排序方式： 共有28条查询结果，搜索用时 0 毫秒
21.
P. Vaníček W. Sun P. Ong Z. Martinec M. Najafi P. Vajda B. ter Horst 《Journal of Geodesy》1996,71(1):21-34
. The aim of this contribution is to show that mean Helmert's gravity anomalies obtained at the earth surface on a grid of
a `reasonable' step can be transferred to corresponding mean Helmert's anomalies on the geoid. To demonstrate this, we take
the by mean Helmert's anomalies from a very rugged region, the south-western corner of Canada which contains the two main chains
of the Canadian Rocky Mountains, and formulate the problem of downward continuation of Helmert's anomalies for this region.
This can be done exactly because Helmert's disturbing potential is harmonic everywhere outside the geoid, therefore even within
the topography. Then we solve the problem numerically by transforming the Poisson integral to a system of 53,856 linear algebraic
equations. Since the matrix of this system is well conditioned, there is no theoretical obstacle to the solution. The correctness
of the solution is then checked by back substitution and by evaluating the contribution of the downward continuation term
to Helmert's co-geoid. This contribution comes out positive for all the points. We thus claim that the determination of the
downward continuation of mean Helmert's gravity anomalies on a grid of a `reasonable' step is a well posed problem with a
unique solution and can be done routinely to any accuracy desired in the geoid computaion.
Received 27 October 1995; Accepted 9 July 1996 相似文献
22.
Robert Tenzer Pavel Novák Ilya Prutkin Artu Ellmann Peter Vajda 《Studia Geophysica et Geodaetica》2009,53(2):157-167
To reduce the numerical complexity of inverse solutions to large systems of discretised integral equations in gravimetric
geoid/quasigeoid modelling, the surface domain of Green’s integrals is subdivided into the near-zone and far-zone integration
sub-domains. The inversion is performed for the near zone using regional detailed gravity data. The farzone contributions
to the gravity field quantities are estimated from an available global geopotential model using techniques for a spherical
harmonic analysis of the gravity field. For computing the far-zone contributions by means of Green’s integrals, truncation
coefficients are applied. Different forms of truncation coefficients have been derived depending on a type of integrals in
solving various geodetic boundary-value problems. In this study, we utilise Molodensky’s truncation coefficients to Green’s
integrals for computing the far-zone contributions to the disturbing potential, the gravity disturbance, and the gravity anomaly.
We also demonstrate that Molodensky’s truncation coefficients can be uniformly applied to all types of Green’s integrals used
in solving the boundaryvalue problems. The numerical example of the far-zone contributions to the gravity field quantities
is given over the area of study which comprises the Canadian Rocky Mountains. The coefficients of a global geopotential model
and a detailed digital terrain model are used as input data. 相似文献
23.
Truncated geoid and gravity inversion for one point-mass anomaly 总被引：1，自引：0，他引：1
The truncated geoid, defined by the truncated Stokes' integral transform, an integral convolution of gravity anomalies with
the Stokes' function on a spherical cap, is often used as a mathematical tool in geoid computations via Stokes' integral to
overcome computational difficulties, particularly the need to integrate over the entire boundary spheroid. The objective of
this paper is to demonstrate that the truncated geoid does, besides having mathematical applications, have physical interpretation,
and thus may be used in gravity inversion. A very simple model of one point-mass anomaly is chosen and a method for inverting
its synthetic gravity field with the use of the truncated geoid is presented. The method of inverting the synthetic field
generated by one point-mass anomaly has become fundamental for the authors' inversion studies for sets of point-mass anomalies,
which are published in a separate paper. More general applications are currently under investigation. Since an inversion technique
for physically meaningful mass distributions based on the truncated geoid has not yet been developed, this work is not related
to any of the existing gravity inversion techniques. The inversion for one point mass is based on the onset of the so-called
dimple event, which occurs in the sequence of surfaces (or profiles) of the first derivative of the truncated geoid with respect
to the truncation parameter (radius of the integration cap), its only free parameter. Computing the truncated geoid at various
values of the truncation parameter may be understood as spatial filtering of surface gravity data, a type of weighted spherical
windowing method. Studying the change of the truncated geoid represented by its first derivative may be understood as a data
enhancement method. The instant of the dimple onset is practically independent of the mass of the point anomaly and linearly
dependent on its depth.
Received: 26 September 1996 /Accepted: 28 September 1998 相似文献
24.
Far-zone gravity field contributions corrected for the effect of topography by means of molodensky’s truncation coefficients 总被引：1，自引：0，他引：1
Robert Tenzer Pavel Novák Peter Vajda Artu Ellmann Ahmed Abdalla 《Studia Geophysica et Geodaetica》2011,55(1):55-71
A spectral representation of the topographic corrections to gravity field quantities is formulated in terms of spherical height
functions. When computing the far-zone contributions to the topographic corrections, various types of the truncation coefficients
are applied to a spectral representation of Newton’s integral. In this study we utilise Molodensky’s truncation coefficients
in deriving the expressions for the far-zone contributions to topographic corrections. The expressions for computing the far-zone
gravity field contributions corrected for the effect of topography are then obtained by combining the expressions for the
far-zone contributions to the gravity field quantities and to the respective topographic corrections, both expressed in terms
of Molodensky’s truncation coefficients. The numerical examples of the far-zone contributions to the topographic corrections
and to the topography-corrected gravity field quantities are given over the study area situated in the Canadian Rocky Mountains
with adjacent planes. Coefficients of the global elevation and geopotential models are used as the input data. 相似文献
25.
