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991.
992.
Digital Image Based Approach for Three-Dimensional Mechanical Analysis of Heterogeneous Rocks 总被引:2,自引:0,他引:2
Summary This paper presents a digital image based approach for three-dimensional (3-D) numerical simulation and failure analysis of
rocks by taking into account the actual 3-D heterogeneity. Digital image techniques are adopted to extract two-dimensional
(2-D) material heterogeneity from material surface images. The 2-D image mesostructures are further extrapolated to 3-D cuboid
mesostructures by assuming the material surface as a representation of the inner material heterogeneity within a very small
depth. The iterative milling and scanning system is set up to generate the 3-D rock mesostructures. A Hong Kong granite specimen
is used as an example to demonstrate the procedure of 3-D mesostructure establishment. The mechanical responses and failure
process under the conventional Brazilian tensile test condition are examined through numerical analyses. The stress distribution,
crack propagation process and failure model of heterogeneous material cases are simulated with a finite difference software.
The numerical results indicate that material heterogeneity plays an important role in determining the failure behavior of
rocks under external loading. 相似文献
993.
Application of horizontal-to-vertical (H/V) Fourier spectral ratio for analysis of site effect on rock (NEHRP-class B) sites in Taiwan 总被引:1,自引:0,他引:1
Vladimir Yu. Sokolov Chin-Hsiung Loh Wen-Yu Jean 《Soil Dynamics and Earthquake Engineering》2007,27(4):314-323
The frequency-dependent amplification for rock (NEHRP-class B) sites was studied using earthquake ground-motion database collected in Taiwan during implementation of the Taiwan Strong Motion Instrumentation Program. The database used includes several hundred records from earthquakes of ML 4.0–7.3 occurred between 1993 and 2004. The characteristics of amplification were evaluated using the well-known technique of horizontal-to-vertical Fourier spectral ratio (H/V) of the S-wave phase [Lermo J, Chavez-Garcia FJ. Site effect evaluation using spectral ratios with only one station. Bull Seism Soc Am 1993;83:1574–94]. The study allows us to analyze peculiarities of rock sites amplification in Northern and Eastern Taiwan. It was suggested to divide the NEHRP-class B site amplification into four types based on frequency of maximum amplification and the shape of amplification function. The applicability of the technique was also checked for a few stiff and soft soil sites (NEHRP-classes D and E). 相似文献
994.
A formula for computing the gravity disturbance from the second radial derivative of the disturbing potential 总被引:6,自引:0,他引:6
J. Li 《Journal of Geodesy》2002,76(4):226-231
A formula for computing the gravity disturbance and gravity anomaly from the second radial derivative of the disturbing potential
is derived in detail using the basic differential equation with spherical approximation in physical geodesy and the modified
Poisson integral formula. The derived integral in the space domain, expressed by a spherical geometric quantity, is then converted
to a convolution form in the local planar rectangular coordinate system tangent to the geoid at the computing point, and the
corresponding spectral formulae of 1-D FFT and 2-D FFT are presented for numerical computation.
Received: 27 December 2000 / Accepted: 3 September 2001 相似文献
995.
H. Nahavandchi 《Journal of Geodesy》2002,76(6-7):345-352
It is suggested that a spherical harmonic representation of the geoidal heights using global Earth gravity models (EGM) might
be accurate enough for many applications, although we know that some short-wavelength signals are missing in a potential coefficient
model. A `direct' method of geoidal height determination from a global Earth gravity model coefficient alone and an `indirect'
approach of geoidal height determination through height anomaly computed from a global gravity model are investigated. In
both methods, suitable correction terms are applied. The results of computations in two test areas show that the direct and
indirect approaches of geoid height determination yield good agreement with the classical gravimetric geoidal heights which
are determined from Stokes' formula. Surprisingly, the results of the indirect method of geoidal height determination yield
better agreement with the global positioning system (GPS)-levelling derived geoid heights, which are used to demonstrate such
improvements, than the results of gravimetric geoid heights at to the same GPS stations. It has been demonstrated that the
application of correction terms in both methods improves the agreement of geoidal heights at GPS-levelling stations. It is
also found that the correction terms in the direct method of geoidal height determination are mostly similar to the correction
terms used for the indirect determination of geoidal heights from height anomalies.
Received: 26 July 2001 / Accepted: 21 February 2002 相似文献
996.
