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- Volume 21, Issue 4, 1973
Geophysical Prospecting - Volume 21, Issue 4, 1973
Volume 21, Issue 4, 1973
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RESISTIVITY TYPE CURVES OVER OUTCROPPING VERTICAL DYKE—II *
By RAKESH KUMARAbstractThe theoretical horizontal resistivity profiles Over an outcropping vertical dyke with two‐electrode and Schlumberger electrode systems are discussed. The two‐electrode array seems very useful in locating the highly conducting thick or thin vein, while the Schlumberger (gradient) array is suited to detect the conducting vein of low and moderately high resistivity contrast and as well as the resistive vein of all widths and resistivity contrasts. Besides, the inline gradient array has a distinction of establishing a clue to evaluate the resistivity contrast of the vein.
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COMPARATIVE FIELD PERFORMANCE OF ELECTRODE ARRAYS IN TIME‐DOMAIN INDUCED POLARIZATION PROFILING *
Authors A. ROY and SHIKHAR C. JAINAbstractFive test profiles in time‐domain induced polarization measurements over conducting sulphide and graphite mineralisations were run with the two‐electrode, three‐electrode, modified unipole, pole‐dipole, dipole‐dipole, Schlumberger, and Wenner α and β configurations. The results show that, compared to the other electrode systems, the simplest two‐electrode array produces the largest anomalies with the smallest of spacings.
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INDUCTIVE SOUNDING OF A STRATIFIED EARTH WITH TRANSITION LAYER RESTING ON DIPPING ANISOTROPIC BEDS *
Authors J. G. NEGI and P. D. SARAFAbstractThe generalised three layer boundary value problem with a transition layer sand‐witched between an isotropic overburden and dipping anisotropic substratum is discussed assuming that plane electro‐magnetic waves are incident normally over the air‐earth inter‐face. The tangential electric (Ey) and magnetic (Hx) fields and the expression for surface‐impedance (Ey/Hx) have been evaluated at the earth's surface. Through numerical analysis it is shown that changes in the values of the parameters m (coefficient of anisotropy), h (thickness of the transition layer), α (angle of inclination of the dipping beds), and b (conductivity ratio between substratum and upper layer) modify the amplitude and phase‐variation curves (with skindepth) significantly.
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INLINE AND BROADSIDE EM DIPOLE DIPOLE PROFILING OVER A THIN VERTICAL INFINITELY CONDUCTING VEIN *
More LessAbstractMaster diagrams for electromagnetic responses of nine dipole dipole systems are presented for various depths of a vertical infinitely conducting vein. The YY system gives the minimum anomaly for both the inline and broadside arrays. Among other inline systems, it is difficult to decide the clearcut superiority of one system over the other, whereas the XX system in broadside array gives maximum anomaly.
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THE ESTIMATION OF SIGNAL SPECTRA AND RELATED QUANTITIES BY MEANS OF THE MULTIPLE COHERENCE FUNCTION *
By R. E. WHITEAbstractA seismic trace recorded with suitable gain control can be treated as a stationary time series. Each trace, χj(t), from a set of traces, can be broken down into two stationary components: a signal sequence, αj(t) *s(t—τj), which correlates from trace to trace, and an incoherent noise sequence, nj(t), which does not correlate from trace to trace. The model for a seismic trace used in this paper is thus χj(t) =αj(t) * s(t—τj) +nj(t) where the signal wavelet αj(t), the lag (moveout) of the signal τj, and the noise sequence nj(t) can vary in any manner from trace to trace. Given this model, a method for estimating the power spectra of the signal and incoherent noise components on each trace is presented.
The method requires the calculation of the multiple coherence function γj(f) of each trace. γj(f) is the fraction of the power on traced at frequency f that can be predicted in a least‐square error sense from all other traces. It is related to the signal‐to‐noise power ratio ρj(f) by
where Kj(f) can be computed and is in general close to 1.0. The theory leading to this relation is given in an Appendix.
Particular attention is paid to the statistical distributions of all estimated quantities. The statistical behaviour of cross‐spectral and coherence estimates is complicated by the presence of bias as well as random deviations. Straightforward methods for removing this bias and setting up confidence limits, based on the principle of maximum likelihood and the Goodman distribution for the sample multiple coherence, are described.
Actual field records differ from the assumed model mainly in having more than one correctable component, components other than the required sequence of reflections being lumped together as correlated noise. When more than one correlatable component is present, the estimate for the signal power spectrum obtained by the multiple coherence method is approximately the sum of the power spectra of the correlatable components. A further practical drawback to estimating spectra from seismic data is the limited number of degrees of freedom available. Usually at least one second of stationary data on each trace is needed to estimate the signal spectrum with an accuracy of about 10%. Examples using synthetic data are presented to illustrate the method.
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MAGNÉTOMÈTRE DIFFÉRENTIEL À HAUTE SENSIBILITÉ*
Authors C. R. COLLIN, A. SALVI, D. LEMERCIER, P. LEMERCIER and F. ROBACHAbstractThe aim of this article is to describe the double resonance magnetometer system designed and built by the Division de Magnétométrie of the C.E.N. Grenoble, and used by the Département des Recherches Minières du CE.A.
