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- Volume 29, Issue 4, 1981
Geophysical Prospecting - Volume 29, Issue 4, 1981
Volume 29, Issue 4, 1981
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THE AMPLITUDE AND PHASE RESPONSE OF A SEISMIC VIBRATOR*
By W.E. LerwillAbstractThe amplitude and phase response of a simple model is compared with the performance of a real vibrator working in the field. The field results show a characteristic phase response which confirms that the real drive force applied to the baseplate and its load impedance is faithfully represented by the acceleration of the reaction mass. It follows that all the parameters necessary to calculate the load impedance and the true power dissipated in the earth can be measured at the output of the vibrator. It also follows that the current method of baseplate phase compensation should be reconsidered.
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THE KUNETZ RELATIONS*
By E. EISNERAbstractIn discussion of the one‐dimensional wave equation the Kunetz “energy relations’ are often invoked. Since the treatment often involves lengthy algebraic calculations which yield a simple result, it may be of interest to present an elementary derivation from first principles. A generalization to higher‐dimensional wave equations follows readily.
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THE RATIO OF THE PP‐ TO THE SS‐REFLECTION COEFFICIENT AS A POSSIBLE FUTURE METHOD TO ESTIMATE OIL AND GAS RESERVOIRS*
Authors R. MEISSNER and M.A. HEGAZYAbstractOn the basis of known relationships between various elastic moduli and porosity in a porous medium and of various parameters from well‐logs, the reflection coefficient for normal incidence of P‐waves is compared to that of S‐waves. If pores contain gas, the reflection coefficient in S is much smaller than that of P but shows large differences for different gas concentration in one of the two layers while the reflection coefficient in P is insensitive to the gas concentration. It is found that the ratio of the two reflection coefficients provides a powerful tool for discriminating between layers of different degrees of gas concentration and for the extension of the sensitivity of the bright‐spot technique to greater depths.
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SHOVER * SHEAR‐WAVE GENERATION BY VIBRATION ORTHOGONAL TO THE POLARIZATION **
More LessAbstractThe radiation distribution of a single vibrator can be described using the formulas given by Miller and Pursey. From these the displacement characteristic for the radial component and also for horizontal component can be derived. This horizontal component is strengthened by adding another vibrator with opposite polarity near the first one. When this horizontal component is oriented perpendicular to the X‐Z plane (plane spanned by the direction of the spread and the vertical axes), an SH‐wave can be generated. Survey results indicate that this method works efficiently.
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SYSTEMATIC CLASSIFICATION OF LAYER‐INDUCED TRANSVERSE ISOTROPY*
By K. HELBIGAbstractWaves propagating through a sequence of layers that are thin compared with the wavelength show effects of anisotropy: velocity and displacement direction depend on the angle between the plane of layering and the wave normal, and shear waves split up into two distinct types of different velocity. The layered medium can thus be replaced by a transversely isotrophic medium the parameters of which depend on the parameters of the individual constituent layers.
A survey of the anisotropy effects possible in such a medium is generally done by varying the layer parameters in order to obtain different replacement media. This approach guarantees that the replacement medium is realistic, but it does not guarantee adequate sampling of the set of replacement media. To this end one has to begin by selecting the replacement media and then check whether the chosen media possess stable (and eventually realistic) representations by layer sequences. In general, there is an infinite number of layer representations for any transversely isotropic medium that can at all be represented. However, if one restricts the solutions to those requiring the minimal number of layers and the minimum number of different layer parameters, the set of solutions has only one free parameter (i.e., it is a one‐dimensional manifold), and an important subset even has a unique solution. A simple algorithm exists for the determination of these “simplest representations”.
Aside from sampling the set of representable transversely isotropic media for survey purposes, the method can be applied to the problem of determining the cause of observed anisotropy effects (or lateral changes in such effects). If this method can be applied to real data, it would for instance allow to determine changes in relative thickness or lithology on a scale smaller than the limit of resolution of the seismic method.
