Madsen  Christian Grøn  Ida Lind  John Engell 《Organic Geochemistry》1996,24(12):1151-1155

The wettability of hydrocarbon reservoirs depends on how and to what extent the organic compounds are adsorbed onto the surfaces of calcite, quartz and clay. A model system of synthetic calcite, cyclohexane and the three probe molecules: benzoic acid, benzyl alcohol and benzylamine, have been studied by adsorption experiments. The results clearly demonstrate the differences in the adsorption behaviour between probes with different functional groups of varying polarity and acidity. The maximum adsorption decreases in the order: benzoic acid, benzyl alcohol and benzylamine. The order of magnitude of ΔG° for the adsorption process implies the formation of a strong bond between the calcite surface and the adsorbate molecules.  相似文献   

Significance of subtidal sediments to heterotrophically-mediated oxygen and nutrient dynamics in a temperate estuary     

William M. Rizzo  Robert R. Christian 《Estuaries and Coasts》1996,19(2):475-487

Sediment-water exchanges of ammonium (NH₄ ⁺), nitrate + nitrite (NO_x ^?), filterable reactive phosphorus (FRP, primarily ortho-phosphate), and oxygen (O₂) under aphotic (heterotrophic) conditions were determined at 2–5 stations in the Neuse River Estuary, from 1987 to 1989. Shallow (1 m), sandy stations were sampled along the salinity gradient. Fluxes from deep (>2 m) sites were compared to the shallow sites in two salinity zones. Grain size became finer and organic content increased with depth in the oligohaline zone but not in the mesohaline zone. Net release of NH₄ ⁺ and FRP occurred at all sites. Fluxes varied from slight uptake to releases of 200–500 μmol m^?2 h^?1 (NH₄ ⁺) and 150–900 μmol m^?2 h^?1 (FRP). Net NO_x ^? exchange was near zero, but were ±100 μmol m^?2 h^?1 over the year. Release of NH₄ ⁺ and FRP from the shallow sandy stations decreased with distance down the estuary, but O₂ uptake did not change. The deeper oligohaline site had twofold higher rates of NH₄ ⁺ and FRP release and O₂ uptake than the shallow site, but no differences occurred between depths in the mesohaline zone. Temperature and organic content were important controls for all fluxes, but water column NO_x ^? concentration was also important in regulating NO_x ^? exchanges. Ratios of oxygen consumption to NH₄ ⁺ release were near the predicted ratio (Redfield model) at oligohaline sites but increased down estuary at mesohaline sites. This may be due to greater nitrification rates promoted by autotrophy in the sediments.  相似文献   

地球静止轨道闪电探测   总被引：4，自引：0，他引：4  

H.J.Christian  R.J.Blakeslee  S.J.Goodrnan  王惠均  易文 《气象科技》2001,29(1):42-48

利用高空NASAU 2飞机的探测资料与其他科学研究方法 ,开发了一种空间探测器。其空间分辨率为 10km ,检测效率 90 % ,具有昼夜映像地球静止轨道的云内和云对地放电的能力。这种闪电图像探测器 (LMS)可连续监视风暴。闪电探测器中新的固态马赛克聚焦平面和较强信号处理相结合 ,为检测微弱的背景受污染信号提供具有权威性的技术 ,并满足白天监测闪电。LMS的视场为10 .5° ,覆盖全美国大陆 ,大范围海域、中美洲、包括安第斯山脉及亚马逊河流域在内的南美大部以及热带辐合区的大部分地区。 2 0世纪 90年代中期LMS会随GOES卫星飞行。该文介绍LMS的特性和设计以及应用这种仪器进行科学研究的可能性  相似文献   

Geochemistry and petrography of impact breccias and target rocks from the 145 Ma Morokweng impact structure, South Africa     

Christian Koeberl  Wolf Uwe reimold 《Geochimica et cosmochimica acta》2003,67(10):1837-1862

