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- Volume 3, Issue 3, 1955
Geophysical Prospecting - Volume 3, Issue 3, 1955
Volume 3, Issue 3, 1955
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RELATIVE DETERMINATION OF THE DENSITY OF SURFACE ROCKS AND THE MEAN DENSITY OF THE EARTH FROM VERTICAL GRAVITY MEASUREMENTS*
By W. DOMZALSKIAbstractThe gravity difference between two stations, one at the surface and the other underground vertically below the former and at a given distance from it, depends on the mean density of the earth, σm as well as on the density of the layer of rock contained between the two stations. When one of these densities is known, the other can be computed from this gravity difference. The reliability of this determination depends on the relative accuracies with which σm and σ can be obtained.
These accuracies are discussed in the paper. The mean density of. the earth has been determined with an accuracy of approximately 0.01 gr/cm3. The determination of the density of a layer of rock depends on density determinations of rock samples which are not representative of the layer as a whole. Thus the accuracy of the value of σ based on a number of sample determinations will depend on many factors, including the method of averaging the density values obtained from the samples and the degree of uniformity in the geology.
To investigate the problem discussed above, three sets of gravity measurements were made under differing conditions. In each instance a number of pairs of stations vertically above each other were occupied on the surface and underground. The results computed from the data on each pair of stations in a set of measurements were considered as repeated measurements of the same quantity, and the most probable value was calculated.
The results demonstrated that the accuracy varied with the conditions prevailing in the area where the observations were made. In Godstone Quarries the dip of the strata was negligible, the rocks fairly uniform and structural conditions undisturbed. Consequently, although the rock layer between the surface and underground stations was only of the order of a hundred feet, the mean density of the earth computed from the average density of the rock samples, was very close to the accepted standard value of 5.52 gr./cm3. This agreement, however, was easily upset when only one random sample density was assumed as representative of a given formation.
In a different locality in Cumberland the observations were made in a mine and on the surface. The rock layer between the surface and the underground stations was approximately a thousand feet thick. One set of measurements followed a line parallel to a fault, the other a line crossing this fault. The results differed appreciably from the standard value of σm, particularly in the latter case.
It is concluded that the gravity difference between a surface and an underground station can be used satisfactorily to determine the average density of a rock layer in situ and en bloc, using the standard value for the mean Earth density.
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UN INTÉGRATEUR POUR LE CALCUL DES CORRECTIONS TOPOGRAPHIQUES*
More LessAbstractWhen gravimetric prospecting is concerned with a region featuring broken relief the calculation of topographical corrections becomes vitally important, for these rectify apparent anomalies completely. It is essential that they should be calculated as exactly as possible. By the classic method this takes an extremely long time and may in the long run involve an outlay actually higher than that expended on operations on the terrain.
It has thus been felt necessary to seek methods which would speed up calculation, and, bearing that in mind, the use of a polar integrator has been envisaged. With this the contour lines of a map can be followed between two fixed radii, the pole corresponding to the location of the position. Two integrator roulettes will give the values of the first two terms of the expansion in terms of the quotient of the height divided by the distance, the second of the terms resulting from the curvature of the earth. Unless there are escarpments very close to the position, the use of these two terms will be sufficient.
Polar integration will deal successively with different contour lines and a subsidiary graph will be plotted, giving the value of each of the integrals in terms of the height. By means of this subsidiary graph the values of the corrections will be obtained with a planimeter and a moment integrator. It is envisaged that the same operation could be repeated in the case of two maps with different scales, for instance, 1–20,000 and 1–200,000 the effect of the areas in the immediate neighbourhood of the position being taken into account by the observer. It is hoped that in this way the calculation of topographical corrections will be made quicker, more exact and less laborious.
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RAPPORTEUR POUR LA CONSTRUCTION RAPIDE DU RAYON REFRACTE*
Authors J. SCHOEFFLER and E. DIEMERAbstractIn this paper we describe a protractor which provides a very simple method of constructing refracted rays in a vertical plane. Another advantage of this apparatus is that it can be made locally in any computing office.
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ETUDE THEORIQUE DE LA VITESSE DES ONDES DE COMPRESSION DANS LES VASES*
More LessAbstractIt has been noted that in water‐saturated silt, the velocity of propagation of longitudinal waves is very low. The purpose of this paper is to suggest a theoretical explanation for this phenomenon.
