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- Volume 19, Issue 2, 1971
Geophysical Prospecting - Volume 19, Issue 2, 1971
Volume 19, Issue 2, 1971
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ANALYSIS OF SOME MULTI‐FREQUENCY, MULTI‐SEPARATION ELECTROMAGNETIC SURVEYS*
More LessAbstractIn the interpretation of anomalies obtained with moving source‐receiver dipole electromagnetic surveys, use is frequently made of the so‐called phasor or vector diagrams. Most such diagrams are based on model experiments or theoretical calculations on bodies of simple geometry. If the anomaly at some given coil‐separation and frequency is known it is possible to predict the anomaly at some other separation and frequency. While the results of such predictions often agree admirably in the laboratory experiments, wide, systematic discrepancies are observed in full‐scale field work. Some of the discrepancies can be explained by various phenomenological arguments and in this respect the effect of overburden conductivity appears to be more serious than is generally supposed. The effect of phase rotation due to the overburden is discussed in detail.
The paper is mainly a cautionary note based on some full‐scale data of multi‐frequency, multi‐separation surveys against the blind use of vector diagrams, but it also discusses the extent of the extra information and interpretation aid provided by such surveys.
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TWO‐DIMENSIONAL MASS DISTRIBUTIONS FROM GRAVITY ANOMALIES: A COMPUTER METHOD*
Authors I. R. QURESHI and H. G. MULAAbstractA gravity anomaly profile, considered to be associated with a two‐dimensional mass distribution of constant density contrast, may be interpreted with the help of a digital computer. Using the infinite slab formula and anomaly values at selected points of the profile, an initial model of the anomalous mass is built. The gravity effect of the model is computed with a simplified version of the formula derived by Talwani, Worzel and Landisman (1959) for line‐integration. After the effect is computed at a point, the model is adjusted depending upon the residual anomaly. The effect of the adjusted model is computed at the following point. Within one cycle of computation the model is adjusted as many times as the number of the selected points. Five such cycles are sufficient to minimize residuals in most cases. A final cycle is computed without adjustment.
Local anomaly profiles have been interpreted to obtain the shape of a sedimentary basin and of a faulted block. A regional anomaly profile has been interpreted to obtain crustal structure.
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THE APPLICATION OF LINEAR FILTER THEORY TO THE DIRECT INTERPRETATION OF GEOELECTRICAL RESISTIVITY SOUNDING MEASUREMENTS*
By D. P. GHOSHAbstractKoefoed has given practical procedures of obtaining the layer parameters directly from the apparent resistivity sounding measurements by using the raised kernel function H(λ) as the intermediate step. However, it is felt that the first step of his method—namely the derivation of the H curve from the apparent resistivity curve—is relatively lengthy.
In this paper a method is proposed of determining the resistivity transform T(λ), a function directly related to H(λ), from the resistivity field curve. It is shown that the apparent resistivity and the resistivity transform functions are linearily related to each other such that the principle of linear electric filter theory could be applied to obtain the latter from the former. Separate sets of filter coefficients have been worked out for the Schlumberger and the Wenner form of field procedures. The practical process of deriving the T curve simply amounts to running a weighted average of the sampled apparent resistivity field data with the pre‐determined coefficients. The whole process could be graphically performed within an quarter of an hour with an accuracy of about 2%.
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APPROXIMATION OF THE GRAVITATIONAL ATTRACTION OF GEOLOGICAL BODIES*
Authors R. BOTEZATU, M. VISARION, F. SCURTU and G. CUCUAbstractThe gravitational attraction produced by a geological body of irregular shape can be easily determined by dividing it into cubes of small size. The exact expression of the effect of a cube is very complicated, but it can be calculated by using an electronic computer.
4851 values of the gravitational attraction were determined for different positions of a cube with the side of 2l and the center in M(x0, y0, z0), for x0∈[0;20], y0∈[0;20] and z0∈[0;10]. Using these values, templates in double logarithmic representation were drawn, with x0 and z0 as parameters and y0 as abscissa, and with x0 and y0 as parameters and z0 in abscissa; this double set of templates permits a good interpolation for all cubes in the considered domain of M.
The use of templates was tested to approximate the effect produced by a theoretical model of spherical shape and in a real case of a three‐dimensional salt body of known shape based on a large number of boreholes. In both cases very good results were obtained.
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CONTRIBUTION TO SIGNAL BY ELEMENTS OF THIN VERTICAL CONDUCTING VEIN IN DIPOLAR EM SYSTEMS*
Authors A. ROY and SHEEL C. JAINABSTRACTAs an extension of the eddy current pattern computed by Koefoed and Kegge (1968) for a thin vertical infinitely‐conducting half‐plate in presence of oscillating magnetic dipoles, this paper computes the contribution to the (electro‐) magnetic anomaly at the receiver by each element of the vein separately. Twelve contour diagrams for such elemental contributions are presented as samples. Two points of general interest stand out: (1) The contributions from different portions of the vein are not all of one sign; the contribution from one part may cancel that from another; and (2) The portion of the vein nearest to the transmitter‐receiver system does not necessarily make the largest contribution to the total measured signal.
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MAPPING OF TECTONICS OF SEDIMENTS BY AIRBORNE MAGNETICS*
By H. LINSSERAbstractThe considerable increase of sensitivity of new magnetometers allows the elaboration of magnetometric maps with contour intervals of fractions of one gamma. On these maps, small anomalies appear which had never been visible before. They are caused by susceptibility contrasts within the sediments.
The areal distribution of these small undulations of the magnetic field depends on tectonics, therefore their analysis can give useful information about the tectonic pattern of the sediments. This paper discusses the automatic transformation of these small anomalies into tectonic units. The problem is solved by a method called Digital Template Analysis which has already yielded very useful results in gravity interpretation.
The application of the method described is restricted to surveys executed by high sensitivity magnetometers. Therefore it is supposed that in most oil exploration problems these magnetometers will replace instruments with lower sensitivity. The improved magnetometer surveys in combination with the interpretation method described represent a most efficient, fast and unexpensive reconnaissance method.
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PREDICTION ERROR FILTERS, WHITE NOISE AND ORTHOGONAL COORDINATES*
By E. J. DOUZEAbstractOptimum filters can be computed using orthogonal coordinates obtained from the eigenvalues and eigenvectors of the autocorrelation matrix. The method is used to obtain unit distance prediction error filters. The output of a unit distance prediction error filter when applied to the input wavelet is an impulse at zero time. The effect on the output of added white noise is easily obtained using the approach through the orthogonal coordinates. The added white noise results in output wavelets which are no longer impulses at zero time. The decrease in time resolution gives a filter that does not increase undesirable high frequency noise as much as filters computed without white noise.
Orthogonal coordinates with little signal energy can be omitted from the filter computation resulting in output wavelets resembling those computed using added white noise.
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RAPID NUMERICAL EVALUATION OF TWO‐DIMENSIONAL GRAVITY PROFILES*
Authors M. J. LONGMAN and D. E. LEAMANABSTRACTA precise method of gravity interpretation is described which uses tables for the attraction of rectangular prisms in all positions relative to a given point. Hence the attraction of any body or combination of bodies may be assessed without complicated mathematical procedures.
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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)