Far-Zone Contributions to Topographical Effects in the Stokes-Helmert Method of the Geoid Determination

In the evaluation of the geoid done according to the Stokes-Helmert method, the following topographical effects have to be computed: the direct topographical effect, the primary indirect topographical effect and the secondary indirect topographical effect. These effects have to be computed through integration over the surface of the earth. The integration is usually split into integration over an area immediately adjacent to the point of interest, called the near zone, and the integration over the rest of the world, called the far zone. It has been shown in the papers by Martinec and Vaníek (1994), and by Novák et al. (1999) that the far-zone contributions to the topographical effects are, even for quite extensive near zones, not negligible.Various numerical approaches can be applied to compute the far-zone contributions to topographical effects. A spectral form of solution was employed in the paper by Novák et al. (2001). In the paper by Smith (2002), the one-dimensional Fast Fourier Transform was introduced to solve the problem in the spatial domain. In this paper we use two-dimensional numerical integration. The expressions for the far-zone contributions to topographical effects on potential and on gravitational attraction are described, and numerical values encountered over the territory of Canada are shown in this paper.     

Résultats préliminaires sur la sédimentation pélagique de l'Atlantique tropical au Crétacé et au Tertiaire (plateau de Demerara,Leg ODP 207)

Five sites located on a bathymetric transect of the distal Demerara Rise were studied by ODP Leg 207. Albian sediments of essentially terrigenous nature (clay, siltstone, sandstone) are the oldest drilled stratigraphic levels and form apparently the top of the synrift sequence. They are overlain by Cenomanian to Santonian finely laminated black shales, rich in organic matter of marine origin, which accumulated on a thermally subsiding ramp. Early Campanian hiatuses are thought to be the result of final disjunction of Demerara Rise (South America) from Africa and the onset of deep water communication between the two Atlantic basins (south and central). The overlying Uppermost Cretaceous–Oligocene chalk includes rich and diversified calcareous plankton assemblages, as well as two radiolarian-rich intervals (Late Campanian and Middle Eocene). A complex erosional surface developed during the Late Oligocene–Early Miocene. Sedimentation was impeded since then on the intermediate and deep sites of Demerara Rise, possibly due to the action of deep submarine currents. To cite this article: T. Danelian et al., C. R. Geoscience 337 (2005).     

On dynamic and thermodynamic components of cloud changes

Clouds are sensitive to changes in both the large-scale circulation and the thermodynamic structure of the atmosphere. In the tropics, temperature changes that occur on seasonal to decadal time scales are often associated with circulation changes. Therefore, it is difficult to determine the part of cloud variations that results from a change in the dynamics from the part that may result from the temperature change itself. This study proposes a simple framework to unravel the dynamic and non-dynamic (referred to as thermodynamic) components of the cloud response to climate variations. It is used to analyze the contrasted response, to a prescribed ocean warming, of the tropically-averaged cloud radiative forcing (CRF) simulated by the ECMWF, LMD and UKMO climate models. In each model, the dynamic component largely dominates the CRF response at the regional scale, but this is the thermodynamic component that explains most of the average CRF response to the imposed perturbation. It is shown that this component strongly depends on the behaviour of the low-level clouds that occur in regions of moderate subsidence (e.g. in the trade wind regions). These clouds exhibit a moderate sensitivity to temperature changes, but this is mostly their huge statistical weight that explains their large influence on the tropical radiation budget. Several propositions are made for assessing the sensitivity of clouds to changes in temperature and in large-scale motions using satellite observations and meteorological analyses on the one hand, and mesoscale models on the other hand.     

Deposits of depletive high-density turbidity currents: a flume analogue of bed geometry, structure and texture

Flume experiments were performed to study the flow properties and depositional characteristics of high‐density turbidity currents that were depletive and quasi‐steady to waning for periods of several tens of seconds. Such currents may serve as an analogue for rapidly expanding flows at the mouth of submarine channels. The turbidity currents carried up to 35 vol.% of fine‐grained natural sand, very fine sand‐sized glass beads or coarse silt‐sized glass beads. Data analysis focused on: (1) depositional processes related to flow expansion; (2) geometry of sediment bodies generated by the depletive flows; (3) vertical and horizontal sequences of sedimentary structures within the sediment bodies; and (4) spatial trends in grain‐size distribution within the deposits. The experimental turbidity currents formed distinct fan‐shaped sediment bodies within a wide basin. Most fans consisted of a proximal channel‐levee system connected in the downstream direction to a lobe. This basic geometry was independent of flow density, flow velocity, flow volume and sediment type, in spite of the fact that the turbidity currents of relatively high density were different from those of relatively low density in that they exhibited two‐layer flow, with a low‐density turbulent layer moving on top of a dense layer with visibly suppressed large‐scale turbulence. Yet, the geometry of individual morphological elements appeared to relate closely to initial flow conditions and grain size of suspended sediment. Notably, the fans changed from circular to elongate, and lobe and levee thickness increased with increasing grain size and flow velocity. Erosion was confined to the proximal part of the leveed channel. Erosive capacity increased with increasing flow velocity, but appeared to be constant for turbidity currents of different grain size and similar density. Structureless sediment filled the channel during the waning stages of the turbidity currents laden with fine sand. The adjacent levee sands were laminated. The massive character of the channel fills is attributed to rapid settling of suspension load and associated suppression of tractional transport. Sediment bypassing prevailed in fan channels composed of very fine sand and coarse silt, because channel floors remained fully exposed until the end of the experiments. Lobe deposits, formed by the fine sand‐laden, high‐density turbidity currents, contained massive sand in the central part grading to plane parallel‐laminated sand towards the fringes. The depletive flows produced a radial decrease in mean grain size in the lobe deposits of all fans. Vertical trends in grain size comprised inverse‐to‐normal grading in the levees and in the thickest part of the lobes, and normal grading in the channel and fringes of the fine sandy fans. The inverse grading is attributed to a process involving headward‐directed transport of relatively fine‐grained and low‐concentrated fluid at the level of the velocity maximum of the turbidity current. The normal grading is inferred to denote the waning stage of turbidity‐current transport.     

