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- Volume 15, Issue 2, 1967
Geophysical Prospecting - Volume 15, Issue 2, 1967
Volume 15, Issue 2, 1967
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GRAPHICAL EVALUATION OF MAGNETIC AND GRAVITY ATTRACTION OF THREE‐DIMENSIONAL BODIES*
More LessABSTRACTA nomogram is presented which enables evaluation of the components of magnetic attraction of a homogeneous finite rectangular prism, and of gravitational attraction due to a uniform rectangular lamina.
In practice any three‐dimensional body could be approximated by a number of right rectangular prisms of varying dimensions governed by the shape of the body. The magnetic attraction of the whole body is then obtained by numerical summation of the effects of the constituent prisms.
For evaluating the gravitational effect, the cross‐section of the body corresponding to each elevation contour is approximated by a number of rectangular laminae (or by a stepping polygon) the attraction of which can be determined with the aid of the same nomogram. The total gravitational attraction of the body is obtained by a process of graphical integration along the vertical axis.
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TWO YEARS' SURVEY WITH CESIUM VAPOUR MAGNETOMETER*
By G. ROYERABSTRACTThe present paper shows that the techniques developed and exploited by qualified geophysicists have brought high sensitivity magnetic surveys to an industrial stage.
It will appear that for most mining and oil problems, either on land or sea, the necessary modern equipment already exists without developing it further in the future. In particular, the digitalization of data is practically finished for the magnetometer itself as well as for some navigation techniques.
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MEASUREMENTS OF THE VERTICAL GEOMAGNETIC FIELD GRADIENT BENEATH THE SURFACE OF THE ARCTIC OCEAN*
More LessABSTRACTSimultaneous measurements made on an ice island and about 1000 ft. below show that magnetic anomalies can be detected in the presence of large time variations of the magnetic field. Attenuation and phase lag of time variations at depth are measurable but do not limit the utilization of the vertical gradient of geomagnetic total intensity for defining crustal anomalies.
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A NEW PARAMETER FOR AEROMAGNETIC SURVEYING*
More LessABSTRACTThe author proposes the parameter Δ2/ΔT1 for possible application in aeromagnetic surveying making use of already available gradiometer systems equipped with sensors separated vertically.
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FITTING OBSERVED PROFILES TO A MAGNETIZED DYKE OR FAULT‐STEP MODEL*
By D. W. POWELLABSTRACTThe symmetry shown, by the function describing the magnetic anomaly due to a dyke or vertical fault step leads to a method by which an observed profile may be adjusted to conform with that symmetry. By making these adjustments a minimum the ideal dyke or fault profile closest to the observations is constructed. In three of four possible situations this is interpreted by simple geometry.
Parallelpiped models may be transformed into two dykes for symmetry analysis. Depth limited models are not so amenable, even in special cases.
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THE REDUCTION OF LATERAL EFFECTS IN RESISTIVITY PROBING*
Authors G. M. HABBERJAM and G. E. WATKINSAbstractTo be able to make use of all existing probe interpretation techniques, it is desirable that field resistivity observations be conducted in such a way that it is possible to construct an equivalent curve for the surface variation of potential about a point source. Further, the usual parallel layer interpretation will only be justified in as much as the observed potential curve is compatible with such a subsurface geometry. Thus, whilst a potential curve may be constructed from suitable finite potential differences obtained using a Wenner configuration of electrodes, it can be shown, using the tri‐potential technique of measurements, that these potential differences may arise partly from lateral resistivity variations.
In this paper, a ladder network technique is employed to display these lateral effects and an adjustment method proposed to reduce them. The adjusted potential differences obtained form a consecutive series suitable for summation to give the potential function. These values are subject to further minimum adjustments required for them to comply with slope and curvature conditions for a layered medium. After forming the potential curve by summation, a final numerical smoothing process is carried out. From this smoothed potential curve, corresponding Wenner and Schlumberger curves can readily be derived for interpretation.
The method proposed thus attempts to extract from the observed data the maximum part which can be reconciled with a purely depth variation of resistivity. A method is also proposed for compounding the ‘errors of closure’ of the network to provide a Lateral Inhomogeneity Index which gives a measure of the departure of the observed data from the basic interpretation requirements.
The method involves no great labour, but can readily be programmed for a computer if desired. Examples are given of the application of the method to field observations. In the interest of objectivity, the final smoothing has been confined to a single stage but in certain very extreme examples a further stage may prove desirable.
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A 1,800 MILLIVOLT SELF‐POTENTIAL ANOMALY NEAR HUALGAYOC, PERU*
More LessABSTRACTA large self‐potential anomaly was outlined in 1963–64 at Tantahuatay near Hualgayoc in the Andes of northern Peru. Peak value recorded was–1842 millivolts–thus making it one of the strongest, or perhaps the strongest, SP anomaly ever measured. A lack of detailed geological data precludes the formulation of an adequate explanation for the Tantahuatay anomaly, but geological and mineralogical similarities with the well‐documented Venencocha anomaly near Cerro de Pasco, Peru (Kruger and Lacy, 1949), suggest that the anomaly arises from sulfuric acid associated with the mineral alunite. The anomaly obviously cannot be explained by the half‐cell mechanism of Sato and Mooney (1960), who place a limit of 700 millivolts on self‐potential anomalies over sulfide bodies. Further study of the Tantahuatay anomaly would be of interest in understanding self‐potential mechanisms in general.
