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- Volume 13, Issue 2, 1965
Geophysical Prospecting - Volume 13, Issue 2, 1965
Volume 13, Issue 2, 1965
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SPONTANEOUS POLARIZATION STUDIES*
Authors ALEX BECKER and W. M. TELFORDABSTRACTSelf potential measurements, both in drill holes and on surface, have been made in and around sulphide mineralization. Similar measurements were carried out on small scale laboratory models. The potential and current configurations around theoretical sulphide bodies are also described. Results of the model and field work seem to verify, in a qualitative manner, the electrochemical mechanism of spontaneous polarization proposed by Sato and Mooney. Possible causes are suggested for the abnormally large surface anomalies frequently found in the field.
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GEOPHYSIQUE ET VOLCANOLOGIE*
Authors PIERRE EVRARD and LOUIS JONESAbstractThe 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.
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A RAPID METHOD OF DETERMINING DIP OR MAGNETIZATION INCLINATION FROM MAGNETIC ANOMALIES DUE TO DYKE‐LIKE BODIES*
By D.W. POWELLABSTRACTAnomalies due to igneous dykes or layering within metamorphic rocks often approximate to those calculated for geometric dyke models. In aeromagnetic surveys over outcrops of these rocks, where the ‘depth’ is generally the flying height, the ratio in which two geometric functions combine to produce such anomalies can be determined from profiles. This ratio is influenced by geomagnetic latitude, the strike and dip of the ‘dyke’ and its direction of magnetization. It is practicable to solve for either unknown, i.e. dip or magnetization inclination if the other is assumed. Since igneous dykes tend to be vertical and metamorphic rocks are generally inductively magnetized it is geologically valid to make these assumptions.
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ANALYTICAL AEROMAGNETIC INTERPRETATION THE INCLINED PRISM*
By J. K. C. MOOABSTRACTThe problem of the magnetic anomaly due to the induced magnetisation, of a body‐having the form of an inclined prism is treated analytically. The formula for the total intensity anomaly is analysed to yield relations between parameters of the anomaly and the depth, width, and dip of the magnetic model. The treatment is extended to include the case of remanent magnetisation. The numerical results are presented as graphs prepared for use in the interpretation of measured anomalies.
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UN NOUVEAU MAGNETOMETRE AERIEN LE MAGNETOMETRE A VAPEUR DE CAESIUM
More LessABSTRACTCompagnie Générate de T.S.F. (C.S.F.) has recently developed a new magnetometer based on the optical pumping of caesium vapour.
After carrying out a systematic programme of ground and in‐flight tests, Compagnie Générale de Géophysique (C.G.G.) has put several of the airborne models into service.
Following a discussion of the fundamental principles involved, the instrument is described and its specifications are given. The two forms of read‐out are: an analog recording, of the conventional type, and a digital version, recorded on magnetic tape.
The instrument measures the total intensity of the earth's magnetic field with a sensitivity of 0.03 gamma. Temperature compensation is provided, making possible operation between — 20 and + 45 degrees centigrade. Performance is not affected by an inhomogeneous field as long as the gradient is less than 10 gammas per centimetre.
The range of the instrument is from 20,000 to 70,000 gammas and the values are obtained automatically, without the intervention of an operator.
There is no danger of instrumental drift, and the selection of caesium vapour and the components of the detector guarantee that the absolute value of the field, throughout the range of the instrument and regardless of the circumstances, is measured to within one gamma.
The first large‐scale project making use of these magnetometers, carried out by C.G.G., is the Magnetic Map of France, which has been compiled for the Centre National de la Recherche Scientifique. The characteristics of this project are given.
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SOLUTION OF SOME INVERSE PROBLEMS OF APPLIED GEOPHYSICS*
By D. ZIDAROVABSTRACTAn iterative process is proposed that computes a distribution of masses giving rise to a certain gravitational field. The possibility of applying a similar procedure to magnetic and electric fields is also discussed.
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A GENERALIZED FORM OF NETTLETON'S DENSITY DETERMINATION*
More LessABSTRACTThe formula for the Bouguer anomaly contains the density of the masses above the datum plane. If we use a wrong value for the density we obtain on the Bouguer map additional anomalies which coincide with the topographic features.
If we do not know the Bouguer density we can compute several Bouguer maps based on different arbitrary densities. Then we select that map which shows the least correlation with the topography and obtain in this way a reasonable value for the density. This method has been found by Nettleton.
Several authors have formulated this process mathematically and have elaborated numerical methods. This paper will present such a method. It differs from the former methods by the following:
- 1 A general formula for the Bouguer density is presented, using a non‐specified linear operator. By choosing special definition for this operator, we can obtain the formulae of several of the known methods.
- 2 It is shown that the determination of the density by “Nettleton Profiles” gives less reliable results than the statistical investigation of the whole area covered by the gravity survey.
- 3 The known methods do not consider the variations of the vertical gradient. It is shown that in areas with a regional gravity anomaly or an extended topographic feature the known methods yield poor results.
- 4 It is shown that the vertical gradient of gravity used as operator gives a most exact and simple formula.
- 5 It is shown how the electronic computer—after reading the Bouguer values based on an arbitrary initial density—can compute the residuals, derivatives etc., which are based on the corrected Bouguer density.
