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- Volume 22, Issue 3, 1974
Geophysical Prospecting - Volume 22, Issue 3, 1974
Volume 22, Issue 3, 1974
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DETAILED ELECTROMETRIC AND THERMOMETRIC OBSERVATIONS IN OFFSHORE AREAS. *
Authors V. A. BOGOSLOVSKY and A. A. OGILVYAbstractThe paper deals with the prerequisites of application, specific peculiarities and methods of electrometric and thermometric investigations aimed at the solution of certain engineering‐geological problems in offshore areas. Practical examples are supplied.
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APPROXIMATE RULES FOR THE COMPOSITION OF APPARENT RESISTIVITY SECTIONS *
Authors G. M. HABBERJAM and A. A. JACKSONAbstractConsideration is given to the form of apparent resistivity sections obtained by varying electrode array position and spacing over two dimensional features.
Since basic solutions exist for horizontally layered media and for vertical or dipping single interfaces, simple rules are proposed whereby these solutions can be directly combined to give solutions for more complex sections. Substantial approximations may be involved as the rules do not fully allow for interactions between the constituents.
The range of error incurred in application of the rules is explored, with particular reference to the square array system of resistivity measurement. The two extremes‐ of orientation of the square array with respect to the line of section are separately considered in this survey.
Within appropriate limits, the rules appear extendable to a wide variety of simple structures for which rigorous solutions would be formidable. The rules are finally discussed in relation to practical applications.
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GEOPHYSICAL PROSPECTING FOR BARITE *
Authors B. B. BHATTACHARYA, SHIKHAR C. JAIN and K. MALLICKAbstractA case history is presented for the exploration of barite with the electrical resistivity method. Altogether eight locations were recommended out of which six correspond to barite bodies. In the present case the gravity method, which is commonly used for the location of barite because of its high density contrast with the host rocks, has failed, perhaps due to small size of the ore bodies.
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TRANSIENT EM RESPONSE OF A LARGE LOOP ON A LAYERED GROUND *
More LessAbstractThe voltage induced in a horizontal loop on a layered ground has been calculated for the case where the loop is excited by a step current and measurements are made during the off‐cycle.
The expressions derived for a uniform ground show that for large time t the induced voltage E(t) is approximately given by E(t)≃— (Ibαμ/20t) (σμ2/t)3/2 where σ is the conductivity of the ground, μ the permeability, b the loop radius, and I the amplitude of the current step. For small times the corresponding result is E(t)≃—Ibμ/2t.
When the ground is composed of a number of layers a numerical procedure for calculating the induced voltage is described.
The calculated responses of various multilayered structures show that at short times the induced voltage is asymptotic to that produced in the case of a uniform ground of conductivity equal to the top layer.
Interference effects in the top layer can lead to anomalous decay curves which may result in the underestimation of the conductivity of a buried layer.
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THEORETICAL BROADSIDE RESISTIVITY PROFILES OVER AN OUTCROPPING DYKE
More LessAbstractTheoretical broadside resistivity profiles over an outcropping dyke of infinite depth extent with three Wenner configurations (namely Alpha, Beta and Gamma), Schlumberger, and two‐electrode configurations of various electrode spacings and for various reflection factors are presented. The broadside profiles qualitatively indicate that the shape of the anomaly is invariant with the electrode configurations. The various electrode configurations can be arranged in decreasing order of magnitude of anomalies as Beta Wenner, Alpha Wenner, Schlumberger, Gamma Wenner, and two‐electrode. The broadside Wenner profiles also show larger anomalies compared to those in inline profiles.
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AN ANALYSIS OF STACKING, RMS, AVERAGE, AND INTERVAL VELOCITIES OVER A HORIZONTALLY LAYERED GROUND *
More LessAbstractA correct derivation of rms, average and interval velocities from one another and from common depth point stacking velocities requires a clear understanding of the relationships between these velocities. We relate the average velocity to the rms velocity through a “heterogeneity factor” which is a quantity that gives a measure of the degree of velocity heterogeneity in the ground. The interval velocity is a quantity which varies according to the method of its derivation. The difference between rms and stacking velocities depends on the heterogeneity factor and on the length of the spread. Unless allowed for, this difference can reverse the advantages of long spreads and cause large errors in interval velocity determinations. It may be removed through a number of techniques. The accuracy of stacking velocities in the presence of random “noise” is independent of the heterogeneity factor. Relevant expressions can be broken down into simple formulae which give the accuracy quickly and with good precision.
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TRANSFORMATION OF DIPOLE RESISTIVITY SOUNDING MEASUREMENTS OVER LAYERED EARTH BY LINEAR DIGITAL FILTERING *
Authors U. C. DAS, D. P. GHOSH and D. T. BIEWINGAAbstractThe technique of linear digital filtering as developed for the direct interpretation of Schlumberger and Wenner soundings (Ghosh 1971) has been applied here for the derivation of the resistivity transform function from the field dipole measurements as the first step in directly interpreting dipole data. Filter coefficients for this transformation have been worked out for the radial‐polar, perpendicular and parallel (30°) arrays of dipole sounding. The procedure combines speed with accuracy.
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COMPUTER PROCESSING OF AIRBORNE ELECTROMAGNETIC DATA *
Authors G. J. PALACKY and G. F. WESTAbstractWith the increasing number of channels in AEM systems, computer data handling is becoming a necessity. The experience gained in processing of seismic and aeromagnetic data cannot be applied directly to low‐frequency (100‐5000 Hz) AEM methods. A novel scheme has been designed for AEM data processing and tested on 2900 km (1800 miles) of Input surveys.
