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- Volume 42, Issue 2, 1994
Geophysical Prospecting - Volume 42, Issue 2, 1994
Volume 42, Issue 2, 1994
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Migration to multiple offset and velocity analysis1
By R.‐G. FerberAbstractThe stacking velocity best characterizes the normal moveout curves in a common‐mid‐point gather, while the migration velocity characterizes the diffraction curves in a zero‐offset section as well as in a common‐midpoint gather. For horizontally layered media, the two velocity types coincide due to the conformance of the normal and the image ray. In the case of dipping subsurface structures, stacking velocities depend on the dip of the reflector and relate to normal rays, but with a dip‐dependent lateral smear of the reflection point. After dip‐moveout correction, the stacking velocities are reduced while the reflection‐point smear vanishes, focusing the rays on the common reflection points. For homogeneous media the dip‐moveout correction is independent of the actual velocity and can be applied as a dip‐moveout correction to multiple offset before velocity analysis.
Migration to multiple offset is a prestack, time‐migration technique, which presents data sets which mimic high‐fold, bin‐centre adjusted, common‐midpoint gathers. This method is independent of velocity and can migrate any 2D or 3D data set with arbitrary acquisition geometry. The gathers generated can be analysed for normal‐moveout velocities using traditional methods such as the interpretation of multivelocity‐function stacks. These stacks, however, are equivalent to multi‐velocity‐function time migrations and the derived velocities are migration velocities.
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A gravimetric 3D Global inversion for cavity detection1
Authors A.G. Camacho, R. Vieira, F.G. Montesinos and V. CuéllarAbstractA gravimetric survey, covering a site 200 m square, was carried out in order to locate karstic cavities. After eliminating the regional trend using a polynomial fit, the residual is modelled by least‐squares prediction. Correlated signals for several wavelengths are detected. The inversion of these anomalies is performed by a global 3D adjustment using spherical bodies as models. The adjustment is repeated in order to obtain a stable configuration. The results show the probable presence of a system of cavities and galleries. Data collected from boreholes and the subsequent appearance of sink‐holes are consistent with the results.
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The fast Hankel transform1
Authors Adel A. Mohsen and E.A. HashishAbstractAn efficient algorithm is presented to compute the Hankel transform. The algorithm yields simultaneously all the required weights for a given order of the Bessel function using the fast Fourier transform. An additional shift is introduced to the filter abscissa besides Koefoed's shift to give a better filter performance.
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Definitions of apparent resistivity for the presentation of magnetotelluric sounding data1
By A.T. BaşokurAbstractA new definition of apparent resistivity for the presentation of magnetotelluric sounding data is proposed. The new definition is based on the frequency‐normalized impedance function. Both the existing and proposed definitions of apparent resistivity are analysed theoretically and are compared using model curves computed for a 1D earth model. Apparent resistivity curves computed using the proposed definition are a better approximation to the true resistivity values of the subsurface layers. In addition, the layers are more noticeable on the apparent resistivity curves, which is an advantage, especially for the ascending and descending type of apparent resistivity curve.
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The relationships between the velocities, attenuations and petrophysical properties of reservoir sedimentary rocks1
Authors A.I. Best, C. McCann and J. SothcottAbstractWe have measured the velocities and attenuations of compressional and shear waves in 29 water‐saturated samples of sandstones and shales at a confining pressure of 60 MPa and at frequencies of about 0.85 MHz. The measurements were made using a pulse echo method in which the samples (diameter 5 cm, length 1.5 cm to 2.5 cm) were placed between perspex buffer rods inside a high‐pressure cell. The velocity of each seismic wave was determined from the traveltime difference of equivalent phase points (corrected for diffraction effects) of the signals reflected from the top and from the base of each sample. Attenuation was determined in a similar way by comparison of the diffraction corrected amplitudes of the signals. The attenuation data are presented as ‘quality factors’: Qp and Qs for compressional and shear waves respectively. The results show that Qs is strongly correlated with Vs, that Qp is weakly correlated with Vp, and that Qp is strongly correlated with Qs. Qp is strongly dependent on the volume percentage of the assemblage of intra‐pore minerals, whether they are clays or carbonates. It is concluded that the attenuation mechanism is due to the local fluid flow arising from the differential dilation of the solid rock frame and the intra‐pore mineral assemblage, which is a result of their very different elastic moduli.
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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)