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ELIMINATION DE L'EFFET DES VAGABONDS INDUSTRIELS DANS UNE PROSPECTION PAR COURANTE TELLURIQUES*
By G. KUNETZAvailable online: 27 April 2006More LessAbstractSeveral methods for eliminating the effects of stray currents on telluric measurements are described.
In the Lyon area the source of the stray currents is formed by the tramways of the city. Since Lyon receives no electric current from outside, its influence may be described as that of a conglomeration of dipoles. The law of decay with distance of a source of this type is derived. The constants which interverne in this law are determined experimentally from the observations close to the city, where the telluric field is small compared with that of the stray currents.
In South Eastern France it is possible, under favourable conditions, to distinguish on the telluric registrations between the variations of the telluric field and those of the stray current field, due to a difference in character between the two. This permits an isocorrection line map to be drawn.
In a survey in Morocco, a base net of telluric values was established by measurements during magnetic storms, when the activity of the telluric field is very strong and supersedes the stray currents. In the routine measurements only those events were utilised which tied in well with the base net.
The method first mentioned is discussed in detail. Of the two others only the principles are indicated and some results shown.
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THE GENERATION OF SEISMIC WAVES BY EXPLOSIONS*
Available online: 27 April 2006More LessAbstractIn the mathematical treatment of the propagation of seismic waves the process in the medium surrounding the point of explosion is introduced as initial condition into the differential equation. It is the purpose of this paper to contribute to the knowledge of this process.
The treatment of the process of explosion is a difficult one because the elastic limit is exceeded in the surroundings of the point of explosion. To gain a better view on the acting forces the space concerned is replaced by a simplifying model. It is shown that a permanent deformation is caused by the explosion. This deformation will be especially intensive if the rock does not offer any resistance to tensile forces.
To determine the extent of such a permanent deformation a sudden release of pressure — the opposite of an explosion — was recorded by seismographs. It has been proved that any seismic waves of measurable magnitude are not produced. This leads to the conclusion that permanent deformation plays an essential part in the generation of seismic waves.
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THE VARIATION OF THE ELASTIC CONSTANTS OF ROCKS WITH FREQUENCY*
Authors J. M. BRUCKSHAW and P. C. MAHANTAAvailable online: 27 April 2006More LessAbstractA method has been developed for the measurement, in the laboratory, of the variation of the elastic properties of rocks in the frequency range 40–120 cycles/sec. In addition, the observations yield information concerning the internal dissipation of energy over this range. A number of rock specimens have been examined and the results are compared with those of other investigators.
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THE SIGNIFICANCE OF DIFFRACTION IN THE INVESTIGATION OF FAULTS*
By THEODOR KREYAvailable online: 27 April 2006More LessAbstractSeismic reflection records taken across faults frequently show an overlapping of reflections from the displaced blocks. It is demonstrated that diffraction of seismic waves is a cause and the effect may be used in interpretation. Overlapping is increased if a seismic profile crosses a fault at an acute angle. Plotted dips will be inaccurate unless diffraction is taken into account. Further, the diffraction oscillation pattern will also be obtained if a reflection horizon terminates for a reason other than faulting, for example, at a wedgeout or reef edge, or at a sudden change of facies. The facts developed are demonstrated by practical examples in which attention is directed to the approximations involved in plotting the boundaries of discontinuities.
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PROPOSALS FOR E.A.E.G. JOURNAL (prepared by the Publications Committee of E.A.E.G.)
Authors D. T. Germain‐Jones, B. Baars and A. A. FitchAvailable online: 27 April 2006More Less
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DEVELOPMENTS IN MARINE GRAVITY SURVEYING*
By B. J. HOFMANAvailable online: 27 April 2006More LessAbstractA description is given of the development, during the previous fifteen years, of gravity surveying methods in marshlands and marine areas. The methods described include the use of marshbuggies, weasels, helicopters, tripods, diving bells and remote control instruments. Special emphasis is placed on remote control instruments and a brief description of their construction is given. Finally, the corrections involved in marine gravity surveying are discussed, and the costs of the several methods are compared.
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EXEMPLES D'APPLICATION DE LA REPRESENTATION CONFORME A L'INTERPRETATION DU CHAMP TELLURIQUE*
Authors G. KUNETZ and J. CHASTENET DE GÉRYAvailable online: 27 April 2006More LessAbstractThe method of conformal transformation is applied to determine the distribution of telluric current density at the surface for several subsurface situations. To this end one starts of from a configuration of boundaries between media of different resistivities and of two equal and opposite current poles, for which the potential distribution can be easily determined. Then a suitable conformal transformation is applied, by which the two current poles are displaced to infinity, and the boundary between the media are made to correspond to typical layer configurations in the subsurface. Several examples of the method are discussed in detail, leading to typical subsurface structures as well as to typical topographic conditions.
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COLLECTION OF FORTRAN II PROGRAMS FOR FILTERING AND SPECTRAL ANALYSIS OF SINGLE CHANNEL TIME SERIES
Available online: 27 April 2006More LessSUMMARYThis report gives a description of Fortran II programs for filtering and spectral analysis. All programs are tested.
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