For the measurement of magnetic anomalies of geological origin it is necessary to make differential measurements between a mobile instrument which scans the region of interest and a fixed compensatory instrument.
The apparatus described here features the following main characteristics:
— it gives a direct numerical measurement of the differential magnetic field between the two instruments with an accuracy of 0.01 gamma (10‐7 Oe).
— it is designed to be easily operated in geological field work (light weight, low power, possibility to make continuous measurements along a given profile, the measuring signals being radio‐linked there are no wires connecting the instruments).
Firstly we describe the components of the magnetometer itself namely: double resonance magnetometer heads and differential numerical magnetometer. Secondly we describe the measuring technique.
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DEFORMATIONS OF NATURAL ELECTRIC FIELDS NEAR DRAINAGE STRUCTURES *
Authors V. V. BOGOSLOVSKY and A. A. OGILVYAbstractThe paper deals with the electro‐filtrational fields formed near the drainage structures. Main laws of deformation of these fields above vertical and horizontal drains are considered and practical examples supplied.
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RAPID DETERMINATION OF CUBE'S GRAVITY FIELD *
By I. R. MUFTIAbstractGravity modeling involves the evaluation of gravity field due to bodies of irregular shape. This is commonly accomplished by dividing the body into smaller units of regular shape whose field can be determined analytically.
Cubes are ideal building blocks for constructing bodies of irregular shape. However, the exact expression defining the gravity field of a cube‐shaped body is very long and requires tedious numerical work. Approximation formulas for rapid evaluation of the cube's gravity field are presented and discussed in detail. Their use for computing the field for small distances leads to large errors in the results; on the other hand, using the exact formula for large distances where approximation formulas yield practically identical results, is waste of time. After a detailed analysis about the nature of the cube's gravity field, certain criteria are established for suitably combining the two alternatives. This results in tremendous reduction in the task of computations without affecting the accuracy of data.
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DÉCONVOLUTION RAPIDE *
By P. BOISAbstractDuring the last ten years data processing has enabled the geophysicist to seriously attack the problem of how to improve the quality of seismic sections. For instance new processing methods have been proposed for improving the resolving power of the seismic pulse. These methods are based on deconvolution and they require lengthy computations.
The method discussed here for pulse contraction is based on the use of Hadamard transforms. It is very fast but nevertheless the quality of the result stands comparison with that of conventional methods.
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SHAPED CHARGE—A POWERFUL EXPLOSIVE SURFACE SEISMIC SOURCE *
Authors R. R. BAHIA and W. F. SILVAAbstractThe directional effect of shaped charge is a well‐known feature used for a long time in military weapons, oil well guns, and steel industry. This charge was successfully applied as a seismic energy source by Petrobrás during the past three years under different surface and geological conditions.
Preliminary amplitude measurements taken with fixed gain shallow refraction instruments showed a consistent difference between conventional and shaped charges. Lately, a similar difference has been noted in deep reflection energy recorded with digital binary gain instruments as well as in deep oil well velocity surveys.
Direct comparisons along more than 50 km of multiple coverage field reflection shooting are in agreement with these results and have proved the practical advantage of this source as compared to conventional dynamite.
This source has been used since 1971 in routine seismic operation in the Amazon jungle with 300 gram unit charges distributed in small and large shot arrays increasing substantially the coverage and halving the cost at a higher record quality. A large amount of production seismic field work has been carried out in several other areas attesting the successful application of the shaped charges.
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PHYSICAL AND COMPUTER MODELING OF INDUCED POLARIZATION *
Authors C. L. AIKEN, D. A. HASTINGS and J. R. STURGULAbstractThe Laboratory of Geophysics of the University of Arizona was presented with an exploration problem by Falconbridge, Ltd. of Canada. Massive mineralized hemispherical “pods” are in the vicinity of a tuff layer of high conductivity and induced polarization response, covered by large thicknesses of resistive volcanics. The initial approach was to utilize electrolytic tank modeling. The extreme resistivity and IP contrasts proved to be difficult to recreate. Two dimensional modeling was attempted next with conductive paper, using copper and silver paint for anomalous masses. This method also proved inadequate. Finally, mathematical equations were solved which could model any arbitrary anomalous body in any steady state electrical field. Plane waves as well as point current sources producing non‐plane waves are possible. Finite difference equations were derived for the non‐linear partial differential equations under consideration. The equations were solved using a digital computer. Initially, the boundary conditions had to be satisfied at the boundaries of resistivity changes, severely restricting possible geometric shapes for anomalous bodies. The final and successful solution was to apply numerical techniques to obtain solutions of equations which require only that the relative resistivities through the area be specified. The Falconbridge problem and its solution are analyzed.
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SERIES APPROXIMATION IN VELOCITY AND TRAVELTIME COMPUTATIONS *
More LessAbstractA rigorous proof is presented to show that over a horizontally layered ground the rms velocity cannot exceed the stacking velocity. The proof helps to illustrate the difference between stacking and rms velocities in a quantitative manner. The series of Taner and Koehler (1969) is used for the purpose. Convergence of this series is tested. Including more terms will not necessarily improve the convergence. Although the series is rapidly convergent when the spread length/depth ratio is small, strong oscillations are observed when this ratio is high.
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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)