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211.
Introduction The gravity anomaly is an indicator of the density distribution of the underground material. Therefore the gravity anomalies have been important data used for studying the deep crustal struc-ture for a long time. Many people have made detailed researches on the regional crustal structure inverted by Bouguer anomalies. In particular some empirical formulae and practical algorithms about the crustal thickness were brought forward, and a series of results were obtained (MENG, 1996)… 相似文献
212.
213.
Sediment accumulation patterns in the Luni basin have been studied on the basis of tubewell lithologs and Bouguer gravity anomaly profiles. East–West geologic transects using these data reveal several sediment-filled graben depressions, for example, the Digrana-Bhawal graben in the northern part, the Mangta-Sindari, Sindari-Bhimgoda, and Bhimgoda-Juna Motisara grabens in the central part; and the Dungari-Ratanpura and Ratanpura-Khanpur grabens in the southern part. Maximum sediment accumulation of more than 300 m is located in the southwestern part of the Luni basin, more specifically in the Ratanpura-Khanpur graben. Minor depressions also occur towards west of Jodhpur and east of Bhadrajun.Sediment filling in these graben depressions commenced with thick clay deposits followed by multistoried fining up sand and gravel sequences. Sediment accumulation centres are coincident with major tectonic lineaments and thus indicate that important loci of sediment accumulation are, possibly, a response to synsedimentary tectonics. 相似文献
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215.
华北重力复测网(火车运输)始建于1972-1973年,三环共45个点,1986年起改用高精度LCR-G重力仪观测(仍保持三环,点位减到15个),至今已积累了16-33期重复观测资料。根据资料,华北网各测点重力值变化可分为三类,即1、有异常变化点;2、持续相对稳定点、3重力值有变化,但非深部物质引起或重力值无变化点。如果观测到的异常持续3-5年,且与背景相符,即有可能与中强震有关,并已被实践证明,因此,重力大跨度复测是中长期地震预报的一种有效手段。 相似文献
216.
A recently proposed method for the computation of the gravitational effect due to the topographic masses defined by a Digital Elevation Model (DEM) involves the representation of the surface relief by means of parts of bilinear surfaces. The so-called bilinear method delivers eventually the mathematical model for the gravitational attraction of a right rectangular prism, whose top is modeled by a bilinear surface. Scope of the paper is to assess the new method by conducting numerical tests using both real and synthetic data. The performance of the bilinear method is evaluated in terms of its computational efficiency as well as its precision by comparing it with other analytical methods available for the practical evaluation of gravitational terrain effects. The techniques considered for the assessment of the bilinear approximation are the vastly applied right rectangular prism method and the polyhedral modeling, a less popular but extremely flexible approach based on the closed expression for the gravity field of an arbitrarily shaped mass distribution defined by planar faces. The different geometric modeling of the topographic relief produces discrepancies to the gravitational attraction of up to several mGal. Thus the choice for the geometric representation of the terrain plays a fundamental role to the numerical computation of potential field quantities especially in the critical region surrounding the computation point. 相似文献
217.
Evaluation of deflections of the vertical on the sphere and the plane: a comparison of FFT techniques 总被引:1,自引:0,他引:1
This paper presents a set of efficient formulas to evaluate the deflections of the vertical on the sphere using gridded data.
The Vening-Meinesz formula, the topographic indirect effect on the deflections of the vertical as well as the terrain corrections
are expressed as both 2D and 1D convolutions on the sphere, and consequently can be evaluated by the 2D and the 1D fast Fourier
transform (FFT). When compared with the results obtained from pointwise integration, the use of the 1D FFT gives identical
results, and therefore these results were used as control values in this paper. The use of the spherical 2D FFT improves significantly
the computational efficiency with little sacrifice of accuracy (0.6″ rms difference from the 1D FFT results). The planar 2D FFT, which is as efficient as the spherical 2D FFT, gives worse results
(1.2″ rms difference from the 1D FFT results) because of the extra approximations.
Received: 27 February 1996; Accepted: 24 January 1997 相似文献
218.
219.
Characterization of periodic variations in the GPS satellite clocks 总被引:11,自引:7,他引:4
The clock products of the International Global Navigation Satellite Systems (GNSS) Service (IGS) are used to characterize
the timing performance of the GPS satellites. Using 5-min and 30-s observational samples and focusing only on the sub-daily
regime, approximate power-law stochastic processes are found. The Block IIA Rb and Cs clocks obey predominantly random walk
phase (or white frequency) noise processes. The Rb clocks are up to nearly an order of magnitude more stable and show a flicker
phase noise component over intervals shorter than about 100 s. Due to the onboard Time Keeping System in the newer Block IIR
and IIR-M satellites, their Rb clocks behave in a more complex way: as an apparent random walk phase process up to about 100 s
and then changing to flicker phase up to a few thousand seconds. Superposed on this random background, periodic signals have
been detected in all clock types at four harmonic frequencies, n × (2.0029 ± 0.0005) cycles per day (24 h coordinated universal time or UTC), for n = 1, 2, 3, and 4. The equivalent fundamental period is 11.9826 ± 0.0030 h, which surprisingly differs from the reported mean
GPS orbital period of 11.9659 ± 0.0007 h by 60 ± 11 s. We cannot account for this apparent discrepancy but note that a clear
relationship between the periodic signals and the orbital dynamics is evidenced for some satellites by modulations of the
spectral amplitudes with eclipse season. All four harmonics are much smaller for the IIR and IIR-M satellites than for the
older blocks. Awareness of the periodic variations can be used to improve the clock modeling, including for interpolation
of tabulated IGS products for higher-rate GPS positioning and for predictions in real-time applications. This is especially
true for high-accuracy uses, but could also benefit the standard GPS operational products. The observed stochastic properties
of each satellite clock type are used to estimate the growth of interpolation and prediction errors with time interval. 相似文献
220.
广域差分GPS的数据处理方法及结果分析 总被引:4,自引:0,他引:4
详细讨论了广域差分GPS数据处理的数学模型和方法,就实际观测数据,用作者研制的软件确定差分修正信息并提供给用户站定位。结果表明,软件可以保证平面位置精度优于1m,高程精度在1.5m左右。 相似文献