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- Volume 9, Issue 1, 1961
Geophysical Prospecting - Volume 9, Issue 1, 1961
Volume 9, Issue 1, 1961
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PROBLEMES POSES A LA SISMIQUE DANS LA REGION PARISIENNE*
By J. MILLOUETAbstractSeismic work near Paris first began in 1954. Two major difficulties characteristic of this region rapidly appeared:
- 1) The existence of silicified limestone beds in the plateaus: these high‐velocity beds, hard to drill, make it difficult to obtain surface corrections and carry strong “ground‐roll” noise waves.
- 2) The feeble structural variations call for a very accurate control of time measurements.
Drilling difficulties, and the need to eliminate parasite waves, have led to the adoption of pattern shooting from hammer‐drilled holes.
The most effective elimination of ground roll has been obtained by shooting with a lateral offset of 600 m. This method has made it possible to work in regions where originally no reflections could be obtained.
The difficulty of obtaining surface corrections has induced us to shoot up‐hole velocity surveys in deep holes at particularly important points. This work is done by a special crew. At other shot points, surface corrections are calculated from structural maps of surface strata and from velocity‐depth curves. The accuracy of these corrections, checked by drilling and velocity surveys, appears sufficient.
The determination of lateral velocity variations by surface velocity shooting (X2/T2) is not feasible in this part of the Paris basin, as the accuracy of this method is insufficient.
The control of reflection times must take into account the deformation of the signal as a function of the type of recording equipment and filters used, and also the modifications of the character of the reflection due to lateral stratigraphic variations. These variations can be observed by comparing the different velocity logs.
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PHENOMENES ANORMAUX EN SISMIQUE REFLEXION*
By D. MICHONAbstractAbnormal phenomenons such as refractions and diffractions are more readily seen on cross‐sections produced in magnetic processing offices than on single records. The result is that the understanding of these phenomenons intervenes more and more in the interpretation of the data.
Our purpose is to proceed here to a restatement of the geometric properties related to these phenomenons and to make an attempt to distinguish between what is useful for the interpretation and what is not.
To begin with we shall call our attention on diffractions and refractions over faults permitting to recognize the faults or to precise them, then on diffractions caused by irregularities of the surface forming the common boundary of heterogeneous media (more or less deep erosion surface, basement rock, …). The following phenomenons are classified as parasites:
- 1) refractions following very shallow horizons, mostly related to topographic accidents,
- 2) diffractions on the upward path caused by shallow faults,
- 3) diffractions in the horizontal plane, found either in marine seismic or in case of heterogeneities “wrapt up” inside of an homogeneous overthrust.
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VARIATION DES VITESSES EN RAPPORT AVEC L'AGE EN LA LITHOLOGIE APPLICATION DE LA LOI DE FAUST*
Authors C. AYNARD, H. MITRANI and R. DHINNINAbstractIt was felt that Faust's first law was the best guide in classifying data concerning velocities and their correlation with lithology, even if, on a local basis, other laws fit the observed values better.
Data have been re‐examined which were provided chiefly by borehole measurements, both conventional and continuous, but also by some particularly reliable velocity profiles. “Faust's coefficient” has been computed for a series of geological formations, i.e. the ratio of true velocity to the value resulting from Faust's law, according to the age and present depth of the formation.
Some graphs are shown, in the first place concerning the Sahara, and especially the Palaeozoic. It can be seen that the only formation showing values notably less than I disappears.
On the other hand, values greater than I can be noticed for the quartzitic Ordovician of limited thickness.
This comparison has been continued on a broader basis, using, in particular, data published by Faust and Wyrobek. The data concerning the very homogeneous Westphalian result in an average value slightly less than I.
The Aquitanian basin has been studied in greater detail, owing to the closer spacing of the measurements.
Generally speaking, the Middle and Upper Jurassic, the Cretaceous and the Eocene‐Oligocene show values notably higher than the Palaeozoic in the Sahara. This seems to be connected with the higher proportion of limestones and dolomites. Values as high as 1.7 are observed in compact dolomites.
