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- Volume 35, Issue 1, 1987
Geophysical Prospecting - Volume 35, Issue 1, 1987
Volume 35, Issue 1, 1987
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TUTORIAL INTERPRETATION OF MAGNETOTELLURIC MEASUREMENTS OVER AN ELECTRICALLY DISPERSIVE ONE‐DIMENSIONAL EARTH*
By D. PATELLAABSTRACTFrequency dispersion of electromagnetic parameters of earth materials has been widely documented in recent years. It is claimed that magnetotellurics (MT) may be significantly affected by dispersion. The paper studies the MT plane‐wave interpretative problem for a one‐dimensional earth characterized by the presence of dispersive layers. The theoretical properties of the MT field under the dispersion hypothesis, andthe main features of the dispersion phenomenon are synthetically reviewed. The examination of previously published MT curve responses over some models of dispersive earth section shows that ambiguity can arise when interpreting MT data with no other source of information. Thus it may be almost impossible to distinguish between the response of a dispersive section and an equally probable dispersion‐free section. The dispersion magnetotelluric (DMT) method is proposed as a means to resolve the ambiguity. The DMT method is based on the execution, at the same site, of an MT sounding and of an always dispersion‐free dc geoelectric deep sounding. The latter technique can be used to compute a synthetic dispersion‐free MT wave impedance, to be compared with the measured MT wave impedance. The apparent dispersion function is introduced as a measure of divergence between the two wave impedances.
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SIGNAL‐TO‐NOISE RATIO ENHANCEMENT IN MULTICHANNEL SEISMIC DATA VIA THE KARHUNEN‐LOÉVE TRANSFORM*
Authors I.F. JONES and S. LEVYABSTRACTThe Karhunen‐Loéve transform, which optimally extracts coherent information from multichannel input data in a least‐squares sense, is used for two specific problems in seismic data processing.
The first is the enhancement of stacked seismic sections by a reconstruction procedure which increases the signal‐to‐noise ratio by removing from the data that information which is incoherent trace‐to‐trace. The technique is demonstrated on synthetic data examples and works well on real data. The Karhunen‐Loéve transform is useful for data compression for the transmission and storage of stacked seismic data.
The second problem is the suppression of multiples in CMP or CDP gathers. After moveout correction with the velocity associated with the multiples, the gather is reconstructed using the Karhunen‐Loéve procedure, and the information associated with the multiples omitted. Examples of this technique for synthetic and real data are presented.
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THE EXACT SEISMIC RESPONSE OF AN OCEAN AND A N‐LAYER CONFIGURATION*
Authors G.G. DRIJKONINGEN and J.T. FOKKEMAABSTRACTThe space‐time acoustic wave motion generated by an impulsive monopole source is calculated with the aid of the Cagniard‐de Hoop technique. Two configurations with plane interfaces are discussed: an air/fluid/solid configuration with the source and the receiver located in the fluid layer; and a stack of n fluid layers between two acoustic half‐spaces where the source and the receiver are located in the upper half‐space. Synthetic seismograms are generated for the pressure of the reflected wavefield, using the source signature of an airgun.
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DISPERSION CALCULATIONS FOR SH AND P‐SV WAVES IN MULTILAYERED COAL SEAMS*
More LessABSTRACTRecent work by Räder, Schott, Dresen and Rüter has provided a means of calculating dispersion and amplitude distribution curves for SH channel waves in multilayered media. An alternative calculation procedure is presented and the P‐SV wave type is also included. Numerical results are given for a coal seam containing a band of dirt of either higher or lower seismic velocities than the coal itself.
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TOPOGRAPHIC REDUCTION OF GRAVITY MEASUREMENTS BY NUMERICAL INTEGRATION OF BOUNDARY INTEGRALS*
By H. GRANSERABSTRACTA numerical method for calculating the topographic reduction of gravity measurements is developed which follows the approximation of the topography by a single valued function. The method involves the conversion of the volume integral for the gravity effect into a two‐dimensional definite integral. The definite integral is partly solved by explicit, and partly by numerical, integration, using the Gauss‐Legendre quadrature formula. This method is well suited to calculating the topographic reduction of 50 to approximately 1000 m from the station–especially for microgravimetric surveys in areas of steeply sloping terrain. To test the method in practice it was applied in an area of rough relief in Keban (East Turkey).
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GEOTHERMAL PROSPECTING BY GEOELECTRIC SOUNDINGS IN NE GREECE*
Authors C. THANASSOULAS, G.‐A. TSELENTIS and N. KOLIOSABSTRACTIn total 77 direct current resistivity soundings were carried out during a geothermal exploration survey of the Genisea, NE Greece, geothermal field. The data revealed a high electrical conductivity zone at the center of the investigated area and suggested that an anomalous heat source lay beneath the study area.
This was confirmed by subsequent drilling data. Temperature measurements, from 11 boreholes, were used for the construction of isotherms that correlated very closely with the geoelectric data.
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RESISTIVITY AND INDUCED POLARIZATION RESPONSES OF ARBITRARILY SHAPED 3‐D BODIES IN A TWO‐LAYERED EARTH*
Authors U.C. DAS and D.S. PARASNISABSTRACTNumerical computations using the integral equation method are presented for resistivity and IP responses due to arbitrarily shaped 3‐dimensional bodies in a layered earth. The unknown surface charge density distribution is expressed as the solution of Fredholm's integral equation of the second kind. Use of moment method (with pulse basis function and point‐collocation) yields the matrix equations for the unknowns. The contributions to Green's function are solved (a) analytically for the primary and (b) by convolution for the secondary contributions resulting in a fast algorithm. The further step of computing potential, apparent resistivity, chargeability etc., for any electrode system, is straightforward. Our results show a good agreement with those from finite difference methods and physical tank experiments. The CPU time is only 138 s on a super‐minicomputer for an apparent resistivity pseudo‐section, even with 96 elementary cells as used for discretization. A large number of models for different geological situations were studied; some are presented here.
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FREQUENCY ELECTROMAGNETIC SOUNDING USING A VERTICAL MAGNETIC DIPOLE*
Authors E. MUNDRY and E.‐K. BLOHMABSTRACTA horizontal transmitter loop (vertical magnetic dipole) is used for frequency electromagnetic (FEM) soundings. The frequency ranges from approximately 6 Hz to about 4000 Hz. The vertical and radial magnetic field components are measured for 20 frequencies per decade several hundred meters from the transmitter loop. A very small bandwidth is selected for amplification using a reference signal. An Apple computer is used for data acquisition.
A computer program for Marquardt inversion optimizes the parameters for the n‐layer case: the resistivities and thicknesses of individual beds and a correction factor for the primary magnetic field. Interpretation of each component individually yields practically the same parameters. Examples from the field are given with interpretation; comparison with dc resistivity measurements is provided.
The ratio of vertical/radial magnetic field components vs. frequency can be transformed simply into apparent resistivity vs. apparent depth. This can be done in the field to obtain an estimation of the depth of the layer boundaries. FEM results are compared with Schlumberger d.c. sounding obtained at the same site.
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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)