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A SEMI‐DIRECT METHOD OF INTERPRETING RESISTIVITY OBSERVATIONS*
By O. KOEFOEDABSTRACTIn this paper a method of interpreting resistivity observations is proposed which consists of two steps. The first of these steps is to approximate the observed resistivity curve by a sum of two‐layer resistivity curves—which are asymptotic to the observed curve—decreased by a constant value. Experience shows that this approximation usually can be made to be reasonably close. In cases where the residue is too large to be neglected, this residue can be accounted for by the addition of the effect of a pair of fictitious current poles of equal and opposite strength.
The approximation by asymptotic two‐layer curves could then be translated either into a distribution of fictitious current poles on the vertical through the current electrode, or into the kernel function in the integral expression for the apparent resistivity. Experience shows, however, that the distribution of fictitious poles derived from the approximation by asymptotic two‐layer curves, may deviate very strongly from the actual distribution of image poles. The error in the kernel function, on the other hand, is shown to be of the same order of magnitude as the relative error in the apparent resistivity. The kernel function is therefore used in the proposed method as an intermediary for determining the resistivity stratification.
From the approximated kernel function some information can be obtained directly on the resistivity stratification in the subsurface. This information, however, is sometimes incomplete and often not very accurate. The step from the approximated kernel function to the resistivity stratification is therefore essentially indirect, i.e. the approximated kernel function is compared with kernel functions computed for different resistivity stratifications. The advantage of this method over comparison of the observed resistivity curve itself with theoretical resistivity curves is that the computation of the kernel function, starting from the resistivity stratification, can be done far more quickly than that of the resistivity curve. The kernel function for any number of resistivity layers is a quotient, of which both thé numerator and the denominator contain only terms which, when plotted on monologarithmic graph paper, are straight lines. This property of the kernel function makes it possible to compute e.g. the kernel function for a three‐layer case in about a quarter of an hour.
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GEOPHONE DISTORTION OF SEISMIC PULSES AND ITS COMPENSATION*
More LessABSTRACTAn electronic analogue was used to obtain geophone distortion of typical seismic pulses. Some of the results were checked by computation and in all cases agreement was extremely close. For late arrival events estimates of attenuation and phase shift may be made from the steady‐state response curves. It is explained why these curves are not applicable to the first peak, for which one must use the results in Table III.
Ground‐geophone coupling introduces the same distortion as a very lightly damped galvanometer with a natural frequency usually in the range 100c/s—200c/s. In swampy terrain this frequency may reduce to 30c/s—40c/s and coupling effects may be severe.
An electronic unit whose characteristics are the inverse of those of a geophone has been used to improve the first peak amplitude of a seismic refraction record. Since 1c/s geophones are often desirable in refraction survey it may well be more economic to use higher frequency geophones followed by their inverse filter.
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A METHOD OF COMPUTING TRUE INTERVAL VELOCITIES FROM EXPANDING SPREAD DATA IN THE CASE OF ARBITRARY LONG SPREADS AND ARBITRARILY DIPPING PLANE INTERFACES*
More LessABSTRACTThe method presented takes the influence of refraction rigidly into account. All travel‐times available are used in an application of the method of least squares. The solution is illustrated by a computer flow diagram.
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BOOK REVIEW
Book reviewed in this article:
M. R. J. Wyllie, The Fundamentals of Well Log Interpretation.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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Volume 64 (2015 - 2016)
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Volume 63 (2015)
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Volume 62 (2014)
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Volume 61 (2013)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 58 (2010)
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Volume 57 (2009)
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Volume 56 (2008)
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Volume 55 (2007)
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Volume 54 (2006)
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Volume 18 (1970 - 2006)
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Volume 53 (2005)
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Volume 52 (2004)
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Volume 51 (2003)
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Volume 50 (2002)
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Volume 49 (2001)
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Volume 48 (2000)
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Volume 47 (1999)
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Volume 46 (1998)
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Volume 45 (1997)
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Volume 44 (1996)
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Volume 43 (1995)
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Volume 42 (1994)
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Volume 41 (1993)
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Volume 40 (1992)
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Volume 39 (1991)
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Volume 38 (1990)
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Volume 37 (1989)
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Volume 36 (1988)
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Volume 35 (1987)
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Volume 34 (1986)
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Volume 33 (1985)
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Volume 32 (1984)
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Volume 31 (1983)
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Volume 30 (1982)
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Volume 29 (1981)
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Volume 28 (1980)
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Volume 27 (1979)
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Volume 26 (1978)
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Volume 25 (1977)
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Volume 24 (1976)
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Volume 23 (1975)
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Volume 22 (1974)
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Volume 21 (1973)
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Volume 20 (1972)
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Volume 19 (1971)
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Volume 17 (1969)
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Volume 16 (1968)
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Volume 15 (1967)
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Volume 14 (1966)
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Volume 13 (1965)
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Volume 12 (1964)
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Volume 11 (1963)
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Volume 10 (1962)
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Volume 9 (1961)
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Volume 8 (1960)
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Volume 7 (1959)
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Volume 6 (1958)
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Volume 5 (1957)
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Volume 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)