bluuebunny/crossref_metadata_2025 huggingface dataset:
EXACT EXPRESSIONS FOR THE GRAVITATIONAL ATTRACTION OF A CIRCULAR LAMINA AT ALL POINTS OF SPACE AND OF A RIGHT CIRCULAR VERTICAL CYLINDER AT POINTS EXTERNAL TO IT
Authors: PARASNIS D. S.
Published: April 2006

Abstract

The gravitational attraction of a circular lamina and a right circular vertical cylinder at any axial point can be found by elementary methods. The attraction at a point P off the axis is usually estimated as follows. The attraction of a lamina, with the axis through P, touching the given lamina externally is subtracted from the attraction of a coaxial lamina which encloses the given lamina. The result is multiplied by the ratio of the area of the given lamina to the area of the annulus. The attraction of the cylinder is estimated by a closely analogous method. The answer is, however, valid only if P is a distant point.

As a step towards calculating exactly the attraction of the cylinder at all points of the space external to it, the corresponding problem for the lamina is dealt with first. It is shown that the anomaly can be expressed as an infinite series in even powers of the horizontal distance of P. The coefficients of the powers consist of exact algebraic expressions containing simple terms of the form (1 +χ2)−n−1/2. These coefficients are determined by exactly summing certain infinite series involving the derivatives of the Legendre polynomials of odd order. The method of summation is believed to be of interest also in other problems where Legendre polynomials are encountered.

The attraction of the cylinder is calculated by appropriately integrating the expression for the attraction of a lamina.

Numerical calculations show that the approximate method everywhere underestimates the gravity anomaly of the lamina as well as of the cylinder. The error at distances one to two times the radius is upto 20 % for the lamina and 8.5 % for an infinitely long cylinder.
MODEL EXPERIMENTS AND SURVEY RESULTS FROM A WING‐TIP‐MOUNTED ELECTROMAGNETIC PROSPECTING SYSTEM*
Authors: BOYD D. , ROBERTS B. C.
Published: April 2006

Abstract

Model experiments at a scale of 1:200 were carried out in order to assess the performance of an E. M. system which was rigidly mounted on the wingtips of an aircraft. The experiments were performed over model conductors of various sizes, shapes and resistivities. From the results the detectability of various natural‐sized conductors was calculated. The results obtained over an unmined massive sulphide body in Cyprus are shown and a comparison made with the model results.
APPROXIMATIONS TO THE REFLECTION AND TRANSMISSION COEFFICIENTS OF PLANE LONGITUDINAL AND TRANSVERSE WAVES*
Authors: BORTFELD R.
Published: April 2006

Abstract

The reflection and transmission coefficients for vertical incidence can be very closely approximated by computing the direct‐reflected and the direct‐transmitted portion for transition layers, and letting the thickness approach zero. The same procedure can then be applied to the general case of oblique incidence, which yields approximate values of the respective coefficients, including those of the converted waves. The approximate values compare well with the exact values calculated by Muskat and Meres (1940).

These approximations might be adequate for all cases occurring in reflection seismic prospecting. They can also be used, possibly even more profitably, for the derivation of some rules governing the reflection process. The influence of Poisson's ratio can be read more or less directly from the approximate formulas. All converted waves depend primarily on the ratio of the shear velocities.
THE SIGNIFICANCE OF DIFFRACTION IN THE INVESTIGATION OF FAULTS*
Authors: KREY THEODOR
Published: April 2006

Abstract

Seismic 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.
DEVELOPMENTS IN MARINE GRAVITY SURVEYING*
Authors: HOFMAN B. J.
Published: April 2006

Abstract

A 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.
TOOLS FOR TOMORROW'S GEOPHYSICS *
Authors: DOBRIN MILTON B. , WARD STANLEY H.
Published: April 2006

Abstract

The introduction of methods for recording and processing field data in digital form on magnetic tape for subsequent processing on electronic computers will result in major improvements in geophysical prospecting techniques. This offers great advantages over analogue recording such as increased dynamic range, greater filter flexibility, more convenience in processing by correlation techniques, freedom from distortion and noise, better adaptability to automatic plotting and contouring, and greater suitability for inverse convolution procedures. Digitally recorded seismic signals can be reliably transmitted long distances by telephone circuits to central data processing centers. Electronic computers facilitate processing of gravity, magnetic, and other potential field data. Smoothing, residual, correlation, and derivative techniques for handling data in digital form can enhance speed and flexibility of processing.
CONTRIBUTION A L'ETUDE DE LA DEFORMATION DU SIGNAL SISMIQUE. ANALOGIES. FILTRAGE EQUIVALENT *
Authors: CHAUVEAU J.
Published: April 2006

Sommaire

Cette publication débute par un essai de synthèse détudes antérieures relatives à la déformation d'un signal sismique, — déformation imputable, soit à la propagation dans les couches superficielles et les niveaux de transition, soit aux réverbérations d'une couche d'eau.

