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- Volume 23, Issue 2, 1975
Geophysical Prospecting - Volume 23, Issue 2, 1975
Volume 23, Issue 2, 1975
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APPARENT RESISTIVITY, ANISOTROPY AND STRIKE MEASUREMENTS*
More LessAbstractIn the search for a descriptive system capable of accounting for the wide variations of earth resistance with orientation of electrode array, which can occur in field measurements, a modified measure of apparent resistivity, based on an anisotropic earth, is proposed.
This measure requires the sampling of earth resistance in, at least, three orientations. A practical system, the crossed square array, which effectively samples in four, is considered in detail. A system of reduction of field observations yields, besides the apparent resistivity, apparent measures of anisotropy and strike.
Field trials show that the anisotropic analysis not only accounts for the major part of the observed orientational variations in resistance, but also yields sounding curves of resistivity, anisotropy and strike, which vary systematically with electrode spacing.
The effects and limitations of applying such techniques in regions of pronounced two dimensional structure are examined.
Finally, to avoid unnecessary labour in deploying the crossed square sounding technique at sites where it is not required, a modified single square technique is proposed, which will normally serve but will also alert the observer when crossing is required.
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A COMBINATION OF GRAVITY AND SEISMIC REFRACTION MEASUREMENTS, APPLIED TO GROUNDWATER EXPLORATIONS NEAR TALTAL, PROVINCE OF ANTOFAGASTA, CHILE*
More LessAbstractIn northern Chile, which is part of the Atacama Desert, groundwater supply and storage are controlled by deep structural phenomena. Several geophysical exploration techniques were used to determine the structure and depth of the basement of a broad valley filled with unconsolidated alluvial deposits. The gravity method was applied to obtain a general picture of the basement. This was followed by seismic refraction measurements along two traverse lines which appeared to be of major interest. Additional experiments with the magnetic and the telluric methods supplemented the geophysical investigations.
In this paper the results of the different geophysical methods are presented, compared, and evaluated with respect to the usefulness in arid regions.
It is concluded that a complementary use of the gravity method and the seismic refraction method is a very fruitful and efficient approach for the geophysical reconnaissance of water‐bearing basins in flat and arid zones like the Atacama Desert.
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ADVANCED METHOD FOR SELF‐ADAPTIVE ESTIMATION OF RESIDUAL STATIC CORRECTIONS*
More LessAbstractThe estimation of residual static corrections is investigated as a linear system. For 1200% coverage shot one way offset weighting functions—or in other words, estimation error operators—are computed. Assuming zero time shift determination errors and zero RNMO it is seen that large residual corrections will distort the effectivity of the estimation.
With an iterative procedure which includes the repeated averaging according to depth point, shot point, and geophone point the effectivity of the method becomes more favourable. This can be learnt from the evaluation of estimation error operators computed by recursive formulae.
In practice, due to the time shift determination errors, the routine does not give improved results in the desired level. Errors can be reduced with a more reliable time shift determination routine.
For detecting and suppressing the unfavourable effect of time shift errors still remaining in the data a self‐adaptive method was developed. In this non linear routine the data are evaluated at all three averaging processes according to limit values and weighting factors computed from the data itself.
Field examples processed in different ways are presented.
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SOME DETAILED INVESTIGATIONS ON INTERFACE WAVES*
By S. K. GUPTAAbstractIn continuation of our earlier work on interface waves (Gupta and Martel 1971) we have extended our studies by using a plexiglas‐steel combination in addition to plexiglas‐brass. Two other emitter positions were also considered: emitter on plexiglas and emitter on the metal edge, a short distance away from the interface. In both cases the presence of an interface wave could be detected from the seismogram. A curved interface between plexiglas and brass was also studied and it was found that the curvature introduces a change in the phase velocity of the interface wave in the same way as a curved surface does for the Rayleigh wave.
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RESOLUTION OF RESPONSES DUE TO CONDUCTIVE OVERBURDEN AND ORE‐BODY THROUGH TIME‐DOMAIN ELECTROMAGNETIC MEASUREMENTS—A FIELD EXAMPLE*
By S. K. VERMAAbstractA technique is suggested to separate the effect of conductive overburden from the recorded time‐domain electromagnetic anomalies. Such a separation leads one to construct a relatively undistorted picture of the conductor underlying a conductive overburden. A parameter τ (with dimensions of time) is introduced to characterize various conductors. To illustrate the applicability of the technique, four ‘Input’ (Mark V) anomalies obtained over the Cuddapah District, A.P., India have been analyzed and different values of the time‐parameter τ corresponding to the overburden and the mineralized zones are obtained.
