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831.
Christopher W. Hamilton Sarah A. Fagents Thorvaldur Thordarson 《Bulletin of Volcanology》2010,72(4):469-485
We have applied quantitative geospatial analyses to rootless eruption sites in the Hnúta and Hrossatungur groups of the 1783–1784
Laki lava flow to establish how patterns of spatial distribution can be used to obtain information about rootless cone emplacement
processes and paleo-environments. This study utilizes sample-size-dependent nearest neighbor (NN) statistics and Voronoi tessellations
to quantify the spatial distribution of rootless eruption sites and validate the use of statistical NN analysis as a remote
sensing tool. Our results show that rootless eruption sites cluster in environments with abundant lava and water resources,
but competition for limited groundwater in these clusters can cause rootless eruption sites to develop repelled distributions.
This pattern of self-organization can be interpreted within the context of resource availability and depletion. Topography
tends to concentrate lava (fuel) and water (coolant) within topographic lows, thereby promoting explosive lava–water interactions
in these regions. Given an excess supply of lava within broad sheet lobes, rootless eruption sites withdraw groundwater from
their surroundings until there is insufficient water to maintain analogs to explosive molten fuel–coolant interactions. Rootless
eruption sites may be modeled as a network of water extraction wells that draw down the water table in their vicinity. Rootless
eruptions at locations with insufficient groundwater may either fail to initiate or terminate before explosive activity has
ceased at nearby locations with a greater supply of water, thus imparting a repelled distribution to observed rootless eruption
sites. 相似文献
832.
A digital procedure for ground water recharge and discharge pattern recognition and rate estimation 总被引:3,自引:1,他引:2
A digital procedure to estimate recharge/discharge rates that requires relatively short preparation time and uses readily available data was applied to a setting in central Wisconsin. The method requires only measurements of the water table, fluxes such as stream baseflows, bottom of the system, and hydraulic conductivity to delineate approximate recharge/discharge zones and to estimate rates. The method uses interpolation of the water table surface, recharge/discharge mapping, pattern recognition, and a parameter estimation model. The surface interpolator used is based on the theory of radial basis functions with thin-plate splines. The recharge/discharge mapping is based on a mass-balance calculation performed using MODFLOW. The results of the recharge/discharge mapping are critically dependent on the accuracy of the water table interpolation and the accuracy and number of water table measurements. The recharge pattern recognition is performed with the help of a graphical user interface (GUI) program based on several algorithms used in image processing. Pattern recognition is needed to identify the recharge/discharge zonations and zone the results of the mapping method. The parameter estimation program UCODE calculates the parameter values that provide a best fit between simulated heads and flows and calibration head-and-flow targets. A model of the Buena Vista Ground Water Basin in the Central Sand Plains of Wisconsin is used to demonstrate the procedure. 相似文献
833.
834.
A method has been developed for computing the gravitational force field of an axially symmetric flat galaxy from its surface mass density. The method is simple to program, fast, and accurate. An inversion formula is derived that allows computation of surface density from rotation curves by use of any method that converts density to force. The method is compared with a method of Clutton-Brock that utilizes Hankel transforms of Laguerre functions. 相似文献
835.
Artificial neural networks (ANNs) were developed to accurately predict highly time-variable specific conductance values in an unconfined coastal aquifer. Conductance values in the fresh water lens aquifer change in response to vertical displacements of the brackish zone and fresh water-salt water interface, which are caused by variable pumping and climate conditions. Unlike physical-based models, which require hydrologic parameter inputs, such as horizontal and vertical hydraulic conductivities, porosity, and fluid densities, ANNs can "learn" system behavior from easily measurable variables. In this study, the ANN input predictor variables were initial conductance, total precipitation, mean daily temperature, and total pumping extraction. The ANNs were used to predict salinity (specific conductance) at a single monitoring well located near a high-capacity municipal-supply well over time periods ranging from 30 d to several years. Model accuracy was compared against both measured/interpolated values and predictions were made with linear regression, and in general, excellent prediction accuracy was achieved. For example, although the average percent change of conductance over 90-d periods was 39%, the absolute mean prediction error achieved with the ANN was only 1.1%. The ANNs were also used to conduct a sensitivity analysis that quantified the importance of each of the four predictor variables on final conductance values, providing valuable insights into the dynamics of the system. The results demonstrate that the ANN technology can serve as a powerful and accurate prediction and management tool, minimizing degradation of ground water quality to the extent possible by identifying appropriate pumping policies under variable and/or changing climate conditions. 相似文献
836.