Morokweng is a large, 145 Ma impact structure in the Northwest Province of South Africa. The impact origin of this structure and its melt rock has been confirmed by ample evidence of shock metamorphism in clasts within the melt rock and samples from granitoid basement below the melt body. The age of this structure is indistinguishable from the biostratigraphic age of the Jurassic-Cretaceous (J-K) boundary. The size of Morokweng, for which diameters ranging from 70 to 165 kilometers have been quoted before, and which has important implications regarding its relation to the J-K boundary, remains an open question.Here we present new results of a detailed petrographic and chemical investigation of impact melt rock and country rock samples. The granophyric melt rock is mostly unaltered and contains a large number of gabbroic and felsic clasts. The occurrence of baddeleyite, formed from high-temperature dissociation of primary zircon, indicates a high-temperature origin. The impact melt rock body, which in the cores investigated here has a thickness of at least 120 m, shows no statistically significant variation or trend in chemical composition with depth or geographic location. Chemical data for impact melt rock, breccia dike/vein breccia samples, granite, quartzite, and basic to mafic clasts were used in harmonic least squares mixing calculations to determine the source rock types and their proportions involved in the formation of the impact melt rock. Granite is the dominant target rock component (50 to 63% by weight; depending on target composition input to the mixing models), with significant (35 to 50%) mafic contributions, and a (possible) minor contribution of quartzite. New platinum group element (Ru, Rh, Pd, Os, Ir, and Pt), Re, and Au data, as well as data for other siderophile elements (Cr, Co, Ni, and Ir), confirm the presence of up to ∼ 5% of a chondritic component in the melt rock. The indigenous contribution of the PGEs from the target rocks is negligible. Normalized PGE abundance patterns and interelement ratios of Morokweng impact melt rock indicate that the projectile was likely of ordinary chondritic (possibly L chondrite) composition, but the choice of the meteoritic compositional data influences this interpretation.  相似文献   

Climatic significance of seasonal trace element and stable isotope variations in a modern freshwater tufa   总被引：1，自引：0，他引：1  

Christian Ihlenfeld  Marc D. Norman  Russell N. Drysdale  John Webb 《Geochimica et cosmochimica acta》2003,67(13):2341-2357

We present a continuous ∼14-yr-long (1985 to 1999) high-resolution record of trace element (Mg, Sr, Ba, U) and stable isotope (δ¹³C, δ¹⁸O) variations in a modern freshwater tufa from northwestern Queensland, Australia. By utilizing the temperature dependence of the δ¹⁸O signal, an accurate chronology was developed for the sampled profile, which allowed a comparison of the chemical records with hydrological and meteorological observations. As a consequence, it was possible to constrain the relevant geochemical processes relating climate variables, such as temperature and precipitation, to their chemical proxies in the tufa record. Temperatures calculated from the Mg concentrations of the tufa samples provide close approximations of average annual water temperature variations. Furthermore, we demonstrate that temporal changes in (Mg/Ca)_water can be estimated using an empirically derived equation relating (Mg/Ca)_water to the (Sr/Ba) ratio measured in the tufa samples. By means of this relationship, it is theoretically possible to determine the (Mg/Ca) ratio of paleowaters, and hence to derive reliable estimates of former water temperatures from the Mg concentrations of fossil tufas from the study area. Sympathetic variations in Sr, Ba, and δ¹³C along the sampled profile record changes in water chemistry, which are most probably caused by variable amounts of calcite precipitation within the vadose zone of the karst aquifer. This process is thought to be markedly subdued whenever the amount of wet-season precipitation exceeds a given threshold. Accordingly, distinct minima in Sr, Ba, and δ¹³C are interpreted to reflect years with above-average rainfall. The pronounced seasonal and annual variability of the U concentration along the profile is thought to primarily record changes in the U flux from the soil to the water table. We suggest that during intensive rain events U is transported to the phreatic zone by complexing organic colloids, giving rise to conspicuous U maxima in the tufa after above-average wet seasons. This study demonstrates the potential of freshwater tufas to provide valuable information on seasonal temperature and rainfall variations. If tufa deposits turn out to be reasonably resistant to secondary processes, combined investigation of speleothems and tufas from the same area could become a promising approach in future research. While speleothems offer continuous records of long-term paleoenvironmental changes, tufas could provide high-resolution time windows into selected periods of the past.  相似文献   

Control of sulfate pore-water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments     

Christian Hensen  Matthias Zabel  Tilmann Schwenk  Natascha Riedinger  Antje Boetius 《Geochimica et cosmochimica acta》2003,67(14):2631-2647

Gravity driven mass-flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western Argentine Basin. In this study we present geochemical data from a total of 23 gravity cores. Pore-water SO₄ is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane (AOM). The different shapes of SO₄ profiles (concave, kink- and s-type) can be consistently explained by sedimentary slides possibly in combination with changes in the CH₄ flux from below, thus, mostly representing transient pore-water conditions. Since slides may keep their original sedimentary signature, a combined analysis and numerical modeling of geochemical, physical properties, and hydro acoustic data could be applied in order to reconstruct the sedimentary history. We present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day. The results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep-sea, depositing high amounts of reactive compounds. The high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by AOM. For the total region we estimate a SO₄ (or CH₄) flux of 6.6 × 10¹⁰ moles per year into the zone of AOM. Projected to the global continental slope and rise area, this may sum up to about 2.6 × 10¹² moles per year. Provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1.2 × 10¹² moles per year as previously estimated. Thus, AOM obviously contributes very significantly to the regulation of global sulfur reservoirs, which is hitherto not sufficiently recognized. This finding may have implications for global geochemical models, as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time.  相似文献