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Tue, 11 Mar 2008 15:10:09 +0100Tue, 11 Mar 2008 15:10:09 +0100On Mathematical Aspects of a Combined Inversion of Gravity and Normal Mode Variations by a Spline Method
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/2038
This paper provides a brief overview of two linear inverse problems concerned with the determination of the Earth’s interior: inverse gravimetry and normal mode tomography. Moreover, a vector spline method is proposed for a combined solution of both problems. This method uses localised basis functions, which are based on reproducing kernels, and is related to approaches which have been successfully applied to the inverse gravimetric problem and the seismic traveltime tomography separately.Paula Berkel; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/2038Mon, 03 Nov 2008 15:10:09 +0100Speech Recognition Support of Assisted Living
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/2029
We present results and views about a project in assisted living. The scenario is a room in which an elderly and/or disabled person lives who is not able to perform certain actions due to restricted mobility. We enable the person to express commands verbally that will then be executed automatically. There are several severe problems involved that complicate the situation. The person may utter the command in a rather unexpected way, the person makes an error or the action cannot be performed due to several reasons. In our approach we present an architecture with three components: The recognition component that contains novel features in the signal processing, the analysis component that logically analyzes the command, and the execution component that performs the action automatically. All three components communicate with each other.Georg Hebinger; Volker Michel; Michael Richter; Andreas Simonpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/2029Wed, 01 Oct 2008 16:07:44 +0200Splines on the 3-dimensional Ball and their Application to Seismic Body Wave Tomography
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1854
In this paper we construct spline functions based on a reproducing kernel Hilbert space to interpolate/approximate the velocity field of earthquake waves inside the Earth based on traveltime data for an inhomogeneous grid of sources (hypocenters) and receivers (seismic stations). Theoretical aspects including error estimates and convergence results as well as numerical results are demonstrated.Abel Amirbekyan; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1854Tue, 17 Apr 2007 12:51:03 +0200Locally Supported Approximate Identities on the Unit Ball
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1837
We present a constructive theory for locally supported approximate identities on the unit ball in \(\mathbb{R}^3\). The uniform convergence of the convolutions of the derived kernels with an arbitrary continuous function \(f\) to \(f\), i.e. the defining property of an approximate identity, is proved. Moreover, an explicit representation for a class of such kernels is given. The original publication is available at www.springerlink.comMuhammad Akram; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1837Sun, 11 Feb 2007 15:03:29 +0100Numerical Aspects of a Spline-Based Multiresolution Recovery of the Harmonic Mass Density out of Gravity Functionals
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1823
We show the numerical applicability of a multiresolution method based on harmonic splines on the 3-dimensional ball which allows the regularized recovery of the harmonic part of the Earth's mass density distribution out of different types of gravity data, e.g. different radial derivatives of the potential, at various positions which need not be located on a common sphere. This approximated harmonic density can be combined with its orthogonal anharmonic complement, e.g. determined out of the splitting function of free oscillations, to an approximation of the whole mass density function. The applicability of the presented tool is demonstrated by several test calculations based on simulated gravity values derived from EGM96. The method yields a multiresolution in the sense that the localization of the constructed spline basis functions can be increased which yields in combination with more data a higher resolution of the resulting spline. Moreover, we show that a locally improved data situation allows a highly resolved recovery in this particular area in combination with a coarse approximation elsewhere which is an essential advantage of this method, e.g. compared to polynomial approximation.Volker Michel; Kerstin Wolfpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1823Wed, 13 Dec 2006 15:19:35 +0100Fast Approximation on the 2-Sphere by Optimally Localized Approximate Identities
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1822
We introduce a method to construct approximate identities on the 2-sphere which have an optimal localization. This approach can be used to accelerate the calculations of approximations on the 2-sphere essentially with a comparably small increase of the error. The localization measure in the optimization problem includes a weight function which can be chosen under some constraints. For each choice of weight function existence and uniqueness of the optimal kernel are proved as well as the generation of an approximate identity in the bandlimited case. Moreover, the optimally localizing approximate identity for a certain weight function is calculated and numerically tested.Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1822Wed, 13 Dec 2006 13:46:52 +0100Time-Dependent Cauchy-Navier Splines and their Application to Seismic Wave Front Propagation
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1760
In this paper a known orthonormal system of time- and space-dependent functions, that were derived out of the Cauchy-Navier equation for elastodynamic phenomena, is used to construct reproducing kernel Hilbert spaces. After choosing one of the spaces the corresponding kernel is used to define a function system that serves as a basis for a spline space. We show that under certain conditions there exists a unique interpolating or approximating, respectively, spline in this space with respect to given samples of an unknown function. The name "spline" here refers to its property of minimising a norm among all interpolating functions. Moreover, a convergence theorem and an error estimate relative to the point grid density are derived. As numerical example we investigate the propagation of seismic waves.Paula Kammann; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1760Wed, 23 Aug 2006 10:19:04 +0200Wavelet Modelling of Regional and Temporal Variations of the Earth´s Gravitational Potential
