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991.
Using Field-Portable XRF to Assess Geochemical Variations Within and Between Dolerite Outcrops of Preseli, South Wales 总被引:1,自引:0,他引:1
Michael C. Jones Olwen Williams-Thorpe Philip J. Potts Peter C. Webb 《Geostandards and Geoanalytical Research》2005,29(3):251-269
Eight dolerite outcrops in Preseli, south Wales were measured in situ using field-portable XRF analysis, in order to compare two different analysis ("sampling") strategies, and to investigate geochemical variability within and between outcrops. A sampling strategy of two (neighbouring but independent) measurements at each of a maximal number of locations dispersed over an outcrop was the more effective in indicating the overall chemical variance of that outcrop. Analysis of variance indicated that much of the observed variance within individual outcrops stems from real geochemical variability rather than from sampling and analytical factors. Standard ANOVA F tests showed that several of the studied outcrops are heterogeneous at the 5% significance level for one or more elements. Geochemical distinctions between some outcrops were demonstrated using discriminant analysis. PXRF analysis offers an alternative approach to conventional characterisation of outcrops, which is often based on laboratory analysis of small numbers of samples. However, PXRF data are affected by rock weathering and may require correction for this if they are to be compared with analyses of fresh rock. 相似文献
992.
The misorientation index: Development of a new method for calculating the strength of lattice-preferred orientation 总被引:5,自引:4,他引:5
Using orientation data from experimentally deformed olivine, we explore some practical problems with the J-index, a commonly applied measure of fabric strength. We show that the J-index is highly dependent on several factors, including the number of discrete data in the orientation distribution function (ODF), and arbitrary numerical parameters specified for its calculation. Because of this non-uniqueness, we conclude that the J-index is difficult to interpret and should only be applied with caution. As an alternative to the J-index, we propose a new measure of fabric strength that is based on the distribution of uncorrelated misorientation angles. This “M-index” is shown to be insensitive to the parameters specified for its calculation. For typical deformed olivine samples, we show that 150 discrete data are adequate to quantify fabric strength using the M-index technique. The M-index correlates well with seismic anisotropy, particularly for materials of the same fabric type. Therefore, we conclude that the M-index technique is well-suited for the quantification of fabric strength and the comparison of like materials. 相似文献
993.
994.
995.
A non-linear two-dimensional vertically stratified cross-sectional model of a constant depth basin without rotation is used
to investigate the influence of vertical and horizontal diffusion upon the wind-driven circulation in the basin and the associated
temperature field. The influence of horizontal grid resolution, in particular the application of an irregular grid with high
resolution in the coastal boundary layer is examined. The calculations show that the initial response to a wind impulse is
downwelling at the downwind end of the basin with upwelling and convective mixing at the opposite end. Results from a two-layer
analytical model show that the initial response is the excitation of an infinite number of internal seiche modes in order
to represent the initial response which is confined to a narrow near coastal region. As time progresses, at the downwind end
of the basin a density front propagates away from the boundary, with the intensity of its horizontal gradient and associated
vertical velocity determined by both horizontal and vertical viscosity values. Calculations demonstrate the importance of
high horizontal grid resolution in resolving this density gradient together with an accurate density advection scheme. The
application of an irregular grid in the horizontal with high grid resolution in the nearshore region enables the initial response
to be accurately reproduced although physically unrealistic short waves appear as the frontal region propagates onto the coarser
grid. Parameterization of horizontal viscosity using a Smagorinsky-type formulation acts as a selective grid size-dependent
filter, and removes the short-wave problem although enhanced smoothing can occur if the scaling coefficient in the formulation
is too large. Calculations clearly show the advantages of using an irregular grid but also the importance of using a grid
size-dependent filter to avoid numerical problems. 相似文献
996.
Ned Philip Smith 《Ocean Dynamics》2005,55(5-6):449-457
Current meter data from a 411-day study are used to characterize the movement of water through Tavernier Creek, a tidal channel
in the Upper Florida Keys that connects northeastern Florida Bay with the narrow continental shelf on the Atlantic Ocean side
of the Keys. The record reveals active tidal and nontidal exchanges. Strongest flood and ebb current speeds commonly reach
50 cm s−1. Low-frequency exchanges are highly coherent with the across-shelf component of local wind stress over time scales in excess
of 2.5 days. Bay-shelf exchanges are investigated in four ways. Current measurements made while a drogue was tracked from
one end of the creek to the other provide a relationship that can be applied to the time series of current meter data. Results
suggest that ocean water reaches the bay end of the creek on 92% of the floods, and bay water reaches the ocean end on 94%
of the ebbs. The Eulerian tidal excursion calculated from the amplitude of the M
2 tidal constituent is 1.42 times the length of the channel, and half-tidal cycle Eulerian displacements are commonly 1.5 times
the length of the channel. Salinity measurements over a 165-day period document the arrival of bay and ocean water at a study
site at about the midpoint of Tavernier Creek. Results suggest that the creek becomes completely flushed after about 450,000 m3 of water have entered from either end. Histograms of ebb and flood volume transports indicate that half-tidal cycle transports
are commonly between 800,000 and 1,100,000 m3. The long-term movement of water through Tavernier Creek is a net outflow from Florida Bay. Results support the idea that
Tavernier Creek serves as an effective conduit for exchanging bay and ocean water, and especially for draining the northeast
corner of Florida Bay. 相似文献
997.
