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- Volume 15, Issue 1, 1967
Geophysical Prospecting - Volume 15, Issue 1, 1967
Volume 15, Issue 1, 1967
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UNITS IN GEOPHYSICAL PROSPECTING*
By O. KOEFOEDABSTRACTA system of geophysical units is proposed which is based on the International System of Units, using the meter, the kilogram, the second and the ampere as fundamental units. A historical introduction is given and the reasons for preferring the use of the International System of Units in exploration geophysics, are explained.
At the meeting of the European Association of Exploration Geophysicists at Stockholm a resolution will be proposed recommending the use in exploration geophysics of the International System of Units.
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THE TERRAPAK *
By M. PIEUCHOTABSTRACTThe Terrapak is a new seismic energy source of the surface energy source type. Essentially, the Terrapak consists of a compressed air piston which transmits a shock to the ground by means of a striker plate attached to the piston rod. The Terrapak is truck‐mounted together with its ancillary equipment. Shocks can be generated at 8 second intervals, and although the weight of the striker is relatively low (approximately 1400 Ibs) the impact velocity exceeds 66 ft/sec. The energy available is about 85,000 ft. lb., which is equivalent to that produced by conventional weight‐dropping methods. The power spectrum of the seismic wave produced by the Terrapak is spread out towards the higher frequencies because its impact velocity is greater than that of a dropping weight.
There is a noticeable difference between noise records made with the Terrapak and noise records made with a conventional weight‐dropping technique. The same noise frequencies are found in both cases, but low frequency noise is stronger with weight dropping than with the Terrapak, whereas the intermediate frequency reflections obtained using the Terrapak are more suitable for correlation.
The expansion of the compressed air generates a sound wave, the effects of which must be eliminated by the use of multiple geophone arrays. A spectrum analysis of the records has confirmed that the waves produced by the Terrapak are rich in intermediate frequency components. This facilitates the picking of reflections in normal areas where the apparent frequency of the reflections is about 30 to 35 c/s.
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A DISCUSSION ON SEISMIC BINARY GAIN SWITCHING AMPLIFIERS *
Authors LEE SIEMS and FRED W. HEFERABSTRACTGeophysical field equipment has undergone rapid changes in the past decade; from simple AGC amplifiers and galvo cameras to binary gain switching amplifiers and digital recorders, all in an attempt to keep pace with the new geophysical interpretive methods developed, and the growing acceptance of the terminology, methods, and philosophy of communication theory.
The additional tools of the digital recorder and digital computer make it possible to utilize these new techniques in geophysical processing. Accomplishing these new techniques demands severe requirements on the digital field recording process in handling the decreasing energy return from the seismometer, and to fully realize the capabilities of digital techniques in reducing data. Simple automatic gain control may be used. However, in the more sophisticated interpretive methods, such as autoregression and deconvolution, it is necessary to reconstruct the actual energy levels in the computer. Recording the control signal used in master AGC or programmed gain control may prove satisfactory; however the accuracy of control versus gain is limited to I% for such analog instrumentation.
To utilize the computer to its fullest extent, and to accurately perform these new techniques, requires an accuracy of I% or better. This accuracy is obtainable by using a step gain control where the gain is increased by fixed steps in which each step represents a gain in amplification by a constant factor. The accuracy in this case can be made dependent only on the tolerance of resistors used as attenuators or feedback elements. Preferably the constant factor of gain steps should be a number easily handled by the computer. By using 6 db steps it becomes a simple matter to shift binary numbers, such as multiplying or dividing by 10 in the decimal system.
The requirements or parameters for such an amplifier system, and the elements of the amplifier necessary to achieve these requirements are presented.
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THE USE OF THE CONTINUOUS SEISMIC PROFILER TO SOLVE HIDDEN LAYER PROBLEMS*
By N. A. MORGANABSTRACTIntegration of the continuous seismic reflection profiler with the marine refraction method on engineering surveys offers a solution to hidden layer problems, allowing both layer velocity and thickness to be computed.
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SPECIFICATION OF TWO MODULI OF A VISCOELASTIC MATERIAL *
More LessAbstractIn order to adequately understand the complicated, anelastic processes governing wave propagation in soils or highly weathered rock it seems desirable first to have available a convenient laboratory material for such propagation studies the frequency dependent rheological parameters of which are known beforehand. A property measurement program has been conducted to determine the complex Young's modulus and the complex shear modulus of a commercially available viscoelastic material over a broad spectral region.
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MATHEMATICAL DENOTATION OF STANDARD‐GRAPHS FOR RESISTIVITY PROSPECTING IN VIEW OF THEIR CALCULATION BY MEANS OF A DIGITAL COMPUTER*
More LessABSTRACTThis paper is the follow‐up of an earlier paper by the same author on the calculation of standard‐graphs for horizontally stratified subsoils. The method which was developed in that earlier paper was destined for handwork; it was laid down in master‐tables. The present paper deals with the presentation of similar methods to the computer expert in purely mathematical denotation. Two different denotations are described for the calculation of the image pole strengths involved. The first denotation is an improved version of Watsons formulae; it gives the total strengths. The second denotation preserves the character of successive approximations as described in the previous paper. Both denotations have been worked out until a six layer resistivity profile, but actually there is no limitation to the number of layers. There is no restriction to the resistivities of any layer. The only condition is now that all interface depths must be integers.