白垩纪-古近纪(K-Pg)生物大灭绝事件前后的孢粉植物群的变化,可以详细揭示植被对一场全球环境危机(6550万年前小行星撞击现今的墨西哥)的响应。在K-Pg界线处有一些植物门类灭绝了,因此孢粉地层学是确定非海相序列中白垩纪-古近纪界线的主要工具。南半球包括以下植物区:热带至亚热带棕榈植物大区,高纬度Nothofagidites/Proteacidites大区,以及一个含有混合的植物组分的过渡区域。在棕榈植物大区,一些马斯特里赫特期的关键物种在白垩纪-古近纪界线处灭绝了,包括Aquilapollenites magnus、Buttinia andreevi、Crassitricolporites brasiliensis、Proteaci ditesdehaani和Gabonisporis vigourouxii。在Nothofagidites/Proteacidites大区,Tricolporite slilliei、Triporopollenites sectilis、Quadraplanus brossus、Nothofagidites kaitangata和Grapnelispora evansii等物种最后出现于白垩纪-古近纪界线处。由此可见,我们需要对南半球与中国的K-Pg界线处的孢粉信息进行更详尽的分析,才能更详细地了解不同纬度、不同地点的植被对墨西哥尤卡坦撞击事件的响应。 相似文献
26.
The physical meaning of the truncated geoid, which is defined by the convolution of gravity anomalies with the Stokes function on a spherical cap of specified radius, has been studied by the authors. They investigated its relation to the density distribution, generating the surface gravity, and its potential use in inversion. Some progress results for simulated studies on point mass anomalies are presented.
The behavior of the truncated geoid is controlled by the radius of the integration domain, hereinafter referred to as the truncation parameter, which is treated as a free parameter. The change of the truncated geoid in response to the change of the truncation parameter was studied in the context of the simulated mass distributions. By means of such computer simulations we have managed to demonstrate the clear sensitivity of the truncated geoid to the depths, in addition to the horizontal positions, of point mass anomalies generating the synthetic surface gravity. The objective of this paper is to illustrate, with the help of computer simulation as the method of our study, the contribution of the truncated geoid to the solution of the gravimetric inverse problem. Further work towards employing the truncated geoid in gravity exploration is being conducted. 相似文献
27.
This paper focuses on one particular way of linear filtering the gravity data to facilitate gravity inversion or interpretation. With the use of integral transforms the gravity anomalies are transformed into new quantities that allow an easier interpretation with the help of pattern recognition. As the integral transforms are in fact filters, and as the regions of integration are caps with a variable radius, which can be systematically changed as a free parameter, we refer to such methodology as the truncation filtering. Such filters may be understood as weighted spherical windows moving over the surface, on which the gravity anomaly is defined, the kernel of the transform being the weight function. The objective of this paper is to define and deploy the truncation filtering for a planar model, i.e. for a homogenous horizontally infinite layer with embedded anomalous masses, and for a spherical model, i.e., for a homogenous massive sphere with embedded anomalous masses. Instead of the original gravity anomaly, the quantities resulting from the truncation filtering are interpreted/inverted. As we shall see, this approach has certain benefits. The fundamental concept of the truncation filtering methodology is demonstrated here in terms of the model consisting of one point mass anomaly.The relationship between the depth of the point mass and the instant of the onset of the dimple pattern observed in sequences produced by truncation filtering the synthetic gravity data generated by point masses is, for both the planar and spherical models, compiled by computer simulations, as well as derived analytically. It is shown, that the dimple pattern is a consequence of truncating the domain of the filter and is free of the choice of the kernel of the filter. It is shown, that in terms of the mean earth and depths of point masses no greater than some 100 km the spherical model may be replaced by a planar model from the perspective of the truncation filtering methodology. It is also shown, that from the viewpoint of the truncation filtering methodology the rigorous gravity anomaly may be approximated by the vertical component of the gravity disturbance. The relationship between the instant of the dimple onset and the depth of the point mass thus becomes linear and independent of the magnitude (mass) of the point mass. 相似文献
28.
A fluvial sequence near Cave Stream (north of Castle Hill Village, central Canterbury), New Zealand, contains organically-preserved plant macrofossils (cuticles). The almost ubiquitous presence of Araucariaceae macrofossils in the lower part of the section and their stratigraphic disappearance roughly coincident with the appearance of the angiosperm leaf Dryandra comptoniaefolia and conifer taxa only known elsewhere from sediments of Paleogene age, indicates that the section spans the Cretaceous-Paleogene (K-T) boundary. This was subsequently confirmed by a palynological study that demonstrated the disappearance of Late Cretaceous index species within the investigated exposure. The sequence supports the pattern recognized elsewhere in New Zealand where Araucariaceae macrofossils either disappear or become very rare in end-Cretaceous and the leaf Dryandra comptoniaefolia is an important component in the early Cenozoic. The Cave Stream K-T boundary is one of the very few in the world with organically-preserved plant macrofossils and confirms the dramatic turnover in macroflora, which is known from North America. 相似文献