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- Volume 45, Issue 3, 1997
Geophysical Prospecting - Volume 45, Issue 3, 1997
Volume 45, Issue 3, 1997
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Depth of detection of buried resistive targets with some electrode arrays in electrical prospecting[Link]
Authors A. Apparao, R. Sivarama Sastry and V. Subrahmanya SarmaAssuming the minimum detectable anomaly to be 10%, depths of detection of a 2D vertical resistive sheet of thickness t are found to be 4.0t, 3.0t, 4.0t and 4.0t with Wenner, two‐electrode, three‐electrode and dipole‐dipole (β‐Wenner) arrays, respectively, when the array spread is in‐line. On the other hand, the depths of detection obtained with a broadside spread of the arrays right over the sheet are much less and are correspondingly 2.5t, 2.0t, 2.5t and 2.5t. An increase in the depth extent W of the sheet from 10t to 20t does not increase its depth of detection with the arrays. The depths of detection of an infinitely resistive horizontal cylinder of radius R are respectively 1.5R, 1.8R, 2.0R and 2.0R with the above‐listed arrays when the array spread is in‐line. With broadside spread of any of the arrays, the depth of detection is seen to be 2.5R. In the case of a spherical target of radius R, the detection depths of any of the arrays are found to be small and to vary between 0.8R and 1.1R. Comparatively, the detection depths of resistive targets are much lower than those of conductive targets of the same size and shape, with any electrode array. Among all the arrays studied, the two‐electrode array performs worst in the detection of resistive targets while it performs best in detecting conducting targets of limited lateral extent. In the case of a spherical target, either resistive or conductive, there is no distinct change in its detection depth with array.
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Spectral factorization technique for estimation of an ARMA operator for multichannel deconvolution of seismic data[Link]
Authors R.K. Pathak, S. Sengupta and S. SinhaA new spectral factorization method is presented for the estimation of a causal as well as a causally invertible ARMA operator from the correlation sequence of seismic traces. The method has been implemented for multichannel deconvolution of seismic traces with the aim of exploiting the trace‐to‐trace correlation that exists within seismograms. A layered earth model with a small reflectivity sequence has been considered, and the seismic traces have been considered as the output of a linear system driven by white noise reflection coefficient sequences. The present method is the concatenation of three algorithms, namely Kung's method for state variable (F,G,H) realization using a singular value decomposition (SVD) algorithm, Faurre's technique for computation of the strong spectral factor and Leverrier's algorithm for ARMA representation of the spectral factor. The inverted ARMA operator is used as a recursive filter for deconvolution of seismic traces. In the example shown, two traces with a covariance sequence of 160 ms length have been considered for multichannel deconvolution of stacked seismic traces. The results presented, when compared with those obtained from a conventional deconvolution algorithm, have shown encouraging prospects.
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The detection of King Xerxes' Canal by the use of shallow reflection and refraction seismics — Preliminary results
Authors V.K. Karastathis and St. P. PapamarinopoulosShallow reflection and refraction seismic studies were carried out in Greece in the eastern leg of the Chalkidiki peninsula, in order to test the validity of reports in history books which describe a legendary canal built by the engineers of King Xerxes during the major Persian invasion of Europe through Greece in the 5th century B.C. In the narrowest part of the Athos peninsula, where it is 2 km wide, an 85 m profile was topographically defined almost centrally between the two coastlines. The position of this profile was based on palaeogeographical, geomorphological and topographic studies and observations. A sledgehammer was used as the seismic source for the shallow target. Despite the presence of significant urban and coherent noise, a final stacked section was produced by a suitable choice of acquisition and processing parameters. Both the reflection and refraction seismic studies illustrated clearly the existence of a channel‐like structure of trapezoid cross‐section, almost midway between the two opposite sides of the peninsula.