The medium under study is assumed to consist of identical solid spheres piled one upon the other in a regular array, with the fluid filling the interstices. First the velocity of propagation is calculated for the system of the solid particles alone; then, utilizing the results obtained for this case, the velocities for the solid‐fluid system are calculated. Two velocities are obtained, one of them being higher and the other lower than in the case where no fluid is present.
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REFLECTION SURVEY IN ROUGH TOPOGRAPHY*
By A. M. SELEMAbstractThe problems connected with reflexion surveys in areas of rough topography consist of choosing proper equipment and of making careful near‐surface corrections. This paper illustrates some solutions to these problems that have been adopted in a survey which has recently started in a hily coastal area, in Southern Italy.
As the area is characterized by a youthful and very actively eroded topography of Post‐Miocene clays and litoral clastic sediments, both rotary and percussion drilling were necessary whilst all the seismic equipment had to be light to allow straight lines to be shot as far as possible. Details are given of the truckmounted, light equipment which has been designed for this special purpose.
In connection with the youthful topography of the area being surveyed, the criteria for the proper computation of the near‐surface corrections is discussed. The great differences in elevation, combined with variations in the surface and near‐surface materials, necessitated a careful analysis before deciding the thickness of the correction zone. The results are shown of the preliminary tests carried out in order to determine the average thickness of the low‐velocity zone. The solution adopted for drawing the reference surface determining the base of the correction zone for the whole of the area is subseqently explained.
Reference datum is drawn so as to follow gently, and with segments of constant slopes, the general features of the rough topographic relief at a depth ranging from 30 to 60 metres from the surface. Criteria for the routine checking of velocity variations in the correction zone are illustrated. Such checks are based on both uphole time analyses and refraction “first breaks” plots, with deeper shot holes at constant intervals.
Unsatisfactory results necessitate longer spreads for “low velocity tests” and uphole shooting from deeper holes and the reference datum is deepened for the particular area where this becomes necessary.
The final cross‐sections are shown and although, as yet, no deep holes, are available to check the seismic data, the general appearance of the cross‐sections seems to indicate that subsurface data are free from the influence of topography and of the velocities in the near‐surface formations.
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RESISTIVITY CURVES FOR A CONDUCTING LAYER OF FINITE THICKNESS EMBEDDED IN AN OTHERWISE HOMOGENEOUS AND LESS CONDUCTING EARTH*
By O. KOEFOEDAbstractA method is described for computing master resistivity curves. It is a modification of that devised by Ehrenburg and Watson, but is superior as a shorter time is required for the computations. A collection of 31 master curves for the three layer case is presented.
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A PRACTICAL METHOD OF CALCULATING GEOELECTRICAL MODEL GRAPHS FOR HORIZONTALLY STRATIFIED MEDIA*
By H. FLATHEAbstractFor the quantitative interpretation of field resistivity curves it is necessary to possess theoretically calculated standard graphs. The problem of calculating such graphs for certain parallel stratified media has been solved long ago. The methods developed so far, however, require facilities, which are not normally available to the geophysicist in the field. It has been shown by experience, that the catalogues of graphs, which have been made available for practical use, are inadequate, when the number of layers exceeds three. For this reason endeavours are made to find a method, to allow the geophysicist to calculate model graphs suited to any given special geological situation, with ordinary field facilities and without too great an expenditure of time. The principle of equivalence implies that such a model graph be known with a high accuracy.
To this end, a method for the calculation of model graphs for a series of parallel beds is described. This method is sufficiently simple – no series need to be summed up – to be applied by the geophysicist in practice, even during field work. The study of the kernel function in Stefanescu's integral representation of the “apparent” resistivity leads to a basis for the set of graphs. The prime elements of this basis form a one‐parametric curve‐system, by means of which the graph for any multi‐layer sequence of strata can be obtained to any degree of approximation by linear combination.
The importance of this method in practice is demonstrated with the help of examples.
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BOOK REVIEW
Book review in this article
H. Faul Nuclear Geology. A Symposium on Nuclear Phenomena in the Earth Sciences, John Wiley and Sons, Inc., New York, 1954 Price $ 7.00.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 69 (2021)
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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 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 18 (1970)
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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)