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- Volume 3, Issue 2, 1955
Geophysical Prospecting - Volume 3, Issue 2, 1955
Volume 3, Issue 2, 1955
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POSSIBILITIES AND LIMITATIONS IN APPLYING GEOELECTRICAL METHODS TO HYDROGEOLOGICAL PROBLEMS IN THE COASTAL AREAS OF NORTH WEST GERMANY*
By H. FLATHEAbstractOn the basis of results of extensive geoelectrical investigations in East Frisia as well as of theoretically calculated data, it is shown to what extent geoelectrics can furnish a contribution to success during the preliminary work for the establishment of water works, or for the enlargement of existing water works, in coastal areas. One of the most important tasks in this sphere is the search for permeable beds suitable for intake, the delimitation of their thickness and extent (catchment areas), and then the location of favourable places for wells. This task is rather difficult because of thickness variations in and change of facies of the old‐pleistocene basin clay (Lauenburg clay) and the presence of Tertiary clay and fine clayey sands which, acting as impervious layers, often divide a permeable bed into several storage horizons. Further difficulties arise from the occurrence of brine bearing deposits near the coast, and water from the depths with a high salt content as a result of leaching processes around salt domes.
The problems arising in this connection are illustrated with the aid of examples from the Rheiderland. The following questions seem to be the most important:
a) the physical interpretation of the measurements with special regard to the principle of equivalence;
b) the correlation of the physical data with the subsurface geology.
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DÉTERMINATION DES SALINITÉS DES EAUX DANS LE SOUS‐SOL DU ZUIDERZEE PAR PROSPECTION GÉOPHYSIQUE*
Authors A. VOLKER and J. DIJKSTRAAbstractThe paper describes the determination of the chlorine content of deep ground water by means of the direct resistivity method using the Wenner electrode configuration. The purpose was to investigate the distribution of salinity in the brackish waters within a permeable sand of thickness 250 metres and which forms the deeper subsoil of the Ysselmeer areas.
Such a knowledge is of great value, firstly because the seepage of salt water into the proposed deep “polders” may reduce the crop yield, and secondly because it allows areas, where potable waters may occur, to be delineated.
To test the method, measurements were made near existing boreholes which extended to a depth of 350 metres. From these borings water samples from various depths were collected and it was found that the resistivity pl of the porewater was closely related to their chlorine content. The resistivity p of the saturated sand was determined from the geo‐electrical measurements. It was found that the resistivity p was given by p= 4, 5 pl. From this relation, which is independent of the salinity, the chlorine content of the porewater could be estimated. These results are in close agreement with those of other observers. In the area p varied from 1 to 200 ohmmetres corresponding to a chlorine content changing from 12.000 to 15 mg per litre.
The method was adapted for working over water. Each of fourteen insulated conductors in a special cable terminated in an electrode, and these electrodes were spaced along the cable so that nine different Wenner spacings could be selected. The electrode separations varied from 44 metres up to 306 metres.
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KONSTRUKTIVE DARSTELLUNG VON SEISMISCHEN HORIZONTEN UNTER BEROCKSICHTIGUNG DER STRAHLENBRECHUNG IM RAUM*
By J. BAUMGARTEABSTRACTThe paper describes a geometrical construction which allows the reflecting or refracting Tsoundaries to be derived from seismic time observations.
In reflection work the construction makes use of the ray reflected from the boundary at normal incidence. For constant velocities of the beds, or for a linear increase of velocity with depth, the method leads to the construction of successive envelopes from which the reflecting interface and the rays are obtained. By using the idea of “stretching fictitious rays”, the construction requires ruler and compasses only and is applicable to both plane and curved boundaries with any strike direction and in any number.
The same principles can be applied to refraction problems and again the. construction of successive envelopes allows the indirect or Mintrop wave to be defined. The position of the refracting boundary and the velocity can be derived immediately. As in the reflection method any number of plane boundaries with any strike can be dealt with.
An appendix discusses the use of the method in reflection work with any type of velocity distribution.
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ELECTRICAL HAZARDS IN SEISMIC PROSPECTING*
Authors A. M. SELEM and E. MERLINIABSTRACTHazards due to powerlines, radiotransmitters and self‐potentials in connection with weather conditions are considered. Voltage and current values are given for several measurements made under the most severe conditions of circuit arrangements. The necessity of general safety precautions is stressed.
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A LOW‐FREQUENCY REFRACTION GEOPHONE*
Authors W. M. JONES and A. T. DENNISONABSTRACTThis paper describes a highly portable geophone, designed for large‐scale refraction surveying, which uses a novel suspension system to obtain a natural frequency of about 1.85 c/s. The nominal output is 0.7 volts/cm/second into a 500 ohm load, the damping then being 0.7 of critical. Some typical recorded arrivals obtained with this geophone are illustrated.
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MESURE DU BRUIT ET SISMOS MULTIPLES (etude experimentale dans les Landes)*
Authors HENRI RICHARD and MARCEL RIMBAUTABSTRACTTo improve the ratio of signal to noise resulting from the shot, seismologists have increased the number of geophones in each recording channel and have also increased the length of each spread. As a rule, however, the effectiveness of this multiple recording is based on theoretical considerations, and it appeared desirable to carry out an experimental investigation to obtain a direct measurement of the reduction in noise as a function of the geophone pattern etc. The work was done in the Landes where experience had shown that recording was difficult.
The experimental arrangement, which allowed the accurate measurement of the noise without interference from reflected energy, is described.
Shots, repeated under apparently identical conditions, revealed the random nature of the noise and pointed to the necessity of a large number of experiments to yield an accurate result. Two geophone patterns were employed, one of 12 geophones in a straight line and the other of two parallel lines of geophones, the directions of the geophone lines being either along the shot line, perpendicular to the shot line or at some intermediate angle.
The results obtained, in two widely spaced locations were in good agreement and allowed definite conclusions to be drawn. When the 12 geophone line is long enough the noise is reduced by a factor of 0.36 and with two lines, each 55 metres long and containing 12 geophones, the reduction was 0.29, if the lines were sufficiently widely separated. The results agree with the factor K N‐½ for the reduction in noise where N is the number of geophones per trace and K is a factor close to unity. The results also show the influence of the length of the geophone line its orientation and its mean distance from the shot.
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BOOK REVIEW
Book review in this article
A. S. Eve and D. A. Keys. – Applied geophysics in the search for minerals. Cambridge University Press. Fourth edition 1954.
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Volumes & issues
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Volume 72 (2023 - 2024)
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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)