On montre comment un modèle analogique convenable permet d'obtenir directement les réflexions et les transmissions d'une couche où la vitesse n'est pas uniforme. Les résultats obtenus sont comparés aux solutions exactes des équations de propagation et aux solutions qui résultent d'un échantillonnage de la distribution des vitesses dans la couche. Les filtres équivalents, à la propagation d'une part, et à léchantillonnage d'autre part, sont déduits de ces comparaisons.
DETERMINATION OF GRAVITATIONAL FIELDS IN DEPTH
Authors: NEDELKOV I. P. , BURNEV P. H.
Published: April 2006

Abstract

A method is developed for determination of the gravitational field deep under the ground at points surrounding on all sides the body responsible for this field. The possibilities for practical applications are discussed in detail.
ELASTIC BEHAVIOUR OF ROCKS AT LOW FREQUENCIES
Authors: USHER M. J.
Published: April 2006

Abstract

An apparatus is described for the measurement of Young's modulus, E, and the energy loss per cycle, ΔW/W, of small beam‐shaped rock samples, over the frequency range 2 to 40 c/s. Results are given for seven sedimentary and two igneous types of rock. An increase of between 1 % and 3 % in E, and of up to 100 % in ΔW/W, occurred over this range. Experiments on rocks containing moisture showed a decrease in the magnitude of E (at 2 c/s), an increased frequency variation of E, and a greater and almost linear increase of ΔW/W with frequency. The effect of oil was similar except that the magnitude of E at 2 c/s was unaffected. It is suggested that the moisture and oil results are due to viscous movements of fluid through the pores in the rock, and that the changes in E and ΔW/W with frequency, in dry rocks, are due to a small viscous mechanism operative at low frequencies.
CONDITIONS D'EMPLOI DE LA GRAVIMETRIE EN EXPLORATION PETROLIERE*
Authors: CHEREAU J.‐Y.
Published: April 2006

Abstract

In gravimetry the observer cannot exert any influence on the quantity he wants to measure, as he can in seismic exploration. This might easily tempt him to ascribe failures of the gravimetric method to natural causes outside his control.

However, he keeps control of a sufficient number of factors to allow him to play a major part in a lot of cases. In this paper the conditions that govern the use of gravimetry are analyzed and subdivided into three classes, viz. natural, technical and economic conditions.
ÜBER DEN ZUSAMMENHANG ZWISCHEN SCHALLGESCHWINDIGKEIT UND POROSITÄT BEI ERDSCHICHTEN*
Authors: WACHHOLZ H.
Published: April 2006

Abstract

There are various methods of measuring the porosity of rocks. The direct methods are based on the definition of the porosity ratio. The indirect procedures, which have often been adopted purely for economic reasons require interpretation methods that relate certain measured quantities to the porosity ratio.

In the present article, the relationship between acoustic velocity and porosity will be examined. The interpretation method most commonly in use today is based on an approximative equation developed by Wyllie, Gregory and Gardner. This equation obtains the travel time of acoustic waves in a porous medium saturated with liquid by linear interpolation with respect to the porosity n between the travel time T0 in the compact medium (n= o) and the travel time T1 in the pure liquid (n= 1).

As observed values of the travel time T differ from those that follow from this linear equation, it seems obvious to attempt a higher degree of approximation, e.g. by means of a polynomial of the third degree.

Besides T0 and T1 other quantities are needed in order to obtain the coefficients of such a polynomial. These quantities must bear some relation to the physical constants of the rock. Only in the neighbourhood of n= o and n= 1 is it possible to make general statements concerning the behaviour of rocks and, in particular of the skeleton. Therefore, in the points n= o and n= i not only the values of the approximative function T=f(n) itself are given, but also those of its derivative with respect to n. The values of these derivatives ∂T/∂/Tn=0 and ∂T/∂/n‐1 depend on the properties of the material filling the pores and on data concerning the structure of the skeleton and the stresses in it. The considerations leading to a mathematical formulation of this dependence are dealt with in this article.

The elastic constants of the skeleton were developed in Taylor series in n, that, for skeletons with low porosity were truncated after the linear term. The coefficients of n are functions of the constants of the material of the skeleton, the shape of the pores and the stresses in the skeleton. These functions were approximately computed for two limiting cases. For a more general application, it is useful to assume that these coefficients are the same for both Young's modulus and Poisson's ratio and then obtain the functional relation between the coefficient and the residual pressure from measurements performed in the laboratory with various kinds of rock. When this relation is established from such laboratory experiments, the cubic interpolation method can be applied.

Whereas the linear interpolation method, as it has been in use up to now, presents a relation between porosity and acoustic velocity only, the cubic interpolation method makes it possible to determine the influence of pressure and the state of cementation of the rocks. Comparison with data obtained from direct measurements shows good agreement.
PRECISION DANS LES ETUDES MAGNETIQUES AERIENNES SON INCIDENCE SUR LA QUALITE DES RESULTATS*
Authors: GIRET RAOUL
Published: April 2006

Abstract

The accuracy of an airborne magnetic survey is limited by the following factors:

Slow (diurnal) variations of the earth's field.

Fast variations of the earth's field (earth field “noise”).

Drift in measuring equipment.

Magnetometer “noise” (electronics, orientors …).

External disturbances, miscompensation.

Irregularities in flight altitude.

Location errors.