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1641
This work is dedicated to the wavelet modelling of regional and temporal variations of the Earth's gravitational potential observed by GRACE. In the first part, all required mathematical tools and methods involving spherical wavelets are introduced. Then we apply our method to monthly GRACE gravity fields. A strong seasonal signal can be identified, which is restricted to areas, where large-scale redistributions of continental water mass are expected. This assumption is analyzed and verified by comparing the time series of regionally obtained wavelet coefficients of the gravitational signal originated from hydrology models and the gravitational potential observed by GRACE. The results are in good agreement to previous studies and illustrate that wavelets are an appropriate tool to investigate regional time-variable effects in the gravitational field.Martin J. Fengler; Willi Freeden; Annika Kohlhaas; Volker Michel; Thomas Peterspreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1641Thu, 02 Jun 2005 14:48:00 +0200Local Multiscale Approximations of Geostrophic Flow: Theoretical Background and Aspects of Scientific Computing
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1634
In modern geoscience, understanding the climate depends on the information about the oceans. Covering two thirds of the Earth, oceans play an important role. Oceanic phenomena are, for example, oceanic circulation, water exchanges between atmosphere, land and ocean or temporal changes of the total water volume. All these features require new methods in constructive approximation, since they are regionally bounded and not globally observable. This article deals with methods of handling data with locally supported basis functions, modeling them in a multiscale scheme involving a wavelet approximation and presenting the main results for the dynamic topography and the geostrophic flow, e.g., in the Northern Atlantic. Further, it is demonstrated that compressional rates of the occurring wavelet transforms can be achieved by use of locally supported wavelets.Willi Freeden; Dominik Michel; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1634Mon, 09 May 2005 16:04:38 +0200Harmonic Spline-Wavelets on the 3-dimensional Ball and their Application to the Reconstruction of the Earth´s Density Distribution from Gravitational Data at Arbitrarily Shaped Satellite Orbits
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1623
We introduce splines for the approximation of harmonic functions on a 3-dimensional ball. Those splines are combined with a multiresolution concept. More precisely, at each step of improving the approximation we add more data and, at the same time, reduce the hat-width of the used spline basis functions. Finally, a convergence theorem is proved. One possible application, that is discussed in detail, is the reconstruction of the Earth´s density distribution from gravitational data obtained at a satellite orbit. This is an exponentially ill-posed problem where only the harmonic part of the density can be recovered since its orthogonal complement has the potential 0. Whereas classical approaches use a truncated singular value decomposition (TSVD) with the well-known disadvantages like the non-localizing character of the used spherical harmonics and the bandlimitedness of the solution, modern regularization techniques use wavelets allowing a localized reconstruction via convolutions with kernels that are only essentially large in the region of interest. The essential remaining drawback of a TSVD and the wavelet approaches is that the integrals (i.e. the inner product in case of a TSVD and the convolution in case of wavelets) are calculated on a spherical orbit, which is not given in reality. Thus, simplifying modelling assumptions, that certainly include a modelling error, have to be made. The splines introduced here have the important advantage, that the given data need not be located on a sphere but may be (almost) arbitrarily distributed in the outer space of the Earth. This includes, in particular, the possibility to mix data from different satellite missions (different orbits, different derivatives of the gravitational potential) in the calculation of the Earth´s density distribution. Moreover, the approximating splines can be calculated at varying resolution scales, where the differences for increasing the resolution can be computed with the introduced spline-wavelet technique.Martin J. Fengler; Dominik Michel; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1623Wed, 23 Mar 2005 13:00:11 +0100Wavelet Deformation Analysis for Spherical Bodies
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1612
In this paper we introduce a multiscale technique for the analysis of deformation phenomena of the Earth. Classically, the basis functions under use are globally defined and show polynomial character. In consequence, only a global analysis of deformations is possible such that, for example, the water load of an artificial reservoir is hardly to model in that way. Up till now, the alternative to realize a local analysis can only be established by assuming the investigated region to be flat. In what follows we propose a local analysis based on tools (Navier scaling functions and wavelets) taking the (spherical) surface of the Earth into account. Our approach, in particular, enables us to perform a zooming-in procedure. In fact, the concept of Navier wavelets is formulated in such a way that subregions with larger or smaller data density can accordingly be modelled with a higher or lower resolution of the model, respectively.Willi Freeden; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1612Thu, 17 Feb 2005 18:43:34 +0100A Model for Spherical SH-Wave Propagation in Self-reinforced Linearly Elastic Media
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1412
The original publication is available at www.springerlink.com. This original publication also contains further results. We study a spherical wave propagating in radius- and latitude-direction and oscillating in latitude-direction in case of fibre-reinforced linearly elastic material. A function system solving Euler's equation of motion in this case and depending on certain Bessel and associated Legendre functions is derived.Amares Chattopadhyay; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1412Tue, 15 Jul 2003 10:45:21 +0200Satellite-to-Satellite Tracking and Satellite Gravity Gradiometry