Yifei Sun 《Journal of Geographical Systems》2002,4(4):359-370
Based on a four-point evaluation system consisting of accuracy, consistency, power, and chance to commit type I errors, this study compares Tango's minimum p (MinP) and Stone's maximum relative risk (MaxRR) methods for detecting focused cluster size through simulations in GIS. It
reveals that the MinP method is more effective than the MaxRR method. The MinP method exhibits higher levels of accuracy and
consistency; and its power and chance to commit type I errors are similar to the MinP method. The MaxRR method has a tendency
to underestimate the cluster size, while the MinP method tends to overestimate the cluster size, particularly when the clusters
are relatively big and have high relative risk levels. In addition, the MinP method seems to be most effective in revealing
the size of clusters when clusters are neither too strong nor too weak. The lowest detection rates for clustering occur when
the clustering signal is relatively weak, which is easily understandable. In practice, it might be useful to use both the
methods to estimate a range of possible cluster sizes, where the MaxRR method indicates the lower estimate, while the MinP
method gives the higher estimate of the cluster size.
Received: 24 August 2002 / Accepted: 20 December 2002 相似文献
997.
Stochastic assessment of GPS carrier phase measurements for precise static relative positioning 总被引:17,自引:11,他引:17
Global positioning system (GPS) carrier phase measurements are used in all precise static relative positioning applications.
The GPS carrier phase measurements are generally processed using the least-squares method, for which both functional and stochastic
models need to be carefully defined. Whilst the functional model for precise GPS positioning is well documented in the literature,
realistic stochastic modelling for the GPS carrier phase measurements is still both a controversial topic and a difficult
task to accomplish in practice. The common practice of assuming that the raw GPS measurements are statistically independent
in space and time, and have the same accuracy, is certainly not realistic. Any mis-specification in the stochastic model will
inevitably lead to unreliable positioning results. A stochastic assessment procedure has been developed to take into account
the heteroscedastic, space- and time-correlated error structure of the GPS measurements. Test results indicate that the reliability
of the estimated positioning results is improved by applying the developed stochastic assessment procedure. In addition, the
quality of ambiguity resolution can be more realistically evaluated.
Received: 13 February 2001 / Accepted: 3 September 2001 相似文献
998.
Prediction of surface horizontal displacements, and gravity and tilt changes caused by filling the Three Gorges Reservoir 总被引:11,自引:0,他引:11
Horizontal displacements, and gravity and tilt changes induced by filling the Three Gorges Reservoir are modeled using elastic
loading Green functions. When the water surface reaches its highest level, the effects become maximum on the reservoir banks.
The longitudinal and latitudinal components of the horizontal displacements reach −8.2 and 7.7 mm respectively, gravity is
increased by up to 3.4 mGal, and the prime vertical and meridian components of the tilt changes are −7.8 and −17.5 arcseconds
respectively. Accordingly, the filling of the reservoir will influence values observed from global positioning system (GPS),
gravimetry and tilt measurements in the area. The results given can be used to provide important corrections for extracting
earthquake-related signals from observed data.
Received: 19 January 2001 / Accepted: 3 September 2001 相似文献
999.
Eric J.E. Molin Harmen Oppewal Harry J.P. Timmermans 《Journal of Geographical Systems》2002,4(4):343-358
In this paper, two approaches for measuring residential group preferences, based on the method of Hierarchical Information
Integration (HII), are compared. In particular, the hypothesis that group-based preference models estimated from integrated
HII experiments better predict group preferences than part individual-based group models estimated from classical HII experiments
is tested. To that effect, the models' ability to predict group preferences for new residential alternatives is compared in
a study of residential preferences of co-ops. Results indicate that integrated HII group experiments indeed result in better
predictions of residential preferences. 相似文献
1000.
Geostatistical incorporation of spatial coordinates into supervised classification of hyperspectral data 总被引:1,自引:0,他引:1
P. Goovaerts 《Journal of Geographical Systems》2002,4(1):99-111
This paper presents a methodology to incorporate both hyperspectral properties and spatial coordinates of pixels in maximum
likelihood classification. Indicator kriging of ground data is used to estimate, for each pixel, the prior probabilities of
occurrence of classes which are then combined with spectral-based probabilities within a Bayesian framework. In the case study
(mapping of in-stream habitats), accounting for spatial coordinates increases the overall producer's accuracy from 85.8% to
93.8%, while the Kappa statistic rises from 0.74 to 0.88. Best results are obtained using only indicator kriging-based probabilities,
with a stunning overall accuracy of 97.2%. Significant improvements are observed for environmentally important units, such
as pools (Kappa: 0.17 to 0.74) and eddy drop zones (Kappa: 0.65 to 0.87). The lack of benefit of using hyperspectral information
in the present study can be explained by the dense network of ground observations and the high spatial continuity of field
classification which might be spurious.
Received: 12 April 2001 / Accepted: 7 September 2001 相似文献