The relative importance of these different causes of errors is analyzed from two viewpoints:

Direct interpretation of anomalies on actual records,

Compilation of isogram maps and their quantitative interpretation (second derivative, reduction to the pole, …).

In conclusion, these errors do not affect the direct interpretation of profiles of standard quality. For instance, if the average altitude of the plane above the magnetic bodies is 1000 m, a variation of its flight level of about 100 m does not introduce serious errors in the interpretation.

On the other hand, compilation of contoured isogam maps is often difficult, and compensation of the previous errors is not always possible except by approximate, rule‐of‐thumb methods. Furthermore, when the interval between lines is greater than the depth of magnetic masses below the plane of flight, the contours are interpolated and do not represent the magnetic field. Sometimes the contouring can offer different solutions, and the map is therefore not objective; this could then result in errors larger than those from the other causes.

To evaluate the quality of the map and its interpretation we must know the location of flight lines and the compilation process employed. It is also wise to avoid quantitative interpretation of small anomalies and limit oneself to a qualitative interpretation.
A NOTE ON THE REFLECTION OF SEISMIC PULSES WITH APPLICATION TO SECOND EVENT REFRACTION SHOOTING*
Authors: O'BRIEN P. N. S.
Published: April 2006

Abstract

Richards (1961) and several others have shown that wide angle reflections may attain large amplitudes. This note extends the plane wave calculations of Richards to include the effect of the phase changes which occur at angles greater than critical. On seismic pulses this introduces a time lead of up to one half period and alters peak‐to‐peak amplitudes by up to 15‐20 %.

It is pointed out that the plane wave reflection coefficient is not applicable at angles very close to critical where the true reflection coefficient is reduced by a factor depending on (R/λ) ¼ In ultrasonic experiments this factor reduced the reflection coefficient by between ½ and ⅓.

Neither does the plane wave coefficient apply at grazing incidence, when it has to be reduced by a factor depending on (R/λ)1‐0.

Graphs are given of the amplitudes of wide angle reflections and head waves for two cases and it is concluded that, except for shallow refractors and angles close to critical, the reflection is always significantly greater than the head wave.
LES FILTRES EN EVENTAIL*
Authors: FAIL J. P. , GRAU G.
Published: April 2006

RESUME

On étudie le principe de filtres séparateurs de vitesses apparentes ou filtres en éventail. L'opérateur de filtrage est déterminéà partir des caractéristiques spectrales bidimensionnelles (fréquence – nombre ?onde) de tels filtres.

Les possibilités pratiques des filtres en éventail sont décrites en ce qui concerne l'amélioration du rapport signal/bruit. On évoque également les modifications éventuelles de la technique ?enregistrement sur le terrain qui pourraient être entraînées par l'utilisation systématique des filtres en éventail.
ORIGIN OF OIL*
Authors: KREJCI‐GRAF KARL
Published: April 2006

Abstract

All sorts of organic matter preserved under anaerobic conditions and transformed in reducing environments may form components of natural crude oils; such oils are linked to sapropel deposits by their contents in porphyrins and trace metals.
QUELQUES EXEMPLES D'ANALYSE D'ENREGISTREMENTS SISMIQUES*
Authors: KUNETZ G.
Published: April 2006

RESUME

Condition nécessaire et suffisante pour qu'un sismogramme obtenu à la surface ?un milieu stratifé puisse être considéré comme résultant de l'ensemble des réflexions, directes et multiples, subies par une onde plane de compression se propageant normalement aux strates, supposés parfaitement élastiques. Modification minimale à faire subir à un sismogramme donné pour qu'il satisfasse à cette condition.

Possibilité, quand cette condition est satisfaite, de déduire du sismogramme la loi de variation de la vitesse des ondes élastiques dans le milieu considéré.

Exposé détaillé du processus pratique sur un exemple et quelques autres résultats de son application.
SOME PHYSICAL ASPECTS OF SEISMIC SCALING LAWS FOR UNDERWATER EXPLOSIONS*
Authors: BURKHARDT H.
Published: April 2006

The paper deals with quantitative seismic measurements near underwater explosions. The blastings were carried out in an artificial small pond, the records were taken in adits, situated vertically below the charge. The range of charges extended from one detonator to 200 g of Ammon‐Gelit explosive.

Conventional seismic scaling by comparing amplitude and size of charge does not yield a simple power law, the exponent being larger for smaller charges and vice versa. This result is due to the different frequency‐spectra of the seismic pulses, as shown by harmonic analysis. In contrast to this, the seismic energy as calculated from the records is directly proportional to the size of charge over the whole range.

During the Lago Lagorai experiments in 1962 some quantitative seismic measurements near the shot point were also carried out. The relevant results are compared with the preceding ones and some differences are discussed.

Some conclusions can be drawn:

The normal methodes, investigating the amplitude only, gives in many cases no clear description; this is especially true for nearby measurements with widely varying frequencies. Therefore it is suggested to extend conventional seismic scaling by including information on the frequency‐spectrum and the kinetic energy of the seismic signal.

In order to get clear experimental data for theoretical investigations, it is necessary to extend the recording over a half‐space, i.e. in adits or boreholes.

