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- Volume 11, Issue 4, 1963
Geophysical Prospecting - Volume 11, Issue 4, 1963
Volume 11, Issue 4, 1963
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THE USE OF GEOPHONE GROUPS TO IMPROVE THE SIGNAL‐TO‐NOISE RATIO OF THE FIRST ARRIVAL IN REFRACTION SHOOTING*
By DAVID DENHAMAbstractFor a group of n geophones at a single station, a signal‐to‐noise improvement of √n can only be expected when the background noise, between the individual detectors in the group, is completely uncorrected. This paper gives results, from four different sites, for the variation of correlation coefficient with distance between the detectors. It is found that at these locations the geophone spacings required to reduce the correlation coefficient to zero varied from 30 to 300 feet.
These results are in agreement with the predictions of the theoretical model of a two‐dimensional isotropic noise field described by Hills and Faran (1952). This model provides estimates of the spacing required between geophones to reduce the correlation coefficient to zero, provided that the velocity of the noise is known and that its spectrum is of a simple form.
Often it is impractical to reduce the correlation coefficient to zero because of the large spacing required between the geophones in the group. For this case a simple method is described in which the signal‐to‐noise improvement can be calculated given that the correlation coefficients between the geophones in the group are known. This procedure can easily be used in the field because the correlation coefficients can rapidly be computed from paper records by using Tomoda's method (Tomoda, 1956) which gives values of sufficient accuracy to be used for on‐the‐spot calculation.
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QUELQUES EXEMPLES D'ANALYSE D'ENREGISTREMENTS SISMIQUES*
By G. KUNETZAbstractNecessary and suficient conditions are established, which allow a seismic trace, recorded at the surface of a stratified, perfectly elastic medium, to be considered as resulting from all the direct and multiple reflections of a plane, compressional wave propagating normally to the strata.
The minimum modifications to be carried out on an observed trace in order that the above conditions be satisfied, is discussed.
When this condition is satisfied, it is possible the variation of the velocity of the elastic waves in the medium to deduce, from the seismic trace.
The complete process is shown in detail by a practical example. Some other results are given.
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ELASTISCHE ANISOTROPIE IN TEKTONISCH VERFORMTEN SEDIMENTGESTEINEN*
By W. GIESELAbstractSedimentary rocks deformed by tectonic forces, especially sandstones, show anisotropy in their elastic behaviour. The velocity of longitudinal ultrasonic pulses in them varies with direction. In three directions perpendicular to each other, velocities are extreme. These directions are parallel to the tectonic axes.
In such rocks grains are arranged according to their shape. This special orientation developed during the tectonic deformation, which can also be recognized by deformed fossils.
A close relation exists between the anisotropy and the arrangement of the grains. A model of the sediment with grains arranged in a similar manner as in natural sandstones is tested mathematically for its elastic behaviour. Anisotropy similar to that of natural sandstones is exhibited by this model.
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WAVE FRONT INVESTIGATIONS IN MODEL SEISsMOLOGY*
Authors H. BERCKHEMER and J. ANSORGEAbstractA method is described which allows to measure the elastic wave motion in the “interior” of two‐dimensional solid models. It is based on the measurements of the lateral deformations of a plate that are related to the state of compression in the plane of the plate. A capacitive detector measuring the motion has been developed.
Wave front studies have been carried out in a half space, in two half spaces in contact and in a single‐layered half space. The experimental results are compared with theory. Special attention has been paid to the head wave.
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FIVE‐LAYER MASTER CURVES FOR THE HYDROGEOLOGICAL INTERPRETATION OF GEOELECTRICAL RESISTIVITY MEASUREMENTS ABOVE A TWO‐STOREY AQUIFER*
By H. FLATHEAbstractOne of the main tasks of geoelectrical techniques in their application to hydrogeological objects is the detection of aquifers with fresh water and the delimitation of such aquifers from impermeable or salt‐water‐bearing beds. In practice it frequently occurs that intercalated clay horizons divide an aquifer into several “storeys” of various thicknesses.
The present paper deals with an aquifer consisting of two storeys (sand and gravel), above a substratum of good electrical conductivity (clay, salt water). By means of a set of 72 computed master curves, the possibilities of interpretation of geo‐electrical measurements above such a substratum are examined with respect to the specific resistivities of the overburden and the intercalated clay horizon. Different thicknesses of the two groundwater storeys are considered. The result gives an idea of the extent to which geoelectrical measurements may help to elucidate hydrogeological conditions; it also provides the geophysicist with practical hints for the performance and the interpretation of such measurements.
