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- Volume 54, Issue 4, 2006
Geophysical Prospecting - Volume 54, Issue 4, 2006
Volume 54, Issue 4, 2006
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Fast‐decaying IP in frozen unconsolidated rocks and potentialities for its use in permafrost‐related TEM studies
Authors N.O. Kozhevnikov and E.Y. AntonovABSTRACTWe investigate the early time induced polarization (IP) phenomenon in frozen unconsolidated rocks and its association with transient electromagnetic (TEM) signals measured in northern regions. The distinguishing feature of these signals is the distortion of the monotony or sign reversals in the time range from a few tens to a few hundreds of microseconds. In simulating TEM data, the IP effects in frozen ground were attributed to the dielectric relaxation phenomenon rather than to the frequency‐dependent conductivity. This enabled us to use laboratory experimental data available in the literature on dielectric spectroscopy of frozen rocks. In our studies we focused on simulating the transient response of a coincident‐loop configuration in three simple models: (i) a homogeneous frozen earth (half‐space); (ii) a two‐layered earth with the upper layer frozen; (iii) a two‐layered earth with the upper layer unfrozen. The conductivities of both frozen and unfrozen ground were assumed to exhibit no frequency dispersion, whereas the dielectric permittivity of frozen ground was assumed to be described by the Debye model. To simplify the presentation and the comparison analysis of the synthetic data, the TEM response of a frozen polarizable earth was normalized to that of a non‐polarizable earth having the same structure and resistivities as the polarizable earth. The effect of the dielectric relaxation on a TEM signal is marked by a clearly defined minimum. Its time coordinate tmin is approximately three times larger than the dielectric relaxation time constant τ. This suggests the use of tmin for direct estimation of τ, which, in turn, is closely associated with the temperature of frozen unconsolidated rock. The ordinate of the minimum is directly proportional to the static dielectric permittivity of frozen earth. Increasing the resistivity of a frozen earth and/or decreasing the loop size results in a progressively stronger effect of the dielectric relaxation on the TEM signal. In the case of unfrozen earth, seasonal freezing is not likely to have an appreciable effect on the TEM signal. However, for the frozen earth, seasonal thawing of a near‐surface layer may result in a noticeable attenuation of the TEM signal features associated with dielectric relaxation in a frozen half‐space. Forward calculations show that the dielectric relaxation of frozen unconsolidated rocks may significantly affect the transient response of a horizontal loop laid on the ground. This conclusion is in agreement with a practical example of inversion of the TEM data measured over the permafrost.
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Vector attenuation: elliptical polarization, raypaths and the Rayleigh‐window effect
More LessABSTRACTWaves in dissipative media exhibit elliptical polarization. The direction of the major axis of the ellipse deviates from the propagation direction. In addition, Snell's law does not give the raypath, since the propagation (wavevector) direction does not coincide with the energy‐flux direction. Each of these physical characteristics depends on the properties of the medium and on the inhomogeneity angle of the wave. The calculations are relevant for multicomponent surveys, where the receivers are placed on the ocean‐floor. An example of the role played by inhomogeneous waves is given by the Rayleigh‐window effect, which implies a significant amplitude reduction of the reflection coefficient of the ocean‐bottom.
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Self‐potential data interpretation using standard deviations of depths computed from moving‐average residual anomalies
Authors E.M. Abdelrahman, K.S. Essa, E.R. Abo‐Ezz and K.S. SolimanABSTRACTWe have developed a least‐squares minimization approach to determine simultaneously the shape (shape factor) and the depth of a buried structure from self‐potential (SP) data. The method is based on computing the standard deviation of the depths determined from all moving‐average residual anomalies obtained from SP data, using filters of successive window lengths for each shape factor. The standard deviation may generally be considered a criterion for determining the correct depth and shape factor of the buried structure. When the correct shape factor is used, the standard deviation of the depths is less than the standard deviations computed using incorrect shape factors. This method is applied to synthetic data with and without random errors, complicated regionals and interference from neighbouring sources, and is tested on a known field example from Turkey. In all cases, the shape and depth solutions obtained are in a good agreement with the actual values.
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Linear demultiple solution based on the concept of bottom‐multiple‐generator (BMG) approxiamtion: some new results
Authors Abiola O. Oladeinde and Luc T. IkelleABSTRACTRecent advances in the demultiple technique have shown that a multidimensional convolution of a portion of data containing only primaries with the whole data (containing both primaries and multiples) can allow us to predict and attenuate all orders of free‐surface multiples that are relevant for practical purposes. One way of constructing the portion of the data containing only primaries is by muting the actual data just above the first free‐surface multiple to arrive. The location of the mute is generally known as the bottom‐multiple‐generator (BMG) reflector; the portion of the data containing only primaries required for constructing the free‐surface multiples is located above the BMG. The outstanding question about this method is how effective can the technique be when the BMG cuts through several seismic events, as is the case in long‐offset data or in very complex shallow geology. We present new results which demonstrate the fact that the BMG location may cut through several seismic events without affecting the accuracy or the cost of demultiple.