Anaerobic Biodegradation of MTBE at a Gasoline Spill Site 总被引:4,自引:0,他引:4
998.
Cross-matching with interpreted warping of 3D streamer and 3D ocean-bottom-cable data at Valhall for time-lapse assessment 总被引:1,自引:0,他引:1
Stephen A. Hall Colin MacBeth Olav I. Barkved Philip Wild 《Geophysical Prospecting》2005,53(2):283-297
Legacy streamer data and newer 3D ocean‐bottom‐cable data are cross‐matched and analysed for time‐lapse analysis of geomechanical changes due to production in the Valhall Field. The issues relating to time‐lapse analysis using two such distinctly different data sets are addressed to provide an optimal cross‐matching workflow that includes 3D warping. Additionally an assessment of the differences between the imaging using single‐azimuth streamer and multi‐azimuth ocean‐bottom‐cable data is provided. The 3D warping utilized in the cross‐matching procedure is sensitive to acquisition and processing differences but is also found to provide valuable insight into the geometrical changes that occur in the subsurface due to production. As such, this work also provides a demonstration of the use of high‐resolution 3D interpreted warping to resolve the 3D heterogeneity of the compaction and subsidence. This is an important tool for Valhall, and possibly other fields, where compaction and subsidence (and monitoring thereof) are key factors in the reservoir management since the predominant observed production‐induced changes are compaction of the soft, high‐porosity chalk reservoir, due to pore‐pressure reduction, and the resultant overburden subsidence. Such reservoir compaction could have significant implications for production by changing permeabilities and production rates. Furthermore the subsidence effects could impact upon subsea installations and well‐bore stability. Geomechanical studies that have previously been used to model such subsidence and compaction are only constrained by observed surface displacements and measured reservoir pressure changes, with the geological overburden being largely neglected. The approaches suggested herein provide the potential for monitoring and assessment in three dimensions, including the probable heterogeneity and shearing, that is needed for full understanding of reservoir compaction and the resultant effects on the overburden to, for example, mitigate well‐bore failures. 相似文献
999.
1000.
The Mackenzie Delta: sedimentary processes and facies of a high-latitude, fine-grained delta 总被引:2,自引:0,他引:2
Philip R. Hill C. Peter Lewis Simon Desmarais Vassen Kauppaymuthoo & Hasna Rais 《Sedimentology》2001,48(5):1047-1078
The Mackenzie Delta is a large fine‐grained delta deposited in a cold arctic setting. The delta has been constructed upon a flooding surface developed on a previous shelf‐phase delta. There are three principal depositional zones: the subaerial delta plain, the distributary channel mouth region and the subaqeous delta. The subaerial delta plain is characterized by an anastomosing system of high‐sinuosity channels and extensive thermokarst lake development. This region is greatly influenced by the annual cycle of seasonal processes including winter freezing of sediments and channels, ice‐jamming and flooding in the early spring and declining river stage during the summer and autumn. Deposition occurs on channel levees and in thermokarst lakes during flood events and is commonly rhythmic in nature with discrete annual beds being distinguishable. In the channel mouth environment, deposition is dominated by landward accretion and aggradation of mouth bars during river‐ and storm surge‐induced flood events. The subaqeous delta is characterized by a shallow water platform and a gentle offshore slope. Sediment bypassing of the shallow‐water platform is efficient as a result of the presence of incised submarine channels and the predominance of suspension transport of fine‐grained sediments. Facies of the shallow platform include silty sand with climbing ripple lamination. Offshore facies are dominated by seaward‐fining fine sand to silt tempestites. Sea‐ice scouring and sediment deformation are common beyond 10 m water depth where bioturbated muds are the predominant facies. The low angle profile of the shallow‐water platform is interpreted to be the combined response of a fine‐grained delta to (1) storm sediment dispersal; (2) autoretreat as a result of the increasing subaerial and subaqeous area of deposition as the delta progrades out of its glacial valley; (3) limited water depth above the underlying flooding surface; and (4) efficient nearshore bypassing of sediment through subice channels at the peak of spring discharge. Several indicators of the cold climate can be used as criteria for the interpretation of ancient successions, including thermokarst lake development, submarine channel scours, freeze–thaw deformation and ice‐scour deformation structures. Permafrost inhibits compaction subsidence and, together with the shallow‐water setting, also limits autocyclic lobe switching. The cold climate can thus influence stratal architecture by favouring the development of regional‐scale clinoform sets rather than multiple, smaller scale lobes separated by autocyclic flooding surfaces. 相似文献