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THE INTERPRETATION OF MAGNETIC ANOMALIES DUE TO DYKES*
Authors J. M. BRUCKSHAW and K. KUNARATNAMAbstractBased on simple scaling laws, a method is developed for the routine interpretation of magnetic anomalies arising from uniformly magnetised dykes. The method can be applied to any observed component of the field, i.e. the vertical, horizontal or the total field anomaly and it is applicable for any resultant direction of magnetisation of the dyke itself and for any strike direction. The necessary data for the interpretation are given in the form of curves covering a range from o up to 15 for the ratio of the thickness of the dyke to the depth to its top. It is also shown that, by integrating in a suitable manner the observed anomaly due to inclined bodies having a horizontal upper surface and enclosed by two pairs of parallel and inclined planes, a curve is obtained which corresponds to a dyke anomaly and which can be interpreted by the same technique.
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CONTRIBUTION AU CALCUL SIMPLIFIE DES CORRECTIONS DE RELIEF A GRANDE DISTANCE EN GRAVIMETRIE*
By R. NEUMANNAbstractIn a recent paper, H. A. Winkler developed a suitable method for determining longdistance terrain corrections in gravity by interpolation. Such a technique implies that it is necessary to take into account the changes of the correction g not only with the location (X, Y) but also with the elevation Z of a station. Study of the function g(Z) shows that its curve, with amply sufficient accuracy, may be represented as a parabola. Moreover, the parabolas corresponding to different stations are all equal. From these properties a method has been derived which saves computing time and yields long‐distance terrain corrections with convenient accuracy.
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SURFACE‐SHIP GRAVITY MEASUREMENTS IN THE NORTH SEA*
By U. FLEISCHERAbstractModern marine gravimeters enable us to measure gravity at sea with a mean error of 2 to 5 mgals, depending on sea conditions and navigational accuracy.
Good results could be obtained, even in rough seas, by means of Graf's ASKANIA Sea Gravimeter mounted on an ANSCHÜTZ gyrostabilized platform.
When a DECCA Navigator System is used, the gravity measured at the points of intersection agrees within 5 mgals at wind‐force Beaufort 4 and vertical accelerations of about 60.000 mgals.
The southern North Sea has been surveyed with VFS “GAUSS”, the research vessel of the German Hydrographic Institute, Hamburg. Since 1959 about 4.000 miles of continuous gravity measurements combined with simultaneous magnetic measurements have been carried out in this area. Here nearly all remote‐control gravimeter stations of the Netherlands Geodetic Commission were passed once or several times. Hence they could be used as basic points. However, there are a few cases in which the Dutch data could not be confirmed.
The results are given as a free air isogam map covering the area between the meridian of Greenwich and 8° east, and between 54° and 56° latitude, with decreasing density of the measurements from east to west.
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LA RECHERCHE DE PHOSPHATES PAR RADIOMETRIE AEROPORTEE*
Authors R. BOLLO and M. JACQUEMINAbstractAs a rule, deposits of phosphates contain radioactive elements. Studies made in the laboratory of samples from North America and north Africa had shown that this radioactivity is due to the presence of uranium, and occasionally of thorium, either in the form of vaandate or in a diffuse state. Surface measurements carried out with a portable scintillometer had enabled some deposits to be outlined, but radiometric prospecting was only a supplementary method at this stage. From the results of these surface studies, it could be considered that the relations emitted by phosphates were sufficiently intense to be detected from a certain altitude.
Consequently, a scintillometer was installed aboard a helicopter, and experimental tests were carried out on the known deposit of Djebel Honk, Algeria. Three profiles, flown perpendicular to the existing layers at 45 metres, permitted us to ascertain the validity of the method, and to define the working standards. Despite the small width of these outcrops (30 metres at most), it was established that “helicopter radiometry” could be used for phosphate exploration.
Fourteen thousand kilometres of profiles were flown in northern Algeria. Since the phosphate deposits known in this area are located at the contact of the Cretaceous and Eocene (Dano‐Maestrichtian, Thanetian, Ypresian), a systematic coverage of these geological series was undertaken. It was then possible to draw up a rapid inventory of the series.
Airborne radiometry, and helicopter radiometry in particular, now seems to be the most efficient, quickest, and least expensive method that can be applied to systematic phosphate exploration.
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BOOK REVIEWS
Books reviewed in this article:
V. Fritsch, Elektrische Messungen an räumlich ausgedehnten Leitem (besonders in der Angewandten Geophysik), (Electrical measurements on 3‐dimensional extended conductors especially in applied geophysics)
D. S. Parasnis, Principles of Applied Geophysics
N. A. Plotnikov, Ressources en eaux souterraines: Classification et Methodes ?Evaluation
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Volume 72 (2023 - 2024)
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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 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)