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111.
We have carried out numerical simulations of three-dimensional nonisothermal flow around an in situ heat-based flow sensor to investigate how formation heterogeneities can affect the interpretation of ground water flow velocities from this instrument. The flow sensor operates by constant heating of a 0.75-m-long, 5-cm-diameter cylindrical probe, which contains 30 thermistors in contact with the formation. The temperature evolution at each thermistor can be inverted to obtain an estimate of the ground water flow velocity vector using the standard interpretive method, which assumes that the formation is homogeneous. Analysis of data from heat-based flow sensors installed in a sand aquifer at the Former Fort Ord Army Base near Monterey, California, suggested an unexpected component of downward flow. The magnitudes of the vertical velocities were expected to be much less than those of the horizontal velocities at this site because the sensors were installed just above a clay aquitard. Numerical simulations were conducted to examine how differences in thermal conductivities may lead to spurious indications of vertical flow velocities. We found that a decrease in the thermal conductivity near the bottom of the sensor can perturb the temperature profiles along the instrument in such a manner that analyses assuming homogeneous thermal conductivity could indicate a vertical flow component even though flow is actually horizontal. This work demonstrates how modeling can be used to simulate instrument response to formation heterogeneity and shows that caution must be used in interpreting data from such devices. 相似文献
112.
Percolation-theory and fuzzy rule-based probability estimation of fault leakage at geologic carbon sequestration sites 总被引:1,自引:0,他引:1
Yingqi Zhang Curtis M. Oldenburg Stefan Finsterle 《Environmental Earth Sciences》2010,59(7):1447-1459
Leakage of CO2 and displaced brine from geologic carbon sequestration (GCS) sites into potable groundwater or to the near-surface environment
is a primary concern for safety and effectiveness of GCS. The focus of this study is on the estimation of the probability
of CO2 leakage along conduits such as faults and fractures. This probability is controlled by (1) the probability that the CO2 plume encounters a conductive fault that could serve as a conduit for CO2 to leak through the sealing formation, and (2) the probability that the conductive fault(s) intersected by the CO2 plume are connected to other conductive faults in such a way that a connected flow path is formed to allow CO2 to leak to environmental resources that may be impacted by leakage. This work is designed to fit into the certification framework
for geological CO2 storage, which represents vulnerable resources such as potable groundwater, health and safety, and the near-surface environment
as discrete “compartments.” The method we propose for calculating the probability of the network of conduits intersecting
the CO2 plume and one or more compartments includes four steps: (1) assuming that a random network of conduits follows a power-law
distribution, a critical conduit density is calculated based on percolation theory; for densities sufficiently smaller than
this critical density, the leakage probability is zero; (2) for systems with a conduit density around or above the critical
density, we perform a Monte Carlo simulation, generating realizations of conduit networks to determine the leakage probability
of the CO2 plume (P
leak) for different conduit length distributions, densities and CO2 plume sizes; (3) from the results of Step 2, we construct fuzzy rules to relate P
leak to system characteristics such as system size, CO2 plume size, and parameters describing conduit length distribution and uncertainty; (4) finally, we determine the CO2 leakage probability for a given system using fuzzy rules. The method can be extended to apply to brine leakage risk by using
the size of the pressure perturbation above some cut-off value as the effective plume size. The proposed method provides a
quick way of estimating the probability of CO2 or brine leaking into a compartment for evaluation of GCS leakage risk. In addition, the proposed method incorporates the
uncertainty in the system parameters and provides the uncertainty range of the estimated probability. 相似文献
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Richard W. Battarbee Gavin L. Simpson Helen Bennion Christopher Curtis 《Journal of Paleolimnology》2011,45(4):489-505
This paper has two aims: (1) to show for the first time how a natural typology can be established using palaeoecological methods;