Artificial neural networks (ANNs) were developed for accurately predicting potentiometric surface elevations (monitoring well water level elevations) in a semiconfined glacial sand and gravel aquifer under variable state, pumping extraction, and climate conditions. ANNs "learn" the system behavior of interest by processing representative data patterns through a mathematical structure analogous to the human brain. In this study, the ANNs used the initial water level measurements, production well extractions, and climate conditions to predict the final water level elevations 30 d into the future at two monitoring wells. A sensitivity analysis was conducted with the ANNs that quantified the importance of the various input predictor variables on final water level elevations. Unlike traditional physical-based models, ANNs do not require explicit characterization of the physical system and related physical data. Accordingly, ANN predictions were made on the basis of more easily quantifiable, measured variables, rather than physical model input parameters and conditions. This study demonstrates that ANNs can provide both excellent prediction capability and valuable sensitivity analyses, which can result in more appropriate ground water management strategies. 相似文献
837.
838.
T. T. Hamilton 《Astrophysics and Space Science》1995,231(1-2):453-456
If typical gamma-ray bursts [GRBs] have X-ray counterparts similar to those detected byGinga, then sensitive focusing X-ray telescopes will be able to detect GRBs three orders of magnitude fainter than the detection limit of the Burst and Transient Spectroscopy Experiment [BATSE]. If a substantial portion of the burst population detected byBATSE originates in a Galactic halo at distances greater than or equal to 150 kpc, existing X-ray telescopes will be able to detect GRBs in external galaxies out to a distance of at least 4.5 Mpc. As reported in Hamilton, Gotthelf and Helfand (1995) the Imaging Proportional counter [IPC] on board theEINSTEIN Observatory detected 42 transient events with pointlike spatial characteristics and timescales of less than 10 seconds. These events are distributed isotropically on the sky; in particular, they are not concentrated in the directions of nearby external galaxies. For halo models of theBATSE bursts with radii of 150 kpc or greater, we would expect to see several burst events in observations pointed towards nearby galaxies. We see none. We therefore conclude that if theGinga detections are representative of the population of GRBs sampled byBATSE, GRBs cannot originate in a Galactic halo population with limiting radii between 150 kpc and 400 kpc. Inasmuch as halos with limiting radii outside of this range have been excluded by theBATSE isotropy measurements, our result indicates that all halo models are excluded. This result is independent of whether the flashes we do detect have an astronomical origin. 相似文献
839.
Patricia?A.?TesterEmail author Sabrina?M.?Varnam Mary?E.?Culver David?L.?Eslinger Richard?P.?Stumpf Robert?N.?Swift James?K.?Yungel Megan?D.?Black R.?Wayne?Litaker 《Estuaries and Coasts》2003,26(5):1353-1364
Airborne laser-induced fluorescence measurements were used to detect and monitor ecosystem wide changes in the distribution
and concentration of chlorophyll biomass and colored dissolved organic matter in the Pamlico-Albemarle Sound system, North
Carolina, U.S., following massive flooding caused by a series of three hurricanes in the late summer of 1999. These high-resolution
data provided a significantly more detailed representation of the overall changes occurring in the system than could have
been achieved by synoptic sampling from any other platform. The response time for the distribution of chlorophyll biomass
to resume pre-flood conditions was used as a measure of ecosystem stability. Chlorophyll biomass patterns were reestablished
within four mo of the flooding, whereas higher chlorophylla biomass concentrations persisted for approximately 6 mo. The primary trophic level in the Pamlico-Albemarle Sound system
returned to equilibrium in less than a year of a major perturbation. 相似文献
840.
This paper considers the effects of natural and artificial openings of a typical, small, southern California coastal estuarine
lagoon on the adjacent barrier beach. A detailed history of beach profiles and lagoon entrance transects before and after
flood-induced and artificial openings of San Dieguito Lagoon in Del Mar, California, has been analyzed. The results suggest
that there is no statistically significant erosional effect on the adjacent beach when the lagoon inlet is artificially opened
to tidal flow. 相似文献