Four maps are given, concerning the Oligocene, the Eocene‐Paleocene (one of them excluding the Danian) and the Upper Cretaceous (exclusive of Danian), and a cross‐section showing the link with lithology.
Some discrepancies with regard to data from borehole samples are discussed.
The question may be raised as to whether such studies can be carried out at the very beginning of exploration of a basin, on the basis of only a few logged wells. In the Aquitaine basin, data given by velocity profiles are in close agreement with those from wells as regards the Tertiary; but as regards the Mesozoic, information is meagre. In basins of the Sahara type, synthetic records have shown that interference of true and multiple reflections with each other is a major drawback. But on the other hand, the possibilities of a combined use of reflection and refraction in order to define lateral changes in velocities may not have been fully appreciated. Some computed examples are shown, since the results of a survey at present being carried out on this basis cannot yet be published.
Finally, mention is made of the part played by the determination of anisotropy, and an appeal is made for more coöperation between geophysicists in this sphere.
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UTILISATION PRATIQUE DES FILMS SYNTHETIQUES DANS LE SAHARA NORD*
By J. LAHERREREAbstractThe object of this paper is to present 19 synthetic records recorded on 12 wells in the North Sahara by different methods.
A comparaison is made of the synthetic records obtained by different methods:
by analogy: United Geophysical Corporation Seismograph Service Corporation
by computation: Institut Français du Pétrole Compagnie Générale de Géophysique
The method originated by Mr Baranov of the C.G.G. is studied in two cross sections: the first on four wells in the Hassai‐Messaoud field and the second on eight wells in the Anngueur region. The influence of internal and surface multiples on the correlations is very important and explains the reasons for marked variations in the quality of reflections. The problem of the origin of the basis of the stratigraphic correlations furnished by the geologists can be a source of errors.
Finally, the comparison between the synthetic and field records indicates that the best results are not always obtained with a synthetic record including multiples.
In conclusion, on the North Sahara, where the importance of multiples is preponderant and not readily apparent, and where interpretation is primarily based on continuity, the study of synthetic records including multiples on all exploration wells would appear to be indispensable.
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ETUDE SUR MODELE ULTRASONIQUE DU PROBLEME DES COUCHES MINCES EN SISMIQUE REFRACTION
By M. LAVERGNEAbstractDifficulties may arise in refraction seismic surveys in the case of thin layers.
Seismograms have been realized by means of ultrasonic model techniques and have led to some results about the propagation of refracted waves along thin layers. The velocities of propagation have been determined as a function of the layer thickness.
When the layer thickness is small compared to the wave length, it is shown that:
— the velocities of propagation of refracted waves can be up to 6 % less than the velocities along thick layers.
— the attenuation of amplitude with distance becomes large.
— the determination of depth from measurements of delays gives doubtful results.
Experiments have been run on two dimensional models but extension of the results to three dimensional cases is possible.
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AN EXPLANATION FOR THE HIGH ULTRASONIC VELOCITY IN INDIAN ROCKS*
Authors M. HAYAKAWA and S. BALAKRISHNAAbstractIt has been observed in the course of a detailed investigation that Indian granites exhibit high ultrasonic velocities in contrast to the specimens obtained from other countries. An attempt has been made in this paper to offer an explanation of this peculiar behaviour. Firstly, the initial internal pressure and initial velocity at the time of formation of rocks are calculated by using the experimental results for the changes in velocity with pressure in Indian, American, Japanese and Russian rocks. Besides this, standard curves have been computed using the above named parameters. Also, their possible influence on the high velocity values of Indian rocks, even under atmospheric conditions is studied. The effect of time of the formation of rocks at specific depths is also calculated in detail. As one of the results of this investigation, it comes out that under high pressure (deeper depths) it requires longer time to complete the processes of formation of rocks than under low pressure (shallow depths) and under high pressure a lot of stress energy can be stored i.e., the initial velocity becomes high when the rock is formed at great depths while at shallow depths a less stress energy can be stored and consequently less velocity values. Thus the combined effect of time and physical conditions at the time of formation of rock on the velocity in the rock at a given time is investigated. It can be seen here that the elastic behaviour of rocks with pressure will provide a clue to the possible petrogenetic history and also the approximate age of the rock.