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Algorithms for staggered‐grid computations for poroelastic, elastic, acoustic, and scalar wave equations[Link]
Authors Turgut Özdenvar and George A. McMechanHeterogeneous wave equations are more complicated numerically than homogeneous wave equations, but are necessary for physical validity. A wide variety of numerical solutions of seismic wave equations is available, but most produce strong numerical artefacts and local instabilities where model parameters change rapidly. Accuracy and stability of heterogeneous equations is achieved through staggered‐grid formulations. A new pseudospectral staggered‐grid algorithm is developed for the poroelastic (Biot) equations. The algorithm may be reduced to handle the elastic and acoustic limits of the Biot equations. Comparisons of results from poroelastic, elastic, acoustic and scalar computations for a 2D model show that porous medium parameters may affect amplitudes significantly. The use of homogeneous wave equations for modelling of a heterogeneous medium, or of a centred rather than a staggered grid, or of simplified (e.g. acoustic) wave equations when elastic or poroelastic media are synthesized, may produce erroneous or ambiguous interpretations.
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A physical study of low‐frequency dispersion of rock conductivity in time‐domain electromagnetics[Link]
Authors Felix M. Kamenetsky and Peter V. NovikovMost rocks display conductivity dispersion in the low‐frequency range, when the usual displacement currents are neglected. The strong influence this low‐frequency dispersion (LFD), including the response sign reversals, was revealed by field experiments with the coincident‐loop configuration widely used in transient electromagnetics (EM). Mathematical modelling of LFD has been the subject of numerous studies. However, confirmation of the role of LFD or induced polarization (IP) by comparing mathematical modelling and field data is rather poor, because knowledge of the properties of rocks in the area of the field measurements is usually insufficient. For this reason physical modelling of LFD has been carried out at Moscow State Geological‐prospecting Academy (Russia) in 1994‐95. In order to observe criteria of similarity for both induction and polarization transients, a ring‐shaped model was chosen and was represented by an electric circuit, consisting of lumped elements (real rock samples included). Qualitatively different transients for dispersive models and their non‐dispersive ohmic equivalents were observed.
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Detecting lateral resistivity inhomogeneities with the Schlumberger array[Link]
Authors Marian Morris, Jan Steinar Rønning and Ole Bernt LileA method of detecting lateral resistivity inhomogeneities with a multi‐electrode cable for the Schlumberger array is presented. Using such a cable system, two dipole‐dipole soundings are gathered in addition to the Schlumberger sounding at each point. Offset differences calculated from the dipole‐dipole data give qualitative information about lateral resistivity inhomogeneities. Results from 1D modelling can give quantitative information about the 2D resistivity distribution since the dipole‐dipole soundings are made to either side of the Schlumberger sounding point. Examples from two different locations in Norway are shown. At Reinøya, northern Norway, a dipping layer, confirmed by refraction seismic data, was identified. In a sedimentary basin with a contamination plume at Haslemoen, southern Norway, the method has revealed lateral variations in resistivity. In both cases, the Schlumberger soundings could be fitted with a 1D resistivity model.
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Cross‐well seismic tomographic delineation of mineralization in a hard‐rock environment[Link]
Authors Shunhua Cao and Stewart GreenhalghSeismic first‐arrival times from a cross‐well experiment are inverted by an iteratively reweighted least squares method for the velocity distribution between boreholes. The inversion is carried out in a multiresolution manner in which long wavelength features are reconstructed first and short wavelength features are gradually introduced into the solution model. As such, the inverse problem is overdetermined at the beginning and becomes mixed‐determined as the constraint on the solution is relaxed. The reconstruction procedure is independent of the initial model and very robust. An accurate and efficient first‐break traveltime calculation scheme is employed to eliminate the possibility of phase mismatches, which can arise with traditional ray shooting or bending methods. Shot static delays are included with the model parameters as unknowns and are recovered simultaneously with the velocity distribution.
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Inversion of potential field data by genetic algorithms[Link]
Authors Fabio Boschetti, Mike Dentith and Ron ListWe present a genetic algorithm that simultaneously generates a large number of different solutions to various potential field inverse problems. It is shown that in simple cases a satisfactory description of the ambiguity domain inherent in potential field problems can be efficiently obtained by a simple analysis of the ensemble of solutions. From this analysis we can also obtain information about the expected bounds on the unknown parameters as well as a measure of the reliability of the final solution that cannot be recovered with local optimization methods. We discuss how the algorithm can be modified to address large dimensional problems. This can be achieved by the use of a ‘pseudo‐subspace method’, whereby problems of high dimensionality can be globally optimized by progressively increasing the complexity and dimensionality of the problem as well as by subdividing the overall calculation domain into a number of small subdomains. The effectiveness and flexibility of the method is shown on a range of different potential field inverse problems, both in 2D and 3D, on synthetic and field data.
