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Interval Used (interval + used)

Distribution by Scientific Domains

Earth and Environmental Science	50%

Selected Abstracts

Scaling analysis of water retention curves for unsaturated sandy loam soils by using fractal geometry

EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2010
C. Fallico
Fractal geometry was deployed to analyse water retention curves (WRC). The three models used to estimate the curves were the general pore-solid fractal (PSF) model and two specific cases of the PSF model: the Tyler & Wheatcraft (TW) and the Rieu & Sposito (RS) models. The study was conducted on 30 undisturbed, sandy loam soil samples taken from a field and subjected to laboratory analysis. The fractal dimension, a non-variable scale factor characterizing each water retention model proposed, was estimated by direct scaling. The method for determining the fractal dimension proposed here entails limiting the analysis to the interval between an upper and lower pressure head cut-off on a log-log plot, and defining the dimension itself as the straight regression line that interpolates the points in the interval with the largest coefficient of determination, R2. The scale relative to the cut-off interval used to determine the fractal behaviour in each model used is presented. Furthermore, a second range of pressure head values was analysed to approximate the fractal dimension of the pore surface. The PSF model exhibited greater spatial variation than the TW or RS models for the parameter values typical of a sandy loam soil. An indication of the variability of the fractal dimension across the entire area studied is also provided. [source]

Complex attributes of the magnetic signal for multiple sources: Application to signals from buried ditches

ARCHAEOLOGICAL PROSPECTION, Issue 2 2010
Christian M. Milea
Abstract Complex attributes of the magnetic signal are computed using a multiple source approach. Polygonal bodies are considered and the attributes of each corner are summed to produce the overall response; that is a decomposition of the polygonal body into magnetized steps to simulate the vertices. The method is tested on synthetic examples of buried ditches, as well as on real magnetic data. This type of target was selected because it comprises a common objective in ,archaeological geophysics'. The resolution of the method in detecting the edges of the buried ditches depends on their lateral dimensions with respect to the sampling interval used. In general, the signal of the shallowest edges of the buried targets obscures the signal from the deeper corners, thus prohibiting their detection. In some extreme cases, the signals from the deeper corners posed a recognizable signature. The analytic signal may be seen as an anomaly rectification technique. In this context, it is suitable for the presentation of ,archaeological prospection' data since it provides an image that resembles the plane view of the buried antiquities better than the total field anomaly itself. Copyright © 2010 John Wiley & Sons, Ltd. [source]

Systematic and statistical error in histogram-based free energy calculations

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2003
Mark N. Kobrak
Abstract A common technique for the numerical calculation of free energies involves estimation of the probability density along a given coordinate from a set of configurations generated via simulation. The process requires discretization of one or more reaction coordinates to generate a histogram from which the continuous probability density is inferred. We show that the finite size of the intervals used to construct the histogram leads to quantifiable systematic error. The width of these intervals also determines the statistical error in the free energy, and the choice of the appropriate interval is therefore driven by the need to balance the two sources of error. We present a method for the construction of the optimal histogram for a given system, and show that the use of this technique requires little additional computational expense. We demonstrate the efficacy of the technique for a model system, and discuss how the principles governing the choice of discretization interval could be used to improve extended sampling techniques. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1437,1446, 2003 [source]

Underestimation of Pacing Threshold as Determined by an Automatic Ventricular Threshold Testing Algorithm

PACING AND CLINICAL ELECTROPHYSIOLOGY, Issue 9 2006
WILLIAM H. SAUER
In this case report, we describe markedly different pacing thresholds determined by a manual threshold test and the automatic Ventricular Capture Management algorithm. The discrepancy in pacing threshold values reported was due to the difference in the AV intervals used with the different testing methods. We propose that the differences in right ventricular dimensions with altered diastolic filling periods affected the threshold in this patient with a new passive fixation lead in the right ventricular apex. [source]