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The 2-km deep Athboy Borehole (1439/2) together with the lower part of boreholes EP30 and N915 form a standard type section for strata of Dinantian (Courceyan to Asbian) age in west Co. Meath. Above a thin basal red-bed siliciclastic sequence, the marine Courceyan shelf succession is almost 600 m thick. It comprises the Liscartan, Meath, and Moathill Formations of the Navan Group and the Slane Castle Formation of the succeeding Boyne Group. The shallow-water limestones include micrites, oolites, and sandy bioclastic packstones and grainstones with subordinate skeletal wackestones and shales. Lateral facies changes from north to south in the Navan area suggest deepening across a shelf towards a depocentre further to the south around Trim. The deeper-water Waulsortian Limestones of late Courceyan to Chadian age (Feltrim Formation, ca. 213 m thick) form a series of five sheet-like mudbanks, interbedded with generally thin units of nodular crinoidal limestones and shales. The mudbanks are formed of bryozoan-rich peloidal wackestones and lime-mudstones with phase C and D components. Rare soft-sediment breccias occur at the bottom and top of banks. The succeeding Fingal Group commences with a thin interval (3–20 m) of black shales, laminated packstones, and micritic limestones of Chadian age, the Tober Colleen Formation. This is followed by the Lucan Formation (Chadian to Asbian) predominantly of laminated and graded calciturbidites, laminated sandstones, cherts, and black shales, which is over 1300 m thick. Ten sedimentary units have been informally defined, based on lithofacies and facies associations. The oldest unit, the Tara Member, is characterized by proximal debris-flow breccia deposits and nodular mudstones. A thick bioturbated micrite and shale unit (Ardmulchan Member) in the middle of the formation is overlain directly by a coarse oolitic and crinoidal grainstone unit (Beauparc Member). Near the top of the formation is a distinctive unit of coarse-grained laminated sandstones and shales (Athboy Member). The highest rocks in the Borehole are clean thickly-bedded limestones of the Asbian Naul Formation (>90 m thick). The youngest Dinantian strata in the area, the Brigantian Loughshinny Formation, marks a return to shale-dominant basin sedimentation. The significance of this work lies in the fact that the Athboy borehole is the longest continuously cored borehole in the Carboniferous of Ireland and provides a continuous sedimentary and biostratigraphic record for the northern part of the Dublin Basin. Foraminiferal biozones (Cf2–Cf6) have been recognized in this and in borehole N915, and Stage boundaries identified, which can be applied throughout the Basin. The sedimentary record for the Lucan Formation indicates four tectonic pulses during the Viséan, in the late Chadian/early Arundian, mid-Arundian, Holkerian, and late Holkerian/early Asbian. 相似文献
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Investigating a time‐shift extended imaging condition in a Kirchhoff pre‐stack depth migration algorithm 下载免费PDF全文
Gareth Shane O'Brien Sean Joseph Delaney Michael Igoe John Doherty Andrew Colhoun 《Geophysical Prospecting》2018,66(4):688-706
Extracting true amplitude versus angle common image gathers is one of the key objectives in seismic processing and imaging. This is achievable to different degrees using different migration techniques (e.g., Kirchhoff, wavefield extrapolation, and reverse time migration techniques) and is a common tool in exploration, but the costs can vary depending on the selected migration algorithm and the desired accuracy. Here, we investigate the possibility of combining the local‐shift imaging condition, specifically the time‐shift extended imaging condition, for angle gathers with a Kirchhoff migration. The aims are not to replace the more accurate full‐wavefield migration but to offer a cheaper alternative where ray‐based methods are applicable and to use Kirchhoff time‐lag common image gathers to help bridge the gap between the traditional offset common image gathers and reverse time migration angle gathers; finally, given the higher level of summation inside the extended imaging migration, we wish to understand the impact on the amplitude versus angle response. The implementation of the time‐shift imaging condition along with the computational cost is discussed, and results of four different datasets are presented. The four example datasets, two synthetic, one land acquisition, and a marine dataset, have been migrated using a Kirchhoff offset method, a Kirchhoff time‐shift method, and, for comparison, a reverse time migration algorithm. The results show that the time‐shift imaging condition at zero time lag is equivalent to the full offset stack as expected. The output gathers are cleaner and more consistent in the time‐lag‐derived angle gathers, but the conversion from time lag to angle can be considered a post‐processing step. The main difference arises in the amplitude versus offset/angle distribution where the responses are different and dramatically so for the land data. The results from the synthetics and real data show that a Kirchhoff migration with an extended imaging condition is capable of generating subsurface angle gathers. The same disadvantages with a ray‐based approach will apply using the extended imaging condition relative to a wave equation angle gather solution. Nevertheless, using this approach allows one to explore the relationship between the velocity model and focusing of the reflected energy, to use the Radon transformation to remove noise and multiples, and to generate consistent products from a ray‐based migration and a full‐wave equation migration, which can then be interchanged depending on the process under study. 相似文献
46.
