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- Volume 24, Issue 4, 1976
Geophysical Prospecting - Volume 24, Issue 4, 1976
Volume 24, Issue 4, 1976
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AN APPROXIMATE METHOD OF RESISTIVITY SOUNDING INTERPRETATION*
By O. KOEFOEDAbstractAn approximate method of interpretation of resistivity sounding is presented, which may be described as a very crude manner of application of the exact direct interpretation method. The accuracy of this method is fairly low, the errors being in the order of 25%. The method is very fast in application and well suited for application to multilayer cases. The main advantage of the method is that it is in close and clear relation to the exact theory.
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A FAST FOURIER TRANSFORM METHOD FOR RAPID COMPUTATION OF GRAVITY AND MAGNETIC ANOMALIES DUE TO ARBITRARY BODIES*
Authors B. K. BHATTACHARYYA and M. E. NAVOLIOAbstractThe spectrum of a magnetic or a gravity anomaly due to a body of a given shape with either homogeneous magnetization or uniform density distribution can be expressed as a product of the Fourier transforms of the source geometry and the Green's function. The transform of the source geometry for any irregularly‐shaped body can be accurately determined by representing the body as closely as possible by a number of prismatic bodies. The Green's function is not dependent upon the source geometry. So the analytical expression for its transform remains the same for all causative bodies. It is, therefore, not difficult to obtain the spectrum of an anomaly by multiplying the transform of the source geometry by that of the Green's function. Then the inverse of this spectrum, which yields the anomaly in the space domain, is calculated by using the Fast Fourier Transform algorithm. Many examples show the reliability and accuracy of the method for calculating potential field anomalies.
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DIP SELECTIVE MIGRATION VELOCITY DETERMINATION*
Authors J. W. SATTLEGGER, P. K. STILLER and J. A. ECHTERHOFFAbstractDip selective migration velocity determination calculates coherency of energy migrating into an output trace or ‘MVD‐axis’ as a function of the three variables time, velocity, and dip. In doing so, it provides additional information, the dip information, relative to standard MVD.
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AUTOMATIC INTERPRETATION OF GRAVITY ANOMALIES ASSOCIATED WITH TWO‐DIMENSIONAL MASS DISTRIBUTIONS*
Authors I. R. QURESHI and A. KUMARAbstractSeveral new features are described which facilitate an automatic, more meaningful, and more accurate interpretation of gravity anomalies associated with approximately two‐dimensional mass distributions. These include a provision for fixed points on the bounds of a distribution, outward dipping boundaries, calculation of the gravity effect at individual elevations of stations, smoothing of models and end‐corrections for distributions of limited strike length. It is possible to obtain distributions about a median plane; these distributions are sensitive to shape and allow estimates of optimum depths and minimum density contrasts for the anomalous masses.
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INTERPRETATION GRAPH FOR VERTICAL TABULAR CONDUCTORS IN E.M. IN‐LINE DIP‐ANGLE MEASUREMENTS*
Authors A. C. R. KETELAAR and C. P. DE VENTEAbstractA graphical interpretation method is presented by which separately the depth and the product of conductivity and thickness of a vertical plate‐shaped conducting orebody can be determined. The method is based on the result of model experiments.
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HYDROGEOPHYSICAL PROPERTIES OF PARTS OF THE BRITISH TRIAS*
More LessAbstractLaboratory studies of Bunter Sandstone specimens from Northwest Lancashire, Shropshire, West Cumberland, and the Vale of Clwyd have indicated that the parameters effective porosity, intergranular permeability, compressional wave velocity, formation resistivity factor, and effective matrix resistivity have significantly different distributions in each of these four regions. Regression analyses have shown that bivariate and trivariate expressions for the prediction of the two hydrological parameters from petrophysical data vary from region to region. It is concluded that, in quantitative geophysical investigations of these formations, each area must be investigated independently.
For all four aquifers, and for both horizontally and vertically oriented specimens, effective porosity can be most reliably and readily estimated through a bivariate relationship involving formation resistivity factor. On the other hand, the best estimate of intergranular permeability from geophysical data is obtained through a trivariate expression involving both formation resistivity factor and effective matrix resistivity. The use of hydrogeophysical relationships to estimate hydrological parameters in situ is illustrated by reference to field examples.
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RECONNAISSANCE DES HORIZONS SISMIQUES PAR ANALYSE FACTORIELLE DISCRIMINANTE*
By P. BOISAbstractA complicated geological structure may make it difficult to recognize seismic horizons. This may be the case when blind zones are present, when horizons have to be identified on both sides of a fault, or when lithological facies exhibit a lateral variation.