Landslide Susceptibility Zoning North of Yenice (NW Turkey) by Multivariate Statistical Techniques

Dramatic effects resulting from landslides on human life and economy of many nations are observed sometimes throughout the world. Landslide inventory and susceptibility mapping studies are accepted as the first stage of landslide hazard mitigation efforts. Generally, these landslide inventory studies include identification and location of landslides. The main benefit is to provide a basis for statistical susceptibility zoning studies. In the present study, a landslide susceptibility zoning near Yenice (NW Turkey) is carried out using the factor analysis approach. The study area is approximately 64 km² and 57 landslides were identified in this area. The area is covered completely by Ulus Formation that has a flysh-like character. Slope angle, elevation, slope aspect, land-use, weathering depth and water conditions were considered as the main conditioning factors while the heavy precipitation is the main trigger for landsliding. According to the results of factor analysis, the importance weights for slope angle, land-use, elevation, dip direction, water conditions and weathering depth were determined as 45.2%, 22.4%, 12.5%, 8.8%, 8.1% and 3.0% respectively. Also, using these weights and the membership values of each conditioning factor, the membership value for landslide susceptibility was introduced. In the study area, the lowest membership value for landslide susceptibility was calculated as 0.20. Consequently, combining all results, a landslide susceptibility map was obtained. Compared with the obtained map, a great majority of the landslides (86 %) identified in the field were found to be located in susceptible and highly susceptible zones.     

The effect of fault relay and clay smearing on groundwater flow patterns in the Lower Rhine Embayment

Faults strongly impact groundwater flow in the unconsolidated sediments of the Lower Rhine Embayment. Hydraulic head maps show that many individual faults form a barrier to fluid flow whereas relay structures in these faults are sites of hydraulic contact between otherwise separated aquifers. The fluid flow patterns around the Rurrand Fault close to the largest open‐pit mine in the Lower Rhine Embayment is one of the first well‐documented examples of fluid flow around a fault relay zone. The effect of clay smearing could be quantified using the Shale Gouge Ratio (SGR) method that is common in hydrocarbon‐related studies but has not been applied to groundwater flow data so far. The effect of fault relay zones on groundwater flow is analysed using numerical simulations. It is concluded that fault relay needs special consideration in the evaluation of the sealing capacities of faults in sedimentary basins. Moreover, it is demonstrated that the SGR methodology is a promising tool for the estimation of fault zone hydraulic properties in hydrogeological modelling.     

不平地形条件下高密度电阻率法的2.5维反演

讨论了不平地形条件下电阻率法2.5维正演和反演的算法,并在此基础上编制了高密度电阻率法2.5维反演程序,该程序可用于九种常用电极装置观测结果的反演.对理论和实测数据的反演结果表明,本文的算法正确,程序运行稳定,反演效果很好.     

山东新泰太古代层状硫化物中的硫同位素非质量分馏效应

物理、化学和生物作用引起的热力学和巩俐学同位素分馏均为与质量有关的分馏。非质量同位素分馏效应应是由一些特殊的气相光化学反应引起的，它可以解释一些元素浓度，甚至单个同位素比值无法揭示的特殊的作用过程。笔者首次在山东新泰地区新太古代层状硫化物中观测到了明显的硫同位素非质量分馏效应。δ33S、δ34S呈线性排列，且与硫同位素质量分馏线平行，33S小于0。     

Economic Hotspots: Visualizing Vulnerability to Flooding

We simulate a large-scale flooding in the province of South-Holland in the economic centre of the Netherlands. In traditional research, damage due to flooding is computed with a unit loss method coupling land use information to depth-damage functions. Normally only direct costs are incorporated as an estimate of damage to infrastructure, property and business disruption. We extend this damage concept with the indirect economic effects on the rest of the regional and national economy on basis of a bi-regional input output table.We broaden this damage estimation to the concept of vulnerability. Vulnerability is defined as a function of dependence, redundancy and susceptibility. Susceptibility is the probability and extent of flooding. Dependency is the degree to which an activity relates to other economic activities in the rest of the country. Input–output multipliers form representations of this dependency. Redundancy is the ability of an economic activity to respond to a disaster by deferring, using substitutes or relocating. We measure redundancy as the degree of centrality of an economic activity in a network. The more central an activity is, the less it encounters possibilities to transfer production and the more vulnerable it is for flooding. Vulnerability of economic activities is then visualized in a GIS. Kernel density estimation is applied to generalize point information on inundated firms to sectoral information in space. We apply spatial interpolation techniques for the whole of the province of South-Holland. Combining information of sectoral data on dependency and redundancy, we are able to create maps of economic hotspots. Our simulation of a flood in the centre of Holland reveals the vulnerability of a densely populated delta.