This paper is followed by a paper on the relevant computer program by Argelo in this same issue
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TWO COMPUTER PROGRAMS FOR THE CALCULATION OF STANDARD GRAPHS FOR RESISTIVITY PROSPECTING*
By S. M. ARGELOAbstractThis paper includes publication of two algorithms, to be processed on a digital computer. The algorithms are based upon the two mathematical denotations, presented by Dr. J. C. van Dam in this same issue. The computer programs are written in Algol, in the form of procedures (i.e. subroutines to be activated from a surrounding Algol program).
Each of the two procedures is preceded by a numerical analysis. Some remarks are made about the reliability of each of the two procedures, the production and the computing time required.
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SOME RESULTS OF THE GEO‐ELECTRICAL RESISTIVITY METHOD IN GROUND WATER INVESTIGATIONS IN THE NETHERLANDS*
Authors J. C. VAN DAM and J. J. MEULENKAMPAbstractIn the year 1958 the Service for Water Management of the „Rijkswaterstaat” started its program of geo‐electrical resistivity prospecting in the western part of the Netherlands. The aim of this program was to obtain data on the salinity distribution of the ground water.
The ground water regime in this part of the Netherlands is most intricate. This is due to the geological and geo‐hydrological conditions and to the low elevation of the land. Many reclaimed areas are up to several metres below mean sea level.
The resistivity data obtained are closely related to the salinity of the ground water. On the basis of bore hole data it was even possible to arrive at calibration curves for the salinity of the ground water in sand deposits. Under special conditions it was also possible to draw conclusions with respect to the presence of less permeable formations as e.g. clay layers.
Some remarks are given on the practical performance and the interpretation of the measurements.
A review is given of the work done until now. Some results are shown by means of maps of the salinity distribution of the ground water in the western and northern parts of the Netherlands.
Two examples are described of the use of the data obtained during the survey in the province of Zuid‐Holland.
Another two examples are presented of detailed investigations for special purposes in relatively small areas.
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LA GRAVIMETRIE DE HAUTE PRECISION APPLICATION AUX RECHERCHES DE CAVITES*
More LessABSTRACTHigh precision gravity surveys have to be examined at different points of view: the accuracy which one can expect on the final results, regardless to their distribution, the shape and amplitude of the expected anomalies, depending on these factors, which grid of stations is convenient in each particular case.
Under certain condition, using an appropriate procedure, involving strict criteria, it is possible to get on the Bouguer map the same accuracy as the reading one.
However an accuracy closed to 0,01 mgal does not imply that anomalies as 0,01 or 0,02 mgal deserved attention. Numerous factors limit our appreciation of significant anomalies to 0,05 mgal, may be a little less.
When cavites are concerned, it is always necessary to define the extreme dimensions an depth of volumes which may give detectable anomalies.
Some example are given illustrating different aspects of this particular type of geophysical prospecting.
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TWO DIMENSIONAL STRAKHOV'S FILTER FOR EXTRACTION OF POTENTIAL FIELD SIGNAL*
By P. S. NAIDUAbstractA two dimensional linear filter operator based on Strakhov's method for the extraction of a potential field signal (gravity or magnetic) from a background of homogeneous random noise has been developed. An algorithm to solve for the coefficients of the operator is given together with the corresponding computer program in FORTRAN language. It may be used on any machine using FORTRAN IV. A few examples of the operator are also included.
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TOPOGRAPHIC AND ISOSTATIC CORRECTIONS TO GRAVITY SURVEYS IN MOUNTAINOUS AREAS*
Authors V. P. st JOHN and RONALD GREENABSTRACTIn mountainous areas it is an inadequate procedure to reduce gravity observations by merely subtracting the effect of an infinite flat slab of material between the station and sea‐level, and adding a terrain correction. A programme is described which directly computes the effect of masses above sea‐level, and mass‐deficiencies below it; as well as the effect of compensating masses under the Pratt and Airy isostatic systems. As an example, the method has been applied to a regional gravity survey of Papua and New Guinea where it is seen to remove the usually high correlation of the Bouguer anomaly with local topography.
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BOOK REVIEWS
Book review in this article
J. Frihagen, Electron Density Profiles in Ionosphere and Exosphere, North‐Holland Publishing Company, Amsterdam, 1966, price N. fls. 60,–.
J. A. E. Allum, Photogeology and Regional Mapping, Pergamon Press, London, 1966, 107 pages, 16 plates, 23 figures, price 17 s 6 d net.
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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 10 (1962)
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Volume 9 (1961)
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Volume 6 (1958)
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Volume 4 (1956)
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Volume 2 (1954)
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Volume 1 (1953)