A method for the recognition of seismic reflections is described. The process takes the dynamic range of seismic information into account as well as the extensive logical possibilities of modern computers. The method implements a factorial discriminant analysis of seismic traces by means of a stepwise iterative process. The reliability of results is studied by examining the a posteriori discrimination between different horizons. If the classification is good, a percentage of well classified reflections is given for each horizon.
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SOME REMARKS ON COMPUTING TRAVEL TIMES IN PLANE LAYERED MEDIA*
By P. HUBRALAbstractA satisfactory approximation to the travel time of a reflected wave is achieved by finding a ray, which originates in the source and emerges close enough to the receiver. The reason for this is, that the actual travel time function can be approximated in every emergence point of the wave by a time function which is entirely defined by seismic parameters along the ray connecting the source with the point of emergence.
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GROUNDWATER EXPLORATION IN HARD‐ROCK TERRAIN—A CASE HISTORY*
Authors B. N. SATPATHY and D. N. KANUNGOAbstractThe utility of electrical resistivity and magnetic measurements to locate suitable wellsites in hard‐rock terrains intruded by dykes is illustrated. The example is taken from a highly hilly terrain of Karnataka State situated in the Western Ghat hills of Peninsular India. Electrical resistivity profiles and soundings, S.P., and total magnetic field measurements are discussed. Of the fifteen recommended sites, nine have been drilled, eight of which are reported to yield between 6–12 m3/hr (1500 to 3000 gals./hr), which is considered satisfactory in this region.
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LONG PERIOD STATICS DETERMINATION BY INVERSE FILTERING*
Authors D. J. KIRKHAM and E. POGGIAGLIOLMIAbstractThe relative time shift of CDP reflections, caused by long period statics, is treated as the result of a filtering process. The shape of the filter is that of a comb having the magnitude and spacing of its teeth determined by the CDP spread geometry. Convolution of a near surface anomaly with a comb filter representing the CDP spread geometry produces the same time fluctuations at CDP reflections (CDP statics) as the stacked traces across the anomaly. Conversely, the near surface time anomaly is given by the convolution of the CDP statics with the inverse of the comb filter. Provided that CDP statics can be separated from noise, dip, and residual normal moveout, it is pcissible to determine long period statics with a relative wavelength of between half a spreadlength and five spreadlengths.
The object of this paper is to present the theory and practical applications of the method. Several examples based on synthetic and real data will also be discussed.
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THE RADIATION OF ACOUSTIC WAVES FROM AN AIR‐GUN*
By M. H. SAFARAbstractA theoretical model is developed for predicting three important parameters of the pressure pulse radiated by an air‐gun, namely the rise time, the amplitude of the initial pulse, and the period of the bubble pulse. A knowledge of these three parameters is essential for the efficient design of air‐guns arrays.
The prediction of the amplitude of the initial pulse is based on the assumption that the initial pulse is radiated by a spherical source with surface area equal to that of the air‐gun ports and not by a spherical source with initial volume equal to that of the air‐gun chamber, as has been assumed previously.
A simple equation is obtained for predicting the period of the bubble pulsation, taking into account the effect of the air‐gun body, boundaries such as the sea‐surface and seabed and the presence of a number of identical air‐guns placed at the same depth and fired simultaneously.
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EFFICIENT DESIGN OF AIR‐GUN ARRAYS*
By M. H. SAFARAbstractA new technique is developed for shaping the pressure bubble pulse radiated by an array of air‐guns. It involves the proper spacing of identical units. It is shown that considerable shortening of the pressure bubble pulse can be achieved provided there is sufficient mutual coupling between all the air‐guns.
The existing air‐gun array technique for reducing the bubble pulse involves the redistribution of the energy of the bubble pulses which are produced by an array of variable sized air‐guns such that no energy of the bubble pulses is radiated along the perpendicular to the array axis and only the sum of the initial pulses produced by the air‐guns forming the array is radiated along that direction. However, the new air‐guns array technique involves the damping of the bubble pulses which are produced by an array of identical air‐guns by means of mutual interaction and the effective pressure pulse radiated by the array is given by the sum of the damped bubble pulses produced by the mutually coupled identical air‐guns.
Preliminary field trials gave results consistent with the theoretical predictions. A comparison between the waveforms of the pressure bubble pulses radiated by a single air‐gun and by an array of four identical air‐guns shows that, due to the presence of mutual coupling between the four air‐guns, effective damping of the bubble pulse radiated by the array is about 50% greater than that of the bubble pulse radiated by the single air‐gun.
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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)