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991.
剪切带倾角尺度律与局部化启动跳跃稳定研究 总被引:9,自引:5,他引:9
研究了剪切带倾角是如何依赖于岩样高度以及剪切带的不稳定性。建立了准脆性材料试件剪切带倾角尺度效应模型,得到了剪切带倾角尺度效应的解析解,且与实验结果比较相符。研究结果表明:剪切带倾角随着试件高度的增加而增加,但其增加幅逐渐减缓,最终趋于稳定值。笔者还对剪应变局部化启动、跳跃和稳定进行了理论分析,解释了实验所观测到的剪切带跳跃现象和砂岩岩样应变局部化较煤样滞后的原因。剪应变局部化是否发生跳跃,关键取决于全程应力-应变曲线软化段是否存在拐点。对于没有拐点的情形,宏观剪切带图案不跳跃。局部化是导致准脆性材料试件剪切带倾角尺度效应的原因。 相似文献
992.
Giles M. Foody 《Journal of Geographical Systems》2001,3(3):217-232
Neural networks are attractive tools for the derivation of thematic maps from remotely sensed data. Most attention has focused
on the multilayer perceptron (MLP) network but other network types are available and have different properties that may sometimes
be more appropriate for some applications. Here a MLP, radial basis function (RBF) and probabilistic neural network (PNN)
were used to classify remotely sensed data of an agricultural site. The accuracy of these classifications ranged from 86.25–91.25%.
The accuracy of the PNN classification could be increased through the incorporation of prior probabilities of class membership
but the accuracy of each classification could also be degraded by the presence of an untrained class. Post-classification
analyses, however, could be used to identify potentially misclassified cases, including those belonging to an untrained class,
to increase accuracy. The effect of the post-classification analysis on the accuracy of the classification derived from each
of the three network types investigated differed and it is suggested that network type be selected carefully to meet the requirements
of the application in-hand.
Received: 23 March 2000 / Accepted: 9 July 2000 相似文献
993.
Two long time series were analysed: the C01 series of the International Earth Rotation Service and the pole series obtained
by re-analysis of the classical astronomical observations using the HIPPARCOS reference frame. The linear drift of the pole
was determined to be 3.31 ± 0.05 milliarcseconds/year towards 76.1 ± 0.80° west longitude. For the least-squares fit the a priori correlations between simultaneous pole coordinates x
p
, y
p
were taken into account, and the weighting function was calculated by estimating empirical variance components. The decadal
variations of the pole path were investigated by Fourier and wavelet analysis. Using sliding windows, the periods and amplitudes
of the Chandler wobble and annual wobble were determined. Typical periods in the variable Chandler wobble and annual wobble
parameters were obtained from wavelet analyses.
Received: 21 January 2000 / Accepted: 28 August 2000 相似文献
994.
P. J. G. Teunissen 《Journal of Geodesy》2001,75(7-8):399-407
Carrier phase ambiguity resolution is the key to fast and high-precision GNSS (Global Navigation Satellite System) kinematic
positioning. Critical in the application of ambiguity resolution is the quality of the computed integer ambiguities. Unsuccessful
ambiguity resolution, when passed unnoticed, will too often lead to unacceptable errors in the positioning results. Very high
success rates are therefore required for ambiguity resolution to be reliable. Biases which are unaccounted for will lower
the success rate and thus increase the chance of unsuccessful ambiguity resolution. The performance of integer ambiguity estimation
in the presence of such biases is studied. Particular attention is given to integer rounding, integer bootstrapping and integer
least squares. Lower and upper bounds, as well as an exact and easy-to-compute formula for the bias-affected success rate,
are presented. These results will enable the evaluation of the bias robustness of ambiguity resolution.
Received: 28 September 2000 / Accepted: 29 March 2001 相似文献
995.
Based on the Bayesian principle and the fact that GPS carrier-phase ambiguities are integers, the posterior distribution
of the ambiguities and the position parameters is derived. This is then used to derive the maximum posterior likelihood solution
of the ambiguities. The accuracy of the integer ambiguity solution and the position parameters is also studied according to
the posterior distribution. It is found that the accuracy of the integer solution depends not only on the variance of the
corresponding float ambiguity solution but also on its values.
Received: 27 July 1999 / Accepted: 22 November 2000 相似文献
996.
The Somigliana–Pizzetti gravity field (the International gravity formula), namely the gravity field of the level ellipsoid
(the International Reference Ellipsoid), is derived to the sub-nanoGal accuracy level in order to fulfil the demands of modern
gravimetry (absolute gravimeters, super conducting gravimeters, atomic gravimeters). Equations (53), (54) and (59) summarise
Somigliana–Pizzetti gravity Γ(φ,u) as a function of Jacobi spheroidal latitude φ and height u to the order ?(10−10 Gal), and Γ(B,H) as a function of Gauss (surface normal) ellipsoidal latitude B and height H to the order ?(10−10 Gal) as determined by GPS (`global problem solver'). Within the test area of the state of Baden-Württemberg, Somigliana–Pizzetti
gravity disturbances of an average of 25.452 mGal were produced. Computer programs for an operational application of the new
international gravity formula with (L,B,H) or (λ,φ,u) coordinate inputs to a sub-nanoGal level of accuracy are available on the Internet.
