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- Volume 45, Issue 6, 1997
Geophysical Prospecting - Volume 45, Issue 6, 1997
Volume 45, Issue 6, 1997
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Recursive seismic ray modelling: applications in inversion and VSP[Link]
Authors T.J. Moser and J. PajchelThe recursive nature of rays in blocky models can be exploited to solve some difficult problems in seismic modelling. Each segment of a ray travels from an initial point up to a reflecting interface, where it is split into reflected and transmitted ray segments, which each continue in a similar way. The tree structure that thus emanates is conveniently handled by a recursive scheme. Recursion allows an automatic generation of all phases on a seismogram, together with all information necessary to analyse or select them. By operating recursively with a ray cell, bounded by a pair of vicinal rays in 2D, or a triplet of vicinal rays in 3D, and two successive isochrons, the two‐point ray‐tracing problem is reduced to a simple interpolation. Also, the cellular approach allows for a stable and robust evaluation of dynamic ray quantities without any paraxial tracing, which is cumbersome in blocky models of realistic complexity. Geometric shadows are filled by recursively generated diffractions. The recursive ray tracer has found applications in the fast computation of Green's functions in target‐oriented inversion and in phase identification in VSP.
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Elimination of ghost noise in vibroseis data by deconvolution[Link]
By Xiao‐Ping LiA deconvolution approach is presented to process uncorrelated vibroseis data. The recorded ground force signal, known to be a better correlator for the vibrogram than the pilot sweep itself, is used to deconvolve rather than to correlate the vibrogram. In such a way the ghost sweep (correlation noise), produced by severe harmonic distortions in the ground force signal both at negative and positive correlation times if such a signal is used as the correlator, is eliminated automatically. This type of deconvolution can eliminate the ghost sweep caused by both the upsweep or by the downsweep signal. Synthetic and real data are used to demonstrate the application of the deconvolution procedure. The results are compared with the traditional correlation procedure and show the superiority of the deconvolution approach.
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Modelling the 2D terrain effect on MT by the boundary‐element method[Link]
Authors Shi‐zhe Xu and Hui ZhouThe boundary‐element method is used to model the 2D terrain effect on the magnetotelluric (MT) field. Firstly, the boundary‐value problem of a 2D magnetotelluric field is transformed into an integral equation problem by using Green's theorem