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Moveout analysis for tranversely isotropic media with a tilted symmetry axis[Link]
More LessThe transversely isotropic (TI) model with a tilted axis of symmetry may be typical, for instance, for sediments near the flanks of salt domes. This work is devoted to an analysis of reflection moveout from horizontal and dipping reflectors in the symmetry plane of TI media that contains the symmetry axis. While for vertical and horizontal transverse isotropy zero‐offset reflections exist for the full range of dips up to 90°, this is no longer the case for intermediate axis orientations. For typical homogeneous models with a symmetry axis tilted towards the reflector, wavefront distortions make it impossible to generate specular zero‐offset reflected rays from steep interfaces. The ‘missing’ dipping planes can be imaged only in vertically inhomogeneous media by using turning waves. These unusual phenomena may have serious implications in salt imaging. In non‐elliptical TI media, the tilt of the symmetry axis may have a drastic influence on normal‐moveout (NMO) velocity from horizontal reflectors, as well as on the dependence of NMO velocity on the ray parameter p (the ‘dip‐moveout (DMO) signature’). The DMO signature retains the same character as for vertical transverse isotropy only for near‐vertical and near‐horizontal orientation of the symmetry axis. The behaviour of NMO velocity rapidly changes if the symmetry axis is tilted away from the vertical, with a tilt of ±20° being almost sufficient to eliminate the influence of the anisotropy on the DMO signature. For larger tilt angles and typical positive values of the difference between the anisotropic parameters ε and δ, the NMO velocity increases with p more slowly than in homogeneous isotropic media; a dependence usually caused by a vertical velocity gradient. Dip‐moveout processing for a wide range of tilt angles requires application of anisotropic DMO algorithms. The strong influence of the tilt angle on P‐wave moveout can be used to constrain the tilt using P‐wave NMO velocity in the plane that includes the symmetry axis. However, if the azimuth of the axis is unknown, the inversion for the axis orientation cannot be performed without a 3D analysis of reflection traveltimes on lines with different azimuthal directions.
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Automated trend reinforcement of aeromagnetic data[Link]
More LessAeromagnetic maps poorly represent lineaments that are at acute angles to the flight‐line direction. Commonly used gridding algorithms cannot cope with local trends, magnetic anomalies at an angle with the main trend of the map, and tend to generate closed contours around the flight lines. By introducing some a priori information, it is possible to add extra data between the flight lines, i.e. trend lines, to reinforce local trends. The proposed automated technique is based on a nearest neighbour search of the maxima and minima in the aeromagnetic map. The resulting map is more realistic and derived maps, such as vertical gradients and analytical signal maps, are greatly improved. Moreover, this automated procedure is user independent and easy to implement. The technique is demonstrated on aeromagnetic data from the Kirkland Lake region, in north‐eastern Ontario, Canada.
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Seismic inversion for coal‐seam thicknesses: trials from the Belvoir coalfield, England[Link]
Authors Tian Gang and N.R. GoultyWe have applied generalized linear inversion to high‐quality seismic reflection data from the Coal Measures in the East Midlands of England. The purpose was to test whether accurate values for coal‐seam thicknesses could be obtained to benefit longwall coal‐mining and coal‐bed methane production. A seismic line intersecting two logged boreholes was chosen so that an objective evaluation of the results could be made, and inversion was carried out after careful reprocessing of the line with post‐stack migration. As coal‐seams are thin beds in the seismic bandwidth, it was necessary to assume that acoustic impedance values are constant. Of the ten coal‐seams in the sequence, only two were found to be major contributors to the reflection response at the boreholes, but inversion runs were carried out independently for two seams and four seams. It was found that keeping the wavelet fixed was the best strategy when inverting for the interface two‐way times at the seam boundaries. However, all the strategies tested showed inconsistent variations in seam thicknesses which were geologically implausible. We conclude that significant improvements need to be made in acquisition and processing techniques for inversion to give useful results in this application.
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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)