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SOURCES OF ERROR IN A SEISMIC DIGITAL RECORDING SYSTEM*)
More LessABSTRACTThe use of digital recorders and computers in seismic exploration promises major enhancement of the quality of final documents available to interpreters. The ultimate objectives of recording and processing remain what they always have been:
1 Record the reflection wavelet as a function of time; this requirement has been met with satisfactory accuracy for a number of years.
2. Record the reflection wavelets with sufficient fidelity to permit the interpreter to recognize them.
Various factors affect our ability to achieve this second objective. Certain recording errors are associated with digital recording systems. However, an understanding of the sources of error will enable the operator to use his system properly and to estimate the noise level or inaccuracy of field recordings. Field operations do not require rigorous error analysis; in most cases a satisfactory approximation can be obtained from simple calculations.
Three types of “noise”–seismic, instrument and power line–introduce errors. Factors which contribute to over‐al recording system error include specifically input noise, power supply ripple, crosstalk, A‐D conversion error, quantizing noise, aliasing, distortion. Examination of each component of a recording system, permits the determination of its ultimate effect on the over‐all noise level–or error level–of the entire system.
Many of the error sources produce statistically independent noise which is not correlative. Where this is true, error voltages from various sources may be combined by taking the square root of the sum of the mean square noise voltages, giving a result slightly greater than the largest single voltage if one source is much greater than any other source. This simplification can be used to estimate over‐all system noise levels. Distortion and crosstalk depend on signal amplitude and should be added algebraically in each category. Each final sum should be used as a statistically independent noise source with respect to other system noise sources.
Using the foregoing examples and simplified system for estimating over‐all system noise, and assuming that much of the distortion (which limits signal/instrument noise ratio to 54 db) can be removed by filtering, we determine that the combined effect of all sources of error is to reduce the system S/N ratio to approximately 74 db.
With proper care digital field recording systems can produce very good field records, and exotic computer processes can enhance signal and reduce various forms of noise. However, one always must recall that the level of confidence which one can place in an interpretation of seismic data must be dependent on a knowledge of the accuracy of the basic data.
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A THEORETICAL STUDY ON THE REDUCTION OF STATISTICAL PORE SYSTEM PARAMETERS TO MEASURABLE QUANTITIES*
More LessAbstractAs shown previously [50], the relations between formation factor, permeability, and porosity can be expressed in statistical parameters of the microscopic pore system. A number of other macroscopically measurable quantities depend on those same parameters. Thus, by proper combination of these quantities, it should be possible to solve for those parameters, that cannot be retrieved by direct methods. In this article, the dependencies on these statistical parameters of the capillarity with imbibition and drainage, the irreducible water saturation, the flow dispersion with miscible displacement, and quantities that are directly observable at ground surfaces or thin sections, are preliminarily studied as a preparation for possible practical measurements.
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USE OF EXPECTED ERROR IN THE DESIGN OF LEAST‐SQUARES OPTIMUM FILTERS*
More LessABSTRACTThe design of least‐squares optimum filters is based upon minimizing a suitably defined error criterion. The expected value of this error is easily computable after the coefficients of the filter have been determined.
When a particular filtering problem is specified, there are several parameters which are specifically not included in the optimization procedure. However, the magnitude of the expected error may be quite sensitive to these parameters. The examination of the relative values of the expected error for variations of these unspecified parameters may lead to a better definition of the filter problem.
The parameters which are left unspecified by the general least‐square filter definition include:
1. The addition of white noise to the signal autocorrelation to stabilize the filter behavior.
2. The specification of the shape of the desired output of the filter.
3. The specification of the lag between the desired output and the input.
Examples are given showing the relationship between these parameters and the value of the expected error.
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BOOK REVIEWS
Book review in this article
F. S. Grant and G. F. West, Interpretation Theory in Applied Geophysics, McGraw‐Hill Comp., New York, 1965, 583 pp., price $ 17.50.
Archaeo‐Physica. Technische und naturwissenschaftliche Beiträge zur Feldarchäologie. Mit Beitragen von E. Le Borgne, I. Scollar, J. D. Mudie, J. Görier, H.‐J. Appel, G. Strunk‐Lichtenberg, Böhlau Verlag, Köln, Graz, 1965, 202 pp., 124 fig.
H. Takeuchi, Theory of the Earth's Interior, Blaisdell Publishing Company, Waltham (Mass.), Toronto, London, 1966, 131 pp.
G. D. Garland, The Earth's Shape and Gravity, The Commonwealth and International Library, Geophysics Division, Pergamon Press, Oxford, London, Edinburgh, New York, Paris, Frankfurt, 1965, 183 pp., price: 17s 6d.
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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 7 (1959)
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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)