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1134
The purpose of satellite-to-satellite tracking (SST) and/or satellite gravity gradiometry (SGG) is to determine the gravitational field on and outside the Earth's surface from given gradients of the gravitational potential and/or the gravitational field at satellite altitude. In this paper both satellite techniques are analysed and characterized from mathematical point of view. Uniqueness results are formulated. The justification is given for approximating the external gravitational field by finite linear combination of certain gradient fields (for example, gradient fields of single-poles or multi-poles) consistent to a given set of SGG and/or SST data. A strategy of modelling the gravitational field from satellite data within a multiscale concept is described; illustrations based on the EGM96 model are given.Willi Freeden; Volker Michel; Helga Nutzpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1134Thu, 08 Mar 2001 00:00:00 +0100Basic Aspects of Geopotential Field Approximation From Satellite-to-Satellite Tracking Data
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1116
The satellite-to-satellite tracking (SST) problems are characterized from mathematical point of view. Uniqueness results are formulated. Moreover, the basic relations are developed between (scalar) approximation of the earth's gravitational potential by "scalar basis systems" and (vectorial) approximation of the gravitational eld by "vectorial basis systems". Finally, the mathematical justication is given for approximating the external geopotential field by finite linear combinations of certain gradient fields (for example, gradient fields of multi-poles) consistent to a given set of SST data.Willi Freeden; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1116Mon, 21 Aug 2000 00:00:00 +0200Singular Optimal Control - The State of the Art
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/603
The purpose of this paper is to present the state of the art in singular optimal control. If the Hamiltonian in an interval \([t_1,t_2]\) is independent of the control we call the control in this interval singular. Singular optimal controls appear in many applications so that research has been motivated since the 1950s. Often optimal controls consist of nonsingular and singular parts where the junctions between these parts are mostly very difficult to find. One section of this work shows the actual knowledge about the location of the junctions and the behaviour of the control at the junctions. The definition and the properties of the orders (problem order and arc order), which are important in this context, are given, too. Another chapter considers multidimensional controls and how they can be treated. An alternate definition of the orders in the multidimensional case is proposed and a counterexample, which confirms a remark given in the 1960s, is given. A voluminous list of optimality conditions, which can be found in several publications, is added. A strategy for solving optimal control problems numerically is given, and the existing algorithms are compared with each other. Finally conclusions and an outlook on the future research is given.Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/603Fri, 23 Jun 2000 00:00:00 +0200Constructive Approximation and Numerical Methods in Geodetic; Research Today - An Attempt of a Categorization Based on anUncertainty Principle
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/647
This review article reports current activities and recent progress on constructive approximation and numerical analysis in physical geodesy. The paper focuses on two major topics of interest, namely trial systems for purposes of global and local approximation and methods for adequate geodetic application. A fundamental tool is an uncertainty principle, which gives appropriate bounds for the quantification of space and momentum localization of trial functions. The essential outcome is a better understanding of constructive approximation in terms of radial basis functions such as splines and wavelets.Willi Freeden; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/647Mon, 03 Apr 2000 00:00:00 +0200Scale Continuous, Scale Discretized and Scale Discrete Harmonic Wavelets for the Outer and the Inner Space of a Sphere and Their Application to an Inverse Problem in Geomathematics
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1046
In this paper we construct a multiscale solution method for the gravimetry problem, which is concerned with the determination of the earth's density distribution from gravitational measurements. For this purpose isotropic scale continuous wavelets for harmonic functions on a ball and on a bounded outer space of a ball, respectively, are constructed. The scales are discretized and the results of numerical calculations based on regularization wavelets are presented. The obtained solutions yield topographical structures of the earth's surface at different levels of localization ranging from continental boundaries to local structures such as Ayer's Rock and the Amazonas area.Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/1046Thu, 23 Mar 2000 00:00:00 +0100Least-squares Geopotential Approximation by Windowed Fourier and Wavelet Transform
https://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/844
Two possible substitutes of the Fourier transform in geopotential determination are windowed Fourier transform (WFT) and wavelet transform (WT). In this paper we introduce harmonic WFT and WT and show how it can be used to give information about the geopotential simultaneously in the space domain and the frequency (angular momentum) domain. The counterparts of the inverse Fourier transform are derived, which allow us to reconstruct the geopotential from its WFT and WT, respectively. Moreover, we derive a necessary and sufficient condition that an otherwise arbitrary function of space and frequency has to satisfy to be the WFT or WT of a potential. Finally, least - squares approximation and minimum norm (i.e. least - energy) representation, which will play a particular role in geodetic applications of both WFT and WT, are discussed in more detail.Willi Freeden; Volker Michelpreprinthttps://kluedo.ub.uni-kl.de/frontdoor/index/index/docId/844Wed, 08 Sep 1999 00:00:00 +0200