In the first step, the digital flight tapes are merged with digitized flight path recovery to form the primary data set. The validity of the raw data is controlled by the computer, but the interpreter has an option of checking them in perspective plots of channel amplitudes. The primary data set is reduced by processors which determine the location and type of anomalies and discard noise. Unlike the widely used deconvolution, the sequential processor determines first the anomaly location and then estimates parameters, such as peak amplitude, base width, and excess area, which are used as acceptance criteria. Interpretation parameters, such as σt, conductor depth, and dip are estimated by comparison with quantitative models. The recorded channel amplitudes are plotted together with the selected interpretation parameters in a profile form. The secondary data set which includes only the interpretation parameters for selected anomalies is graphically displayed as a schematic map of apparent σt. Elongated features are traced by a fan strike recognition routine and a trend map is automatically compiled and plotted. Disk storage is essential for second pass processing during which parts of the primary data set are searched for undiscovered anomalies matching the analyzed trend. The suggested procedure for AEM data processing is demonstrated on an Input MK V survey, Southern Indian Lake, Manitoba.
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A MODIFIED GEOELECTRICAL PROCEDURE USING POLARDIPOLE ARRAYS—AN EXAMPLE OF APPLICATION TO DEEP EXPLORATION *
By L. ALFANOAbstractDipole soundings are more sensitive to noise caused by lateral and superficial inhomogeneities than Schlumberger soundings. However, the former are preferable for deep explorations in view of the relatively short cables required. The simple solution of carrying out the field work by means of dipole spreads, and to transform the dipole resistivity diagrams into Schlumberger ones by means of proper formulae would be valid only for smooth and regular curves; but often, owing to the presence of lateral noises, the dipole data show a considerable scatter. For such cases a “continuous dipole sounding” method is proposed for which all successive dipoles are contiguous, so that all parts of the profiles are covered and interpolation is not necessary. Obviously the moving dipoles have lengths proportional to their distances, so that they appear equal in the usual bilogarithmic scale. It follows that only polar‐dipole arrays may be used. The transition from a dipole to the corresponding Schlumberger apparent resistivity diagram requires an integration constant which is not unequivocally determined. Therefore, the solution is not unique, but all possible derived Schlumberger diagrams have a common part. Similarly, they have some common interpretative results, which may be referred to the original dipole diagram obtained in the field.
A special measurement technique is required since the dipole‐dipole voltages to be determined are noticeably smaller than the Schlumberger ones. This is true also because dipole soundings are used for great depths and for long distances between the two dipoles.
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NAVIGATION REQUIREMENTS FOR GEOPHYSICAL EXPLORATION *
More LessAbstractMarine geophysical work requires accurate navigation on a continuous basis. A repeatability of 150 m CEP (circular error probable) with sequential positioning of 15 m CEP is usually acceptable. Quoted figures for many presently available navigation systems yield nearly the required accuracy. In actual operations, however, such accuracies may not be achieved, and usually the performance actually realized cannot be determined for want of independent measurements for comparison. If more redundancy in navigation measurements were available on an integrated basis, the limitations of individual systems would no longer be so critical and the accuracy of measurements could be assessed in real time. Failures in navigation continue to occur. Such failures usually result from ignorance of the capabilities of the navigation system and limitations and consequent choice of a system which is not appropriate to the demands of a particular survey. The points to be covered in work standards are listed.
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SEISMIC MODEL STUDIES ON DIFFRACTION OF WAVES BY EDGES OF VARYING RADIUS OF CURVATURE AND DEPTH *
Authors S. DATTA and A. N. BHOWMICKAbstractResults of studies carried out with the help of a three‐dimensional seismic model on waves diffracted from edges of varying radius of curvature R and depth h with respect to wave length λ are described. The amplitude decay, travel time, and apparent velocity of the wave diffracted from a sub‐surface edge of semi‐infinite length are found to depend on the parameters R, h, and distance from the edge on the surface provided the ratio of the parameters to λ are less than some limiting values. The nature of the amplitude decay is independent of R when the depth exceeds 2λ, and independent of h when R exceeds 1.5λ. When these are below the limiting values (h= 2λ and R= 1.5λ), the nature of the decay depends appreciably on R and h. The apparent decay in amplitude on the surface due to geometrical spreading by the diffracting edge is less than that of a cylindrical secondary wave source and decreases with increase in depth of the edge. The nature of the travel time curves of the diffracted waves near the edge depend on R/λ when the depth is within about one λ. Apparent velocity of the wave depends largely on R/λ in the zone of diffraction up to a distance of about one λ from the edge on the surface. Beyond this distance the velocity is almost the same irrespective of R/λ and depend only on h/λ. The width of the zone of diffraction caused by an edge of finite length comparable to λ is more and more narrow as the ratio of the distance of the edge on the surface to its depth increases.
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A CONTRIBUTION TO THE QUANTITATIVE ERROR ANALYSIS IN MAGNETIC AND GRAVIMETRIC INTERPRETATION *
By S. E. HJELTAbstractThe paper discusses the smallest obtainable parameter errors (variances) in the interpretation with the least‐squares method. Useful approximations of the sum of squares contained in the minimum error expressions are obtained using results of numerical integration. The approximations lead to especially simple results for long interpretation profiles, when the parameter errors are proportional to the square root of the point separation.
Formulae are developed and examples shown for minimum error calculation in gravimetric interpretation with the cylindrical model and in magnetic interpretation with the two‐dimensional plate model. Smallest errors are obtained when the interpretation profile is chosen around the anomaly maximum except for dip and depth extent interpretation of magnetic plates.
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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)