Influences of shape and size of the lake, in which the shots are fired, should be taken into account, too.
THE USE OF ISOCHRON CHARTS IN SEISMIC REFRACTION INTERPRETATION*
Authors: NEALE R. N.
Published: April 2006

A graphical method of interpreting refraction data is presented based upon wave front construction in which different overburden velocity distributions can be used. The wave fronts are constructed with the aid of an Isochron Chart. By using two or more Isochron Charts, the effect of velocity variations above the refractor of interest can be allowed for.
GENERALISATION DES OPERATEURS D'ANTI‐RESONANCE A UN NOMBRE QUELCONQUE DE REFLECTEURS*
Authors: KUNETZ G.
Published: April 2006

RESUME

On sait que l'effet perturbateur, sur une trace sismique, d'un fort réflecteur proche de la surface peut être en grande partie éliminé en convoluant la trace par un opérateur, dit “d'anti‐résonance”, très simple, puisqu'il ne comporte que trois termes.

Nous nous proposons de montrer que pour éliminer, de la même manière, l'effet d'un nombre quelconque de réflecteurs, il existe également des opérateurs d' anti‐résonance à un nombre fini de termes, et nous donnerons les formules permettant de construire ces opérateurs.

Les formules obtenues permettent également de vérifier qu'une trace sismique (symé‐trisée par rapport à l' origine) doit être une fonction d' auto‐corrélation, et fournissent, de plus, l'expression et l' interprétation physique d'une fonction dont la fonction d'auto‐corrélation donne la trace sismique.
REFRACTION SEISMICS WITH AN ANISOTROPIC OVERBURDEN: A GRAPHICAL METHOD OF INTERPRETATION*
Authors: HELBIG K.
Published: April 2006

An analytical expression for the time‐distance curve of seismic waves travelling in a medium consisting of intrinsically anisotropic layers with arbitrarily dipping plane interfaces can be given in terms of the “co‐ordinates” of the interfaces (length h of the perpendicular from the shotpoint to the interface, strike ν and dip α of the interface) if for each layer the velocity is given as a function of the orientation of the wave normal. The interpretation of the time‐distance curve is understood as the inverse process, namely finding an expression for the co‐ordinates in terms of some characteristics of the time‐distance curve, e.g. intercept times and apparent velocities. In addition, it is useful to know where the “limiting ray”, which is the ray connecting shotpoint and last geophone, enters and leaves a specific layer, for it is only on the medium between these two points that information can be obtained by interpretation. As ray and wave normal do not generally coincide in anisotropic media, the location of these points cannot be calculated from the co‐ordinates and the direction of the wave normal without recourse to the functional dependence between the directions of ray and wave normal.

An analytical solution of this problem would involve the solution of a number of equations, implicitly containing several parameters. Successive approximation would be rather cumbersome. Instead, a graphical method is proposed which yields all pertinent information without calculation.
CRITICAL‐ANGLE EFFECTS IN SEISMIC EXPLORATION*
Authors: POLEY J. PH.
Published: April 2006

In the case where a medium of velocity α1 is underlain by a medium of higher velocity α2, no propagation of compressional elastic wave energy into the lower medium takes place at angles of incidence exceeding a certain critical value. This so‐called critical angle –which is a measure of the actual velocity‐contrast –is furthermore marked by a sharp increase in the amount of reflected compressional energy. An investigation has been made to find out whether this critical‐angle effect might be usable as a velocity‐contrast indicator in seismic exploration.

Model experiments confirmed the expectation that this effect should be manifest at the surface under ideal conditions.

Two small‐scale field set‐ups showed that the effect is actually measurable in the field (in one case after applying corrections for planting variations).

Finally a seismic line along the shore of the North Sea provided the data for contrast analysis over a considerable distance along the base of the Tertiary. Comparison of the measured changes in contrast and subcrop velocities obtained at those locations where subsurface data from independent sources were available, showed good agreement.

It is concluded that critical‐angle shooting may be used to indicate subcrop changes along a reflector, or even to obtain the same sub‐stratum velocity as is sought in conventional refraction work, at considerably shorter operating distances.
GEOPHYSIQUE ET VOLCANOLOGIE*
Authors: EVRARD PIERRE , JONES LOUIS
Published: April 2006

Abstract

The authors present a review of the activities of the “Centre National de Volcanologie” (Belgium) in the field of the application of geophysical and geodesic techniques in the study of active volcanoes.

The first region of interest is constituted by the African volcanoes: Nyiragongo and Nyamuragira, North of the Kivu lake; the second by the Etna in Sicily.

The methods used were: volcano‐seismology, gravimetry, magnetism, underground clinometry and “tiltmetry”.

The authors close with considerations on geothermic research, which is envisaged for future programmes.
SOME ELECTRONIC SIMULATORS OF GALVANOMETERS AND GEOPHONES, AND THEIR USE FOR INVERSE FILTERING*
Authors: HOSKEN J. W. J.
Published: April 2006

Abstract

Electronic circuits are described which simulate the effects of galvanometers and geophones. By placing these circuits in the feedback loop of an operational amplifier it is possible to obtain transfer functions which are the inverse of those of the transducers. That is, they will remove the distortion on the signal which was produced by the geophone or the galvanometer.