Received: 23 June 2000 / Accepted: 2 January 2001 相似文献
997.
A model for adjustment of differential gravity measurements with simultaneous gravimeter calibration
A mathematical model is proposed for adjustment of differential or relative gravity measurements, involving simultaneously
instrumental readings, coefficients of the calibration function, and gravity values of selected base stations. Tests were
performed with LaCoste and Romberg model G gravimeter measurements for a set of base stations located along a north–south
line with 1750 mGal gravity range. This line was linked to nine control stations, where absolute gravity values had been determined
by the free-fall method, with an accuracy better than 10 μGal. The model shows good consistence and stability. Results show
the possibility of improving the calibration functions of gravimeters, as well as a better estimation of the gravity values,
due to the flexibility admitted to the values of the calibration coefficients.
Received: 15 November 1999 / Accepted: 31 October 2000 相似文献
998.
Deriving rules from activity diary data: A learning algorithm and results of computer experiments 总被引:1,自引:0,他引:1
Theo A. Arentze Frank Hofman Harry J.P. Timmermans 《Journal of Geographical Systems》2001,3(4):325-346
Activity-based models consider travel as a derived demand from the activities households need to conduct in space and time.
Over the last 15 years, computational or rule-based models of activity scheduling have gained increasing interest in time-geography
and transportation research. This paper argues that a lack of techniques for deriving rules from empirical data hinders the
further development of rule-based systems in this area. To overcome this problem, this paper develops and tests an algorithm
for inductively deriving rules from activity-diary data. The decision table formalism is used to exhaustively represent the
theoretically possible decision rules that individuals may use in sequencing a given set of activities. Actual activity patterns
of individuals are supplied to the system as examples. In an incremental learning process, the system progressively improves
on the selection of rules used for reproducing the examples. Computer experiments based on simulated data are performed to
fine-tune rule selection and rule value update functions. The results suggest that the system is effective and fairly robust
for parameter settings. It is concluded, therefore, that the proposed approach opens up possibilities to derive empirically
tested rule-based models of activity scheduling. Follow-up research will be concerned with testing the system on empirical
data.
Received: 31 January 2001 / Accepted: 13 September 2001 相似文献
999.
Local geoid determination combining gravity disturbances and GPS/levelling: a case study in the Lake Nasser area, Aswan, Egypt 总被引:1,自引:0,他引:1
C. C. Tscherning Awar Radwan A. A. Tealeb S. M. Mahmoud M. Abd El-Monum Ramdan Hassan I. El-Syaed K. Saker 《Journal of Geodesy》2001,75(7-8):343-348
The use of GPS for height control in an area with existing levelling data requires the determination of a local geoid and
the bias between the local levelling datum and the one implicitly defined when computing the local geoid. If only scarse gravity
data are available, the heights of new data may be collected rapidly by determining the ellipsoidal height by GPS and not
using orthometric heights. Hence the geoid determination has to be based on gravity disturbances contingently combined with
gravity anomalies. Furthermore, existing GPS/levelling data may also be used in the geoid determination if a suitable general
gravity field modelling method (such as least-squares collocation, LSC) is applied. A comparison has been made in the Aswan
Dam area between geoids determined using fast Fourier transform (FFT) with gravity disturbances exclusively and LSC using
only the gravity disturbances and the disturbances combined with GPS/levelling data. The EGM96 spherical harmonic model was
in all cases used in a remove–restore mode. A total of 198 gravity disturbances spaced approximately 3 km apart were used,
as well as 35 GPS/levelling points in the vicinity and on the Aswan Dam. No data on the Nasser Lake were available. This gave
difficulties when using FFT, which requires the use of gridded data. When using exclusively the gravity disturbances, the
agreement between the GPS/levelling data were 0.71 ± 0.17 m for FFT and 0.63 ± 0.15 for LSC. When combining gravity disturbances
and GPS/levelling, the LSC error estimate was ±0.10 m. In the latter case two bias parameters had to be introduced to account
for a possible levelling datum difference between the levelling on the dam and that on the adjacent roads.
Received: 14 August 2000 / Accepted: 28 February 2001 相似文献
1000.
One of the aims of the Earth Explorer Gravity Field and Steady-State Ocean Circulation (GOCE) mission is to provide global
and regional models of the Earth's gravity field and of the geoid with high spatial resolution and accuracy. Using the GOCE
error model, simulation studies were performed in order to estimate the accuracy of datum transfer in different areas of the
Earth. The results showed that with the GOCE error model, the standard deviation of the height anomaly differences is about
one order of magnitude better than the corresponding value with the EGM96 error model. As an example, the accuracy of the
vertical datum transfer from the tide gauge of Amsterdam to New York was estimated equal to 57 cm when the EGM96 error model
was used, while in the case of GOCE error model this accuracy was increased to 6 cm. The geoid undulation difference between
the two places is about 76.5 m. Scaling the GOCE errors to the local gravity variance, the estimated accuracy varied between
3 and 7 cm, depending on the scaling model.
Received: 1 March 2000 / Accepted: 21 February 2001 相似文献