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- Volume 9, Issue 4, 1961
Geophysical Prospecting - Volume 9, Issue 4, 1961
Volume 9, Issue 4, 1961
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APPROXIMATIONS TO THE REFLECTION AND TRANSMISSION COEFFICIENTS OF PLANE LONGITUDINAL AND TRANSVERSE WAVES*
By R. BORTFELDAbstractThe 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.
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ETUDE STATISTIQUE DE SISMOGRAMMES*
More LessAbstractThe amplitudes of reflection coefficients as computed from well velocity logs are distributed according to approximately normal laws. The distribution in time of the reflection coefficients, the absolute value of which is greater than a certain threshold, is Poissonian. The amplitudes of real seismogram traces obey a normal law.
The autocorrelation of a seismic trace is usually quite different from the autocorrelation of the corresponding impulse.
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NOMOGRAMS FOR DETERMINING MAXIMUM ERRORS AND LIMITING CONDITIONS IN SEISMIC REFRACTION SURVEY WITH A BLIND‐ZONE PROBLEM
Authors L. V. HAWKINS and D. MAGGSAbstractNomograms are presented for the determination of the blind‐zone in seismic refraction surveys. The nomograms are calculated for the case where an intermediate velocity is assumed within the blindzone of a recorded refractor which is overlain by a single recorded layer. The nomograms may be used with time‐term or critical distance calculations and yield the depth to the top of the blind‐zone, the thickness of the blind‐zone, and the corrected depth to the recorded refractor. These values represent the maximum or limiting conditions for a layer of the assumed velocity within the blind‐zone.
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WAVE‐FRONT DIAGRAMS FROM UPHOLE SHOOTING*
By R. MEISSNERAbstractUphole shooting during a reflection or refraction survey provides additional information about the velocities within the weathered layer if recorded with a complete spread of geophones on the surface. Wave‐front diagrams can be constructed which give a genuine picture of the wave paths in the subsurface. Their construction is based on the assumption that the travel time from a shot in the hole to a geophone on the surface is the same as that from a fictitious shot on the surface to a fictitious geophone in the subsurface.
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MESURES D'AMPLITUDE DES ONDES ULTRASONIQUES DANS LES SONDAGES*
Authors J. MORLET and T. SCHWAETZERAbstractSince 1955, the continuous velocity measurements in bore holes, made with one, later with more receivers, have become indispensable for the geophysicist, and can be useful to the geologist and the production engineer. From the beginning we have found that the amplitudes of the signals received vary in the course of a single operation. The variations have for a long time been considered as a disturbing phenomenon. It was tried to suppress them by changing the instrument gain.
With increasing precision of the velocity measurements we have sometimes observed anomalies that were hard to explain, discrepancies between the continuous velocity logs and the conventional well velocity survey, as well as between continuous velocity measurements made with different spacings. These phenomena could be observed in cased as well as in uncased holes. The first attempts to explain these phenomena by considering the velocities only, did not lead to an acceptable theory. A further study of these anomalies of the continuous velocity log has shown that in reality they were caused by strong variations of the amplitude which affected the velocity measurements. This result has led us to study the amplitude variations in open as well as in cased holes.
Up to now, the characteristics of the apparatus used in France and in the Sahara have permitted amplitude measurements with a single receiver only. In addition to the “cement bond log” in cased holes, it seems that in open holes correlations can be observed between the measured amplitude and a) the lithology; b) the fluid contained in the pores; c) the continuity of the matrix or, conversely, the permeability. However, it is now certain that numerous factors affect the amplitudes of the signals simultaneously.
In order to study the attenuation of ultrasonic waves in the different formations traversed by a deep well, we have studied the deformations of the signal in the course of its path in the mud and in the formation, restricting ourselves to the P waves. The factors to be taken in consideration are: a) the geometric energy dispersion, inversely proportional to the square of the distance from the source; b) attenuation by absorption and scattering in the mud and in the formation; c) the transmission coefficients at the transition from mud to formation and conversely, as well as at the transition between two formations having different velocities; d) phenomena of signal composition that are due to the fact that the receivers and the transmitters have a non‐infinitesimal length; these last mentioned phenomena have considerable influenc eon the amplitudes of the received signals.
Finally, the practical method for obtaining an attenuation log in an open hole requires the measurement of the ratio of the amplitudes of the first arrivals at the two receivers. Some examples show the importance of amplitude measurements made with two receivers.
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MODIFIED TRIPOTENTIAL PROSPECTING METHOD*
More LessAbstractA modification to the tripotential method of Carpenter (1955) and Carpenter and Haberjam (1956) is suggested and its theoretical implications and practical advantages are discussed. A specially designed switchbox is described which enables model tank experiments to be made with the modified tripotential method and with conventional methods. The results broadly show that a horizontal distance of the configuration at which the γ—resistance becomes negative directly corresponds to the depth to the interface. Discrepancies between the observed and actual depths are due to the bottom and wall effects of the wooden tank.
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LA RADIOMÉTRIE AÉROPORTÉE, OUTIL D'EXPLORATION GÉOLOGIQUE STRUCTURALE*
By R. GIRETAbstractThis paper gives the results of an airborne radiometric survey flown in France over an area of 1,700 km2 at 150 m. above ground, with parallel north‐south profiles 1 kilometre apart. Besides uranium indications discovered or recovered, this survey shows a close correlation between isorad and geological maps. The southern granitic zone is more radioactive than the northern gneissic one, with a constant step on the boundary. In the northern zone granitic intrusions have high radioactivity, and isorad contours are right on the boundaries of the main granitic units. In addition, the radiometric map shows the connections between these batholith roots and probable new satellite outcrops not shown on the geological map. Such a radiometric map is a help towards completing the geological map, and gives indications regarding probable structural connections between different granites.
The detailed survey of an anomaly discovered during the previous reconnaissance survey is given as another example. It was flown twice, with an aircraft at 75 m elevation with profiles 250 m apart, and with a helicopter at 35 m elevation with profiles 50 m apart. In this area of the main Marche fault, granulites in the south and gneisses in the north have low radioactivity, and the central shearing zone also. But, on both sides of this shearing zone, two crushed areas have high radioactivity, and uranium indications. The geological survey of the northern crushed gneisses area shows small granitic intrusions linked with the radioactive anomalies. These intrusions were unknown until they were discovered with the help of this map. Some mineral indications are connected with them.
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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)