Both a geophone simulator and its inverse filter have been built and used (O'Brien, 1965). The simulator is accurate over at least seven octaves and the inverse filter restores the ground signal to what it would have been if the recording geophone had had a natural frequency √n+1 times that of the actual geophone, where A is the gain of the operational amplifier used in the circuit.
ZUR ENTWICKLUNG EINES KOMPLEXEN ERDUNGSMESSERS*
Authors: ANDERS WILFRIED
Published: April 2006

After a brief reference to the various possibilities of measuring induced polarization, the experimental setup of a transistorized earth resistance meter is shown which allows one to measure the (real) resistance at the frequencies 9 cps and 1.5 cps. As a measure of the induced polarization, it supplies directly the resistivity percentage difference. The possibilities of its use are explained in detail and further development is considered.
A SIMPLE METHOD OF MAGNETOTELLURIC INTERPRETATION *
Authors: JAIN SUDHIR
Published: April 2006

A simple method of magnetotelluric interpretation is derived using the property that the earth currents flow in horizontal sheets. It is shown that when the depth is taken as two‐thirds of the Cagniard's depth of penetration (Cagniard, 1953), the mean resistivity‐over the depth is the same as the apparent resistivity of the medium. From a mean resistivity versus depth of penetration plot, resistivity can be easily computed at all depths of the sounding.

This method gives satisfactory results in a short time and makes it possible to interpret the soundings over media of several layers. However, the results are inaccurate near maximum and minimum points of the plot as well as in the zone of thin layers.
POISSON'S CONSTANT WITH DRY SEDIMENTS AND WITH PACKINGS OF SPHERES*
Authors: WACHHOLZ H.
Published: April 2006

In order to find a relation between Poisson's constant of dry sediments and the porosity, it is necessary to consider the elastic behavior of the sediment's initial state and the final state during the process of consolidation. Sand and other loose materials of more or less granular substances represent the initial state. As substitute for this zone, elastic constants of simple cubic and hexagonal packing are examined. Supposed is the adhesion for the planes of contact. Besides of Hertz's formula for the contact area of spheres, an elastic function of displacement due to a tangential force for the contact area of the spheres is used. This function has been derived. With the aid of these two. elements, the constants of the mentioned packing are calculated. Poisson's constant will then be calculated from the formula being valid for isotropic solids for three different directions of the sound. The mean value is lying at .

With the known equation of Poisson's constant of rock having a small porosity, a third‐degree polynomial is formed. This polynomial agrees well with the average values of known measurements, and therefore can serve as a guide for the calculation of Poisson's constant of water‐saturated sediments.
PART I: THE SECTIONAL AUTO‐CORRELOGRAM
Authors: ANSTEY N. A.
Published: April 2006

The auto‐correlation function of a seismic trace contains information on all the multiple reflection activity present in the trace.

The interpretation of this information is facilitated by the arrangement of autocorrelation functions in cross‐sectional form, in the manner of a normal record section. This is the concept of the Sectional Auto‐Correlogram.

Specifically, the Sectional Auto‐Correlogram will…

Show if the record section does not include significant multiples, thus allowing confident picking of the primary reflections.

Show if the record section does include significant multiples, giving their travel times and inclinations (and, under certain circumstances, their reflection coefficients).

Indicate by what process the multiples should be treated.

Yield an authoritative measure of the success of a multiple‐attenuating treatment.

Delineate shallow horizons, even those whose primary reflections are too early to be recorded satisfactorily.

Give the true travel time of a primary reflector, and the sign of its reflection coefficient.

The Sectional Auto‐Correlogram allows the study of primary reflectors by consideration of the multiples generated by them, and in this sense may be said to turn multiple reflections to advantage. Thus a primary reflection at a certain time is defined if we find that every reflection on the record is followed by a multiple after this certain time. Alternatively, a primary reflection at a certain time is defined if, after that certain time, we can find a repetition of the entire record.

The Sectional Auto‐Correlogram also has secondary uses in fault identification, crustal studies and weathering problems.
A SURVEY OF HIGH RESOLUTION GEOMAGNETICS *
Authors: LANGAN LEE
Published: April 2006

The development of optically pumped and monitored “atomic” magnetometers has allowed the application of greater resolution in geomagnetic measurements for exploration purposes. This paper presents a survey of instrumentation in use, the variety of exploration techniques that are being used in practice, and an estimate of their state of development.

Alkali metals (rubidium and cesium) and helium have been used as active elements to obtain the magnetic‐field dependent Larmor frequency. Each of these instruments is capable of obtaining high resolution data (data with sensitivity numerically smaller than one‐tenth gamma). They have been adapted for airborne, land, and marine usage.

Such sensitivity has enabled the rapid development of geomagnetic gradiometers, high sensitivity aeromagnetics, diurnally reduced ground surveys, broad‐band magneto‐telluric surveys, susceptibility measurements, and convenient magnetic search and location techniques. Outlines of each procedure and an example of its application are given.

Resume

En détection des richesses du sol, “les magnétomètres atomiques” qui fonctionnent selon le principe ?un pompage optique ont permis ?obtenir des mesures plus sensibles. Sont décrits les appareillages actuellement utilisés, les méthodes de mesure différentes, ainsi que la situation technique récente.