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PROPAGATION OF WAVES IN INHOMOGENEOUS MEDIA
More LessAbstractA method is developed for determining the propagation of a wave of arbitrary shape in an inhomogeneous medium in which both the density and the velocity change with depth according to an arbitrary continuous function. The solution is obtained by means of successive approximations. The successive terms in this approximation correspond to the direct wave, single reflections and multiple reflections of successive order. The general formulae are then applied to three particular types of transition layer.
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CALCUL DE L'ATTRACTION D'UN POLYGONE HORIZONTAL DE DENSITÉ UNIFORME*
By JEAN GOGUELAbstractThe vertical component of the attraction exerted by a horizontal homogeneous polygon is obtained as the sum of terms proportional to the difference, for each of the vertices, between the external angle and its spherical perspective. A set of computation charts is presented, each for a fixed value of the angle of the polygon (22° 30′, 30°, 45°, 67° 30’ and 90°). These charts give the difference between the external angle and its spherical perspective as a function of the quotient of the distance from the sides to the vertical through the station over the depth. The chart for the right angle is an alignment chart, permitting a more accurate reading; the others are graphs in rectangular coördinates.
For a remote station the result would be obtained as a small difference between relatively large quantities, and consequently with a not quite satisfactory precision. For these stations it is preferable to assume the entire mass to be concentrated in the centre of gravity.
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VERSUCHE ZUR AUSNUTZUNG VAGABUNDIERENDER STRÖME IN DER GEOPHYSIKALISCHEN PROSPEKTION.*
More LessAbstractThe experimental recording of the three components of the diurnal intensity of the stray currents in a mine shaft outside the region of mining activity of Freiberg has shown the uninterrupted presence of these currents with periods ranging from a few minutes to a few hours. It is demonstrated from theory and by model experiments that prospection by stray currents, utilizing telluric measuring methods, is feasible when the ratio between the distance to the sources of the stray currents and the size of the surveyed area exceeds a limiting value which depends on the accuracy of the measurements. To obtain an economic procedure it is necessary to replace the classical telluric technique by a vectorial recording method.
From a field example in the vein deposit of Freiberg it is shown that stray currents of arbitrary direction, and also telluric currents, are canalized by veins of anomalous conductivity. Other examples are given of underground measurements of stray currents, which were carried out for studying conductivity anomalies.
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DEEP ORE EXPLORATION BY ELECTRICAL METHODS
By H. BRAEKKENAbstractIt has been found that ore prospecting by suitable electromagnetic and electrical methods may under favourable conditions—such as occur in parts of Norway—be carried out to greater depths than have so far been realized. By 500 c.p.s. electromagnetic measurements at the surface there was located a deep ore body, occurring in the continuation of, but separate from, an old ore body, which had been worked down to its end at a depth of some 500 m. The new ore body, starting at a depth of some 700 m and continuing downward with a slightly more dipping axis, has been traced over a considerable distance by further electromagnetic measurements at the surface, with the energizing current led directly to the ore in drill holes. Possibilities, problems and further improvements in deep E.M. techniques of measurement and interpretation are discussed. By D.C. equipotential mapping at the surface, with the current led directly to the ore, the location and direction of the deep ore body was established in agreement with the electromagnetic findings. Problems and developments in deep Charged Potential prospecting and interpretation are discussed. By 500 c.p.s. electromagnetic and D.C. potential measurements in drill holes essential information has been achieved both in locating ore formations beside a drill hole and in deciding whether ore formations in different drill holes are connected or not.
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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 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)