georges jobert - Academia.edu (original) (raw)

Papers by georges jobert

Despite its great practical importance, the theory of inverse problems remains poorly known. Inde... more Despite its great practical importance, the theory of inverse problems remains poorly known. Indeed, physics was built by solving direct problems: from a given model to the corresponding measurable data. The opposite approach: from observed data to model parameters-the solution of an 'inverse' problemis not so easy. It is, however, essential in many fields.

Bulletin of the Seismological Society of America, Oct 1, 1972

Expressions identical to the formulas of a preceding paper are obtained when due allowance is mad... more Expressions identical to the formulas of a preceding paper are obtained when due allowance is made for the variation of quantities which were erroneously taken as constant. A method for the approximate inversion of the travel-time curves is presented for the case where the surfaces of equal velocity are planes with an infinitesimal dip.

Bulletin de la Société Géologique de France, 1980

Thomson’s formulas (1872) that give the position of the magnetic centre of a magnet and the first... more Thomson’s formulas (1872) that give the position of the magnetic centre of a magnet and the first results on the global geomagnetic field were sufficient to show that the source of this field had a disymmetry in the equatorial plane. To confirm this with Schmidt’s (1934) explicit formulas giving the position of the optimal eccentric dipole, numerical calculus was necessary. To obtain their relatively simple formulas, Thomson and Schmidt used lengthy algebra that was avoidable.

Applied Inverse Problems, 1978

A fast method to solve the invers-e problem of local earthquakes foci is presented. The introduct... more A fast method to solve the invers-e problem of local earthquakes foci is presented. The introduction of a quadratic hyperbolic norm in space-time permits the treatment of the non-linearity of the problem by means of a simple scalar equation. The duration of the process is comparable to that of one iteration of Geiger's method. A complete analytical study is given.

Geofisica Pura e Applicata, 1959

SummaryThe hypotheses of a previous note are modified to take into account eventual zones of plas... more SummaryThe hypotheses of a previous note are modified to take into account eventual zones of plastic flow in the ground. A simple model of preseismical strain is used, which enables us to study the repartition of the elementary shear planes. It is possible to deduce the extent of faulting in depth from the combined preseismical and seismical strains. An application to the San Francisco earthquake gives a depth of 7 km only.ResumoNi modifiis la hipotezojn de antaùa noto por reprezenti eventualajan zonojn kun plasteca fluo en la Tero. La simpla modelo uzita por la antaùsisma deformigo permesas studi la liniojn kie elementaj sirplanoj estas paralelaj al fiksa plandirekto. Oni trovas por la responda sisma deformigo simplan proksimuman formulon. Aplikado al la fendego de San Francisco donas profundecon de nur 7 km.

Nature, 1984

A north-south wide-angle fan-profile through the Yarlung Zangbo suture, across the Lhasa block an... more A north-south wide-angle fan-profile through the Yarlung Zangbo suture, across the Lhasa block and through the Bangong-Nujiang suture shows several significant and sharp changes in crustal thickness. Both sutures appear as vertical 20-km steps in the Moho and may have been the loci for eastward strike-slip motion of the Tibetan lithosphere.