La génération de la fréquence Larmor qui dépend du champ magnétique s'obtient par les métaux alcalins (le rubidium, le césium) et par ľhélium. Chacun des instruments utilisés fournit des résultats de résolution fine, inférieure au dixième du gamma. Des équipements furent réalisés pour ľutilisation aéroportée, sur navire, ainsi que sur terre.

De telles sensibilités élevées ont été rendues possibles grâce aux nouveaux gradio‐mètres, aux aéro‐magnétomètres extra‐sensibles, aux mesures des profils terrestres avec correction diurne, aux investigations magnéto‐telluriques large bande, aux mesures de susceptibilityé et aux méthodes pratiques de recherche magnétique.

Le sommaire explique les principes à la base et les applications de tous ces proédés.
UNITS IN GEOPHYSICAL PROSPECTING*
Authors: KOEFOED O.
Published: April 2006

A system of geophysical units is proposed which is based on the International System of Units, using the meter, the kilogram, the second and the ampere as fundamental units. A historical introduction is given and the reasons for preferring the use of the International System of Units in exploration geophysics, are explained.

At the meeting of the European Association of Exploration Geophysicists at Stockholm a resolution will be proposed recommending the use in exploration geophysics of the International System of Units.
MATHEMATICAL DENOTATION OF STANDARD‐GRAPHS FOR RESISTIVITY PROSPECTING IN VIEW OF THEIR CALCULATION BY MEANS OF A DIGITAL COMPUTER*
Authors: VAN DAM J. C.
Published: April 2006

This paper is the follow‐up of an earlier paper by the same author on the calculation of standard‐graphs for horizontally stratified subsoils. The method which was developed in that earlier paper was destined for handwork; it was laid down in master‐tables. The present paper deals with the presentation of similar methods to the computer expert in purely mathematical denotation. Two different denotations are described for the calculation of the image pole strengths involved. The first denotation is an improved version of Watsons formulae; it gives the total strengths. The second denotation preserves the character of successive approximations as described in the previous paper. Both denotations have been worked out until a six layer resistivity profile, but actually there is no limitation to the number of layers. There is no restriction to the resistivities of any layer. The only condition is now that all interface depths must be integers.

This paper is followed by a paper on the relevant computer program by Argelo in this same issue
TWO YEARS' SURVEY WITH CESIUM VAPOUR MAGNETOMETER*
Authors: ROYER G.
Published: April 2006

The present paper shows that the techniques developed and exploited by qualified geophysicists have brought high sensitivity magnetic surveys to an industrial stage.

It will appear that for most mining and oil problems, either on land or sea, the necessary modern equipment already exists without developing it further in the future. In particular, the digitalization of data is practically finished for the magnetometer itself as well as for some navigation techniques.
MEASUREMENTS OF THE VERTICAL GEOMAGNETIC FIELD GRADIENT BENEATH THE SURFACE OF THE ARCTIC OCEAN*
Authors: HEIRTZLER J. R.
Published: April 2006

Simultaneous measurements made on an ice island and about 1000 ft. below show that magnetic anomalies can be detected in the presence of large time variations of the magnetic field. Attenuation and phase lag of time variations at depth are measurable but do not limit the utilization of the vertical gradient of geomagnetic total intensity for defining crustal anomalies.
A 1,800 MILLIVOLT SELF‐POTENTIAL ANOMALY NEAR HUALGAYOC, PERU*
Authors: GAY S. PARKER
Published: April 2006

A large self‐potential anomaly was outlined in 1963–64 at Tantahuatay near Hualgayoc in the Andes of northern Peru. Peak value recorded was–1842 millivolts–thus making it one of the strongest, or perhaps the strongest, SP anomaly ever measured. A lack of detailed geological data precludes the formulation of an adequate explanation for the Tantahuatay anomaly, but geological and mineralogical similarities with the well‐documented Venencocha anomaly near Cerro de Pasco, Peru (Kruger and Lacy, 1949), suggest that the anomaly arises from sulfuric acid associated with the mineral alunite. The anomaly obviously cannot be explained by the half‐cell mechanism of Sato and Mooney (1960), who place a limit of 700 millivolts on self‐potential anomalies over sulfide bodies. Further study of the Tantahuatay anomaly would be of interest in understanding self‐potential mechanisms in general.
THE VERTICAL MAGNETIC FIELD OF AN ALTERNATING CURRENT DIPOLE FOR HORIZONTALLY STRATIFIED MEDIA *
Authors: MUNDRY E.
Published: April 2006

For the computation of the vertical component Hz of the magnetic field of a horizontal A.C. dipole lying on the earth's surface, a recurrence formula is presented for a horizontally stratified half space, to obtain the (n+ 1)‐layer case from the w‐layer case.

By means of several computed diagrams for the two‐layer case, Hz can be determined for different ratios of conductivity of the subsoil and that of the overburden. Thereby the distance from the dipole as well as the layer thickness h are expressed in terms of the wave length A of a plain wave in the overburden. Assuming a sufficiently large conductivity difference, the results show that evidence about the subsurface conditions can be obtained if the distance between the measuring coil and the dipole is of the order of A/3, and if the thickness h of the layer varies within the range A/100 < h < A/6.