and (2) to show how it can be used in lake restoration studies with respect to the definition of recovery targets for acidified
lakes. By defining the characteristic reference assemblages for low alkalinity site types rather than for a specific site
it allows success to be measured more broadly, unconstrained by the specific composition of the pre-acidification flora. We
analyse statistically the pre-acidification diatom assemblages of sediment cores from 121 low alkalinity lakes in the UK in
order to assess whether a reference typology for such lakes can be defined on the basis of their diatom floras. We use samples
dating to ~1850 AD to represent pre-acidification conditions. The results show that three main clusters can be identified,
two dominated by benthic taxa (Clusters 1 and 3) and one dominated by planktonic taxa (Cluster 2). Cluster 1 is characterised
by taxa such as Brachysira vitrea, Cymbella microcephala and Fragilaria spp., Cluster 2 by Cyclotella comensis, C. radiosa, Asterionella formosa, Aulacoseira subarctica and Achnanthes minutissima and Cluster 3 by Eunotia incisa, Frustulia rhomboides var. saxonica, Fragilaria virescens var. exigua, and Cymbella perpusilla. Although environmental data for 1850 AD are not available it is apparent from the contemporary distribution of the taxa
in the different clusters that Cluster 2 represents the most alkaline pre-acidification conditions. Some sites in this cluster
have been acidified, but some, especially the larger, deeper lakes have been enriched. Cluster 1 includes sites that contain
diatoms with relatively high pH optima (pH 6.4–7.4) whereas Cluster 3 sites contain diatoms with the lowest pre-acidification
pH conditions in the data-set. Sites in this cluster also have the lowest base cation concentrations at the present day and
include the sites in the UK that have been most affected by acid deposition. 相似文献
117.
118.
Curtis D.W. Berg P. Gordon D. Harvey P.R. Smith D.M. Zehnder A. 《Solar physics》2002,210(1-2):115-124
The Ramaty High-Energy Spectroscopic Imager (RHESSI) spacecraft is a NASA Small Explorer (SMEX) class mission. RHESSI is designed
to image solar X-rays and gamma rays with high-energy resolution. The Instrument Data Processing Unit (IDPU) serves as the
central RHESSI instrument on-board data-processing element. It controls and monitors the instrument operations, and provides
a flexible telemetry collection and formatting system. The system responds autonomously to optimize science data collection
over a wide dynamic range of conditions, handling up to 40 Mbps of telemetry during solar flares. This paper presents an overview
of the IDPU hardware and software design. 相似文献
119.
Curtis J. Saxton Kinwah Wu João B. G. Canalle Mark Cropper Gavin Ramsay 《Monthly notices of the Royal Astronomical Society》2007,379(2):779-790
We investigate the hydrodynamics of accretion channelled by a dipolar magnetic field (funnel flows). We consider situations in which the electrons and ions in the flow cannot maintain thermal equilibrium [two-temperature (2T) effects] due to strong radiative loss, and determine the effects on the keV X-ray properties of the systems. We apply this model to investigate the accretion shocks of white dwarfs in magnetic cataclysmic variables (mCVs). We have found that the incorporation of 2T effects could harden the keV X-rays. Also, the dipolar model yields harder X-ray spectra than the standard planar model if white dwarf is sufficiently massive (≳1 M⊙ ) . When fitting observed keV X-ray spectra of mCVs, the inclusion of 2T hydrodynamics and a dipolar accretion geometry lowers estimates for white dwarf masses when compared with masses inferred from models excluding these effects. We find mass reductions ≲9 per cent in the most massive cases. 相似文献
120.
Cooper KM Curtis M Wan Hussin WM Barrio Froján CR Defew EC Nye V Paterson DM 《Marine pollution bulletin》2011,62(10):2087-2094
A meta-analysis approach was used to assess the effect of dredging induced changes in sediment composition, under different conditions of natural physical disturbance, for the structure and function of marine benthic macrofaunal communities. Results showed the sensitivity of macrofaunal communities increased as both the proportion of gravel increased and the level of natural physical disturbance decreased. These findings may be explained by the close association of certain taxa with the gravel fraction, and the influence of natural physical disturbance which, as it increases, tends to restrict the colonisation by these species. We conclude that maintaining the gravel content of surface sediments after dredging and, where practicable, locating extraction sites in areas of higher natural disturbance will minimise the potential for long-term negative impacts on the macrofauna. 相似文献