Mass exchange between debris flow and the bed plays a vital role in debris flow dynamics. Here a depth‐averaged two‐phase model is proposed for debris flows over erodible beds. Compared to previous depth‐averaged two‐phase models, the present model features a physical step forward by explicitly incorporating the mass exchange between the flow and the bed. A widely used closure model in fluvial hydraulics is employed to estimate the mass exchange between the debris flow and the bed, and an existing relationship for bed entrainment rate is introduced for comparison. Also, two distinct closure models for the bed shear stresses are evaluated. One uses the Coulomb friction law and Manning's equation to determine the solid and fluid resistances respectively, while the other employs an analytically derived formula for the solid phase and the mixing length approach for the fluid phase. A well‐balanced numerical algorithm is applied to solve the governing equations of the model. The present model is first shown to reproduce average sediment concentrations in steady and uniform debris flows over saturated bed as compared to an existing formula underpinned by experimental datasets. Then, it is demonstrated to perform rather well as compared to the full set of USGS large‐scale experimental debris flows over erodible beds, in producing debris flow depth, front location and bed deformation. The effects of initial conditions on debris flow mass and momentum gain are resolved by the present model, which explicitly demonstrates the roles of the wetness, porosity and volume of bed sediments in affecting the flow. By virtue of extended modeling cases, the present model produces debris flow efficiency that, as revealed by existing observations and empirical relations, increases with initial volume, which is enhanced by mass gain from the bed. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
47.
Thibaut Lery CÉline Combet Gareth Murphy Luke O. C. Drury 《Astrophysics and Space Science》2005,298(1-2):375-378
As network performance has outpaced computational power and storage capacity, a new paradigm has evolved to enable the sharing
of geographically distributed resources. This paradigm is known as Grid computing and aims to offer access to distributed
resource irrespective of their physical location. Many national, European and international projects have been launched during
the last years trying to explore the Grid and to change the way we are doing our everyday work. In Ireland, we have started
the CosmoGrid project that is a collaborative project aimed to provide high performance super-computing environments. This
will help to address complex problems such as magnetohydrodynamic outflows and jets in order to model and numerically simulate
them. Indeed, the numerical modeling of plasma jets requires massive computations, due to the wide range of spatial-temporal
scales involved. We present here the first jet simulations and their corresponding models that could help to understand results
from laboratory experiments. 相似文献
48.
Sub-grid scale parameterisation of 2D hydrodynamic models of inundation in the urban area 总被引:1,自引:0,他引:1
This paper presents preliminary results from a study considering the parameterisation of coarse-grid 2D flood models to take
into account sub-grid scale flow patterns occurring in the urban area. A simulation of a severe flood in an urbanized coastal
floodplain is first run using a fine grid that can resolve the flow around and between buildings. Next, the same model is
run again using the same underlying topography, although stripped from any buildings, and a set of 7 values of the roughness
parameter (Manning’s n), all larger than (or equal to) the value used in the original run. A further set of simulations is carried out using a five-fold
increased grid cell size. It is found that while it may be possible to model the overall effects of the buildings using strongly
increased roughness parameter values, using a coarse grid otherwise has implications related to the loss of information about
the site topography that results in flood flow routes being incorrectly modelled. 相似文献
49.
Gareth S. Jones Jonathan M. Gregory Peter A. Stott Simon F. B. Tett Robert B. Thorpe 《Climate Dynamics》2005,25(7-8):725-738
Volcanic ‘super-eruptions’ have been suggested to have significantly influenced the Earth’s climate, perhaps causing glaciations
and impacting on the human population. Climatic changes following a hypothetical ‘super-eruption’ are simulated using a coupled
atmosphere ocean general circulation model, incorporating scaled volcanic stratospheric aerosols. Assumptions are made about
the stratospheric sulphate aerosol loading, size distribution, lifetime, chemical make up and spatial distribution. As this
study is concentrating on the physical climatological impacts over long timescales, microphysics and chemical interactive
processes are not simulated. Near-surface temperatures fall by as much as 10 K globally for a few months and a considerable
deviation from normal temperatures continues for several decades. A warming pattern is evident over northern land masses during
the winter due to increased longwave forcing and a positive AO mode. The overturning rate of the North Atlantic thermohaline
circulation doubles in intensity. Snow and ice increases in extent to a maximum coverage of 35% of the Earth. Despite these
and other impacts longer term climatic changes that could lead to a transition to a glaciation do not occur, for present day
boundary conditions and one possible plausible aerosol loading. 相似文献
50.
This paper presents the current progress of the River Level Monitoring using GPS Heighting (RiGHt) project. The aim of the
project is to develop an integrated system to allow the continuous monitoring of river heights using a buoy equipped with
Global Positioning System (GPS) and satellite communications and using Geographical Information System (GIS) techniques. ?
2000 John Wiley & Sons, Inc. 相似文献