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E‐POLARIZATION BOUNDARY CONDITIONS ABOVE LATERALLY NON‐UNIFORM CONDUCTORS FOR A SIMPLE INDUCTION MECHANISM*
By S. SPITZAbstractThe induction problem for an inhomogeneous two‐dimensional conductor presenting a vertical contact between two media of diverse conductivities is considered. The general solution of the Helmholtz equation in the atmosphere is constructed for a simple induction mechanism. It is found that in the E‐polarization case the anomalous field behavior in the boundary region and beyond differs from the generally assumed one. The conclusion of this paper is that the boundary conditions widely utilized in numerical computations have to be modified.
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RESISTIVITY PROFILING WITH DIFFERENT ELECTRODE ARRAYS OVER A GRAPHITE DEPOSIT*
More LessAbstractMotivated by several papers by Indian colleagues suggesting that a single‐pole array is the most favorable one for locating electrically conductive targets in the underground, detailed measurements were carried out with different electrode arrays over a well‐known graphite deposit near Pfaffenreuth, Germany. The following arrays were used: (1) Wenner array; (2) single‐pole array; (3) half‐Wenner array; (4) half‐Schlumberger array. Each of the arrays established the location of the main graphite deposit. The half‐Schlumberger array turned out to be the most reliable one since it showed details which were only very poorly indicated by the other arrays or not at all.
Mathematical modeling of three‐dimensional bodies simulating the geological situation for this deposit were carried out. These results show an unexpectedly good correlation with the measured effects.
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FOCUSSED ELECTRICAL RESISTIVITY ARRAYS: SOME THEORETICAL AND PRACTICAL EXPERIMENTS*
By P.D. JACKSONAbstractThe “Laterolog 7′’and the “Microlaterolog’ have been studied theoretically for the case of a conducting halfspace containing a single overburden, using a technique based on the method of images. The results have shown these focussed arrays to be more sensitive to the lower medium (i.e., having greater depth of investigation) than unfocussed ones of the same dimensions, when the “correct’ geometric factor is used. The geometric factor of a focussed electrode array is somewhat involved, and is explained with reference to an ideal focussed array; such an array would pass a measuring current of constant intensity into a fixed geometrical shape of conducting material, irrespective of any layering or any other heterogenities that may be present, using auxiliary current sources of variable magnitude. This concept of a constant amount of current flowing in a beam of fixed geometry and current density, is the basis of focussed arrays, and it is shown that the geometric factor, used to calculate the apparent resistivity, is the one derived when considering the homogeneous case. The value of the constant measuring current, alone, is used to convert the measured potential difference into a resistance, the ratio between this value and the combined intensities of the auxiliary current sources, for the homogeneous case, being incorporated into the geometric factor.
Surprisingly good agreement was found between the theoretical models and practical experiments using a focussing seabed resistivity probe which is a substantial modification of the “Microlaterolog”. Both show similar deviations from the ideal case which are explained in terms of refraction at the overburden interface. All experiments indicated that focussed arrays have a greater depth of investigation than similar unfocussed ones for a single overburden, whether it be resistive or conductive.
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FLUID‐FLOW MONITORING USING ELECTROMAGNETIC PROBING*
Authors R.J. LYTLE, D.L. LAGER, E.F. LAINE, J.D. SALISBURY and J.T. OKADAAbstractHigh‐frequency electromagnetic probing is used to monitor the rate and direction of flow of fluids injected into the ground. This method shows the potential for providing more detailed information than procedures presently used. The experimental technique and the test‐of‐concept experimental results are discussed. This technique has applications in oil‐reservoir engineering and in hydrology studies concerning storage of chemical wastes.
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CONTOUR MAP PRESENTATION OF DIPOLE‐DIPOLE INDUCED POLARIZATION DATA*
By D.C. FRASERAbstractDipole‐dipole induced polarization (IP) data are displayed typically as multi‐level profiles, or as contours on vertical sectional plots referred to as pseudo‐sections. The dipole‐dipole array tends to yield IP anomalies in which the most anomalous values are displaced laterally from the source body. The data patterns are fairly interpretable on pseudo‐sections or on multi‐level profiles but are sufficiently complex to discourage the contouring of the data in plan.
A method was developed for the presentation of dipole‐dipole IP data on a contour map. The method consists of a simple averaging of data which can be performed manually if desired. It yields a single output value per station which reflects all levels of the pseudo‐section, and is suitable for contouring in plan. The advantage of the technique is that it provides a quantitative picture of IP anomalies in their background or regional setting.
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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)