Geophysical Journal International, 1960

I. Introduction Geophysical research in France is conducted by very various teams. The following ... more I. Introduction Geophysical research in France is conducted by very various teams. The following Institutes and Observatories of Physics of the Earth depend on a University : Paris (+ Martinique and Guadeloupe), Strasbourg, Clermont-Ferrand, Pic du Midi, Alger (+Tamanrasset). The National Centre for Scientific Research has recently put in operation a Centre for Geophysical Studies in the Nibvre (at Garchy). Other observatories depend on the Office for Scientific and Technical Research Overseas (ORSTOM) : M'Bour ](SCntgal), Bangui (RCpublique Centrafricaine), NoumCa (Nouvelle Caltdonie). But research in specific fields is also carried on either in universities or by public services as the National Meteorological Office, National Geographical Institute, Bureau of Geological, Geophysical and Mining Researches (BRGGM), or by private companies like the General Geophysical Company (CGG). A French National Committee for Geodesy and Geophysics groups all geophysicists. We have used in preparing our report its yearly proceedings which include a complete bibliography of the work done. We will limit ourselves to presenting-without intending to be exhaustive-the principal researches undertaken in France from 1955 and the principal results obtained in the field of the physics of the Earth's interior. 2. Gravity An absolute value of gravity was determined in 1958 (A. Thulin 1960) by means of a method initiated by Ch. Volet, at the International Bureau of Weights and Measures in Sbvres. A scale of platinum-iridium is photographed as it falls. The precision of the measurement is & I mgal and the value obtained is 12.8 mgal lower than that of the Potsdam system, in very good agreement with pendulum determinations by Clark and Agaletskii. An interferometric method is in preparation.

Geophysical Journal International, 1977

Earth flattening methods are developed to obtain simultaneous approximate expressions of propagat... more Earth flattening methods are developed to obtain simultaneous approximate expressions of propagators and Green matrices for body force and dislocation sources in a gravitating sphere.

Geophysical Journal International, 1975

Kennett has shown how the equations of elastodynamics may be derived from a variational principle... more Kennett has shown how the equations of elastodynamics may be derived from a variational principle (see also Lanczos, Germain) and attempted to extend this method to the ' Minor Matrix ' system (Gilbert & Backus). Unfortunately such a direct extension is impossible as will be shown below. However in the case of a medium composed of a series of homogeneous layers, it is possible to construct a stationary expression by a simple transformation. Our discussion will be limited to P-S V or Rayleigh waves in such a medium. References to the equations of Kennett will be denoted, e.g. (K-4.1).

Geophysical Journal International, 1974

As the structure of P-SV wave equation matrix allows its diagonalization, it is possible to obtai... more As the structure of P-SV wave equation matrix allows its diagonalization, it is possible to obtain the expression of an approximate propagator, the only approximation consisting in using a series of homogeneous layers to represent the stratification of the medium and the effect of sphericity.

Geophysical Journal International, 1982

The introduction of vertical and horizontal slownesses as independent variables in the transport ... more The introduction of vertical and horizontal slownesses as independent variables in the transport equation allows us to obtain an explicit expression for the amplitude variation along a ray in a vertically stratified medium. This expression can be used to introduce the dynamical characteristics in the inverse problem of refraction seismology. For the scalar wave equation the ratio of squared amplitudes is a rational functional of some function of the local slowness, the parametrization of which leads to a system of linear homogeneous equations. For P seismic waves the same functional is obtained for the ratio of squared amplitudes weighted by the density.

Pure and Applied Geophysics, 1959

The theory of a Zöllner's horizontal pendulum with perfect wires of suspension is made. We u... more The theory of a Zöllner's horizontal pendulum with perfect wires of suspension is made. We use the Euler's angles of the axes of a system fixed in the moving body relatively to fixed axes and a fourth angle caracterising the amplitude of a motion of translation. We give the conditions for the decoupling of the four types of motion (pure rotation, pitching, rolling, translation) and the equations of forced motions. A rigorous theory of Lettau's double pendulum is initiated. We suppose that the primary pendulum has only one degree of freedom. First the case of a secondary pendulum not disturbing the primary one is examined, and then the case of an indifferent primary pendulum, wich enables us to understand what happens in the normal case. The potential energy of the secondary pendulum is not always a minimum in the central position when the plane of symmetry is vertical and it is possible that parasitesolutions appear, stabler than the central one. This study is extende...

Geophysical Journal of the Royal Astronomical …, 1975

The use of propagator and matrical elements matrices allows one to derive simultaneously the solu... more The use of propagator and matrical elements matrices allows one to derive simultaneously the solutions to the problems of the excitatior, of a stratified medium by body forces and dislocations. * Contribution IPG No. 150.