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Electromagnetic fields in a non‐uniform steel‐cased borehole
Authors Hee Joon Kim and Ki Ha LeeABSTRACTSince most oil wells are cased in steel, electromagnetic (EM) signals undergo severe attenuation as they diffuse across the casing. This paper examines an effect of non‐uniform casing properties on EM fields measured in a steel‐cased well embedded in a layered formation. We use a finite‐element method for computing secondary azimuthal electric fields in a cylindrically symmetric model, and analytically obtain primary fields for a homogeneous casing in a homogeneous whole space. Although steel casing largely masks EM signals induced into a layered formation, the induced signal is more pronounced in phase than in amplitude. The effect of casing non‐uniformity is quite large in measured fields but is highly localized. When electrical conductivity varies rapidly in the casing wall, the resulting EM fields also vary rapidly. A cross‐correlation function of these variations has strong peaks at two points, the interval between them being equal to the source–receiver distance. The high‐frequency coherent noise event caused by the non‐uniform casing can be greatly suppressed by low‐pass filtering to enhance EM signals indicating formation conductivity.
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Theory and seismic applications of the eigenimage discrete wavelet transform
More LessABSTRACTDiscrete wavelet transforms are useful in a number of signal processing applications. To improve the scale resolution, a joint function of time, scale and eigenvalue that describes the energy density or intensity of a signal simultaneously in the wavelet and eigenimage domains is constructed. A hybrid method, which decomposes eigenimages in the wavelet domain, is developed and tested on field data with a variety of noise types. Several illustrative examples examine the ability of wavelet transforms to resolve features at several scales. Successful applications to time‐lapse seismic reservoir monitoring are presented. In reservoir monitoring, the scale‐dependent properties of the eigenstructure of the 4D data covariance matrix enable us to extract the low‐frequency time‐lapse signal that is the result of internal diffusive losses caused by fluid flow.
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Theoretical development of the differential scattering decomposition for the 3D resistivity experiment
Authors Greg A. Oldenborger and Partha S. RouthABSTRACTIn any numerical solution of the DC resistivity experiment, care must be taken to deal with strong heterogeneity of electrical conductivity. In order to examine the importance of conductivity contrasts, we develop a scattering decomposition of the DC resistivity equation in the sparse differential domain as opposed to the traditional dense integral formulation of scattering‐type equations. We remove the singularity in the differential scattered series via separation of primary and secondary conductivity, thereby avoiding the need to address the singularity in a Green's function. The differential scattering series is observed to diverge for large conductivity contrasts and to converge for small contrasts. We derive a convergence criterion, in terms of matrix norms for the weak‐form finite‐volume equations, that accounts for both the magnitude and distribution of heterogeneity of electrical conductivity. We demonstrate the relationship between the differential scattering series and the Fréchet derivative of the electrical potential with respect to electrical conductivity, and we show how the development may be applied to the inverse problem. For linearization associated with the Fréchet derivative to be valid, the perturbation in electrical conductivity must be small as defined by the convergence of the scattered series. The differential scattering formulation also provides an efficient tool for gaining insight into charge accumulation across contrasts in electrical conductivity, and we present a derivation that equates accumulated surface charge density to the source of scattered potential.
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Rough seas and statistical deconvolution
Authors Ed Kragh and Robert LawsABSTRACTThe rough‐sea reflection‐response varies (1) along the streamer (2) from shot to shot and (3) with time along the seismic trace. The resulting error in seismic data can be important for time‐lapse imaging. One potential way of reducing the rough‐sea receiver error is to use conventional statistical deconvolution, but special care is needed in the choice of the design and application windows.
The well‐known deconvolution problem associated with the non‐whiteness of the reflection series is exacerbated by the requirement of an unusually short design window – a requirement that is imposed by the non‐stationary nature of the rough‐sea receiver wavelet. For a synthetic rough‐sea data set, with a white 1D reflection series, the design window needs to be about 1000 ms long, with an application window about 400 ms long, centred within the design window. Although such a short design window allows the deconvolution operator to follow the time‐variation of the rough‐sea wavelet, it is likely to be too short to prevent the non‐whiteness of the geology from corrupting the operator when it is used on real data.
If finely spatial‐sampled traces are available from the streamer, the design window can be extended to neighbouring traces, making use of the spatial correlations of the rough‐sea wavelet. For this ‘wave‐following’ approach to be fruitful, the wind (and hence the dominant wave direction) needs to be roughly along the line of the streamer.
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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)