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LINEAR TRANSFORMATIONS OF GRAVITY AND MAGNETIC FIELDS*
By P. J. GUNNAbstractThe spectral representation of gravity and magnetic fields shows that the mathematical expressions describing these fields are the result of convolution of factors which depend on the geometry of the causative body, the physical properties of the body and the type of field being observed. If a field is known, it is possible to remove or alter these factors to map other fields or physical parameters which are linearly related to the observed field. The transformations possible are: continuation, reduction to the pole, converting between gravity and magnetic fields, converting between components of measurement, calculation of derivatives, and mapping magnetization and density distribution, relief on interfaces, and vertical thicknesses of layers.
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LOCATING A DIFFRACTOR BELOW PLANE LAYERS OF CONSTANT INTERVAL VELOCITY AND VARYING DIP*
By P. HUBRALAbstractFor a two‐dimensional situation, a direct solution exists to the problem of locating a point diffractor below plane layers of constant interval velocity and varying dip. The required surface measurements are obtainable from an arbitrary portion of the observed diffraction curve on the stacked section. The method resembles the computing of plane dipping layers of constant interval velocity from common depth point surface measurements.
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ON THE POSSIBILITIES OF DETERMINING DIP IN MAGNETIC INTERPRETATION WITH THE INFINITELY DEEP PLATE MODEL*
By S.‐E. HJELTAbstractDip and magnetic susceptibility of very deep magnetic plates can be estimated approximately from either vertical, horizontal or total field measurements. A general accuracy of 2–5 degrees is easily obtained, if the other plate parameters, most notably horizontal position of the plate, are precisely determined. For reliable interpretation, measurements around the anomaly maximum or on the dip‐side flank of the anomaly should be preferred. The depth extent of the plates must be great, some ten times the plate width at least.
The method is best suited to form a part of a plate interpretation scheme, where the other plate parameters are found by some other suitable means. The method can be applied to a simultaneous determination of dips of several plates, but because of its error sensitivity an iterative formulation should then be preferred.
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A NUMERICAL COMPUTATION PROCEDURE FOR THE DIRECT INTERPRETATION OF GEOELECTRICAL SOUNDINGS*
By D. PATELLAAbstractIn this paper a numerical method of direct interpretation of geoelectrical soundings is described. It has similarities with already existing direct methods, but owing to its simplicity and, in particular, to the possibility of applying it also without digital computers, it proves useful mostly in the field, where very often an accurate method for the interpretation of multi‐layer curves is required. The direct interpretation system splits up into three steps: i) the evaluation of the resistivity transform after application of Hankel's inversion theorem; ii) the determination of the layer distribution after application of Koefoed's recurrent procedure; iii) the control of the solution. Each step is considered and the practical procedures suggested. Finally two field examples are presented and discussed.
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VIBROSEIS DECONVOLUTION*
Authors D. RISTOW and D. JURCZYKAbstractVibroseis deconvolution can be performed either before or after correlation. As regards the deconvolution before correlation, the Vibroseis deconvolution operator can be described as convolution of a spike deconvolution operator with a minimum‐phase filter operator with bandpass properties. As regards the deconvolution after correlation, the deconvolution operator can be shown to be the convolution of three operators: spike deconvolution operator and two‐fold convolution with a minimum phase operator. Time‐varying Vibroseis deconvolution can particularly well be described and performed after correlation.
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FUNDAMENTAL NOISE AFFECTING SIGNAL‐TO‐NOISE RATIO OF RESISTIVITY SURVEYS*
More LessAbstractVariations of the spontaneous electrical field appear to be a fundamental noise in geoelectrical resistivity surveying, irrespective of the origin of these variations. The signal‐to‐noise ratio can decrease to such an extent that investigations to large depths are impossible in some regions. Description of the character and magnitude of the noise recorded in Spain and Holland (1973) and the correlation between simultaneous recorded data at different locations form the subject of this paper.
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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)