As an example for the 3‐layer case, a nonconducting intermediate layer is assumed.
TOPICS ON SEISMIC RESEARCH AT THE MONACO OCEANOGRAPHIC MUSEUM *
Authors: LEENHARDT OLIVIER
Published: April 2006

The author comments the techniques and methods used in continuous seismic profiling by the Monaco Oceanographic Museum during the last five years.

Pinger probe give informations on the Mud layering and thickness. On the shelf or in abyssal plains, boomer and sparker are employed and allow to rise various geological conclusions. South coast of France, continental shelf in the Gulf of Lion, abyssal plain of the Ligurian Sea and Lake of Geneva are the areas concerned to with the results.
CARACTERES DE ĽONDE ELECTROMAGNETIQUE DE SURFACE ENGENDREE PAR UN DIPOLE MAGNETIQUE *
Authors: DUROUX J.
Published: April 2006

The electromagnetic field radiated from a magnetic dipole lying on the ground is considered, in the extremely low frequency range (DC to 20 000 Hertz).

Theoretical and experimental data are given on the characteristics of the surface wave (vanishing wave) generated at the air‐ground interface, in the case of an homogeneous subsurface.

The case of a subsurface with electrical resistivity varying with depth is considered.

It is shown how the above‐mentioned characteristics may be applied in the quantitative investigation of the electrical resistivity of the subsurface as a function of depth, in a method using the measurements on the ground of all the components of the radiated field: horizontal electrical components, vertical and horizontal magnetic components.
EXPLORING DEEP INTERFACES BY SEISMIC WIDE ANGLE MEASUREMENTS*
Authors: MEISSNER R.
Published: April 2006

Amplitudes of the vertical ground motion are larger in the critical angle region than in the near vertical region, especially amplitudes coming from deeper boundaries or interfaces with small velocity contrast. Four basic boundary models are used to derive specific travel time patterns for first order and higher order interfaces. The transition from reflected waves to diving waves in the supercritical angle region is shown for the different models.

Examples from wide angle surveys in Bavaria and in the Buchara region in the USSR have many characteristic similarities. Travel time patterns and asymptotic velocities are almost identical. The most important boundaries of the continental crust ‐ the Mohorovč Ić discontinuity and the top of the crystalline basement ‐ correspond to two boundary models mentioned before. Amplitude and frequency investigations have been used to derive items of the M‐discontinuity. It is certainly a gradient zone and seems to have a lenticular structure with material of stepwise different velocities.
ANALYSIS OF DIFFRACTION PATTERN FILTERING OF VARIABLY DENSITY AND VARIABLY AREA RECORDINGS
Authors: SCHEEN W. L. , VAN ASPEREN M. HOOGENDOORN
Published: April 2006

When a small, transparent replica of a seismic section is illuminated by a homogeneous beam of coherent, monochromatic parallel light a diffraction pattern is created that is representation of the double Fourier spectrum of the recorded seismic waves, i.e. their spectrum in terms of frequency, f, and apparent wave number, k.

Masking selected parts of this diffraction pattern causes the spectrum to be filtered: the recomposition of the filtered spectrum then provides a filtered section.

The ideal seismic section for this purpose would be a continuous variable density section obtained from recording made at every point of the seismic line. The light transmission coefficient (in terms of light amplitude) at each spot of the replica should be linearly related to the instantaneous seismic signal strength at the spot on the seismic section to which it refers.

Unfortunately we cannot make recordings at every point of a seismic line and in our practically realisable recordings we have to be content with sampling in the direction of the location coordinate x. This means that with variable density recordings aliasing will be present and evident in the spectrum obtained in the direction of k; furthermore, the aliased spectrum is also multiplied by a sine function of k because the recording obtained at a given station is not presented on the seismic section as a single line along the time axis, but occupies the entire width of the trace.

The diffraction patterns created by variable density recordings of dipping sine waves, including clipped recordings, and of the effect of dip filtering in such sections are discussed. The efficiency of dip rejection is shown to decrease with increasing dip.

The diffraction pattern of a variable density recording is found to be characterised by a relationship between point pairs in the pattern.

No such simple relationship has been found for the diffraction pattern of a variable area section; the spectra of such VAR sections belong to a very special class, because the amplitude transmission coefficient has only two values, viz. 0 and 1.

Consequently, selective masking of the diffraction pattern of a VAR section may give rise to a filtered profile that does not look like a VAR section at all.

General statements about the diffraction pattern of VAR sections are hard to give, because the transmission coefficient at a given point in the replica is not proportional to the signal level in the seismic section at the relevant point.

In the case of VAR presentation of harmonic waves it was found that, as well as the aliasing effect in the k direction, higher harmonics of the frequency are also introduced.

Some synthetic examples are given that show dip filtering to be less effective with VAR than with variable density recordings.

Some arguments are advanced in favour of the opinion that high‐pass filtering of VAR sections will have less success than low‐pass filtering. This is demonstrated by two synthetic examples.
VERTICAL PROFILING OVER A MEDIUM OF CONTINUOUSLY VARYING RESISTIVITY*
Authors: UNZ M.
Published: April 2006

An interpretation method is suggested for apparent resistivity profiles over a medium of continuously varying resistivity. The medium is replaced by an equivalent laminated system of constant resistivity in each layer and evaluated by superposition. The approximation facilitates detailed subsurface profiling as required for underground storage projects or water infiltration schemes.
MAGNETO‐TELLURIC MEASUREMENTS IN THE NORTH GERMAN BASIN*
Authors: VOZOFF K. , SWIFT C. M.
Published: April 2006

Magneto‐telluric measurements were carried out at a group of eight sites, from Braunschweig to Lübeck, to determine the resistivity values associated with the North German conductivity anomaly. The data were analyzed for scalar and tensor apparent resistivities over the period range 5–2000 seconds. The results in general indicate very conductive (˜I Ω‐m) sediments overlying a resistive basement which is strongly lineated.

Interpreted basement depth (˜6 km) is compatible with published seismic refraction data. Principal conductivity axes in the basement appear to be directly related to trends of salt domes and major troughs. A major change in axis direction occurs near the center of the North German anomaly.
THE EAEG'S CHOICE OF UNITS*
Authors: GREEN R.
Published: April 2006

The use of MKS system units in preference to the cgs system facilitates accurate numerical calculation in magnetostatic problems in geophysics and the practice of stating the precise dimensions of every unit guards against confusion. Suggestions are made for the unique definition of quantities such as magnetic potential, etc. for which the undesirable circumstance of arbitrary alternatives still persists.
PNEUMATIC ACOUSTIC ENERGY SOURCE*
Authors: GILES BEN F.
Published: April 2006

In recent years considerable work has been done to devise a satisfactory non‐dynamite seismic system that would replace dynamite in offshore areas. Prior to the advent of digital recording and processing, the non‐dynamite sources have generally not provided the depth of penetration or the resolution required for satisfactory seismic interpretation.

More recent developments in non‐dynamite offshore marine sources include adaptation of the Vibroseis from a land unit to a marine unit, and adaptation of the Dinoseis unit from a land to a marine unit. The SUE (Seismic Underwater Explorer) system is a thermodynamic non‐dynamite source utilizing a mixture of propane and oxygen detonated in a special chamber approximately 15 feet below the water surface. This source gives penetration to more than 4 sec in areas typified by Gulf of Mexico type geology and shows deeper penetration than had previously been obtained by dynamite along the western United States in areas with 20 lb charge limitations. A pneumatic source, the airgun, has been in production use in the United States since June 1966. This non‐dynamite source provides an intriguing amount of versatility and can be expanded to provide additional energy as necessary to obtain the penetration desired. Tests using systems comprised of from eight to twenty‐three airguns show penetration in excess of 5 seconds in many areas. Power spectra comparisons both in amplitude and frequency content demonstrate that this is a controlled source generating a controlled seismic wavelet and a controlled frequency spectrum that can be tailored to fit requirements of particular areas. Sample sections obtained in the Gulf of Mexico and the Pacific Ocean offshore California show adequate penetration to 5.0 seconds reflection time.

Quantitative measurements with the airguns demonstrate the effect of:

Variation of the number of guns in the system;

Shaping the frequency spectrum by using different sizes of airguns in the system;

Effects on signal‐to‐noise ratios as a result of stacking several small energy sources together;

Reproducibility of the initial pulse wavelet from shot to shot.

The improvement in record quality as a result of advanced digital processing with non‐dynamite sources is comparable to that obtained with dynamite sources. Non‐dynamite sources make additional improvements possible where high source multiplicity is advantageous. Excellent dynamic correlations yield accurate velocity control as well as definitions of apparent velocities attributable to multiples and primary‐to‐multiple amplitude relationships.

Non‐dynamite sources are being used more and more extensively in offshore exploration. The advent of digital recording and processing provides a means for improving depth of penetration and resolution of many non‐dynamite sources.
NOMOGRAMS FOR SOLVING EQUATIONS IN MULTILAYER AND DIPPING LAYER CASES*
Authors: CHAN SIEW HUNG
Published: April 2006

Nomograms for solving equations in multilayer and dipping layer cases are presented. The nomograms constructed are used to solve the following equations: I. Intercept‐time formula. 2. Critical distance formula. 3. Critical angle formula. 4. Critical angle and dip angle formula. 5. Vertical depth formula.
ESSAI DE FILTRAGE NON‐LINEAIRE APPLIQUE AUX PROFILS AEROMAGNETIQUES*
Authors: DREYER HENRI NAUDY ET HENRI
Published: April 2006

Digital aeromagnetic profiles recorded with a high accuracy can be filtered with an electronic computer. Linear filtering is not very efficient, because the frequency spectrum of a given anomaly is very wide. But other methods are possible which, through a step by step analysis of the profile, leave some categories of anomalies unaltered while they remove the others completely.

Our method uses as a criterium the width of the anomalies; it distinguishes and processes differently “bell‐shaped” and “multi‐legged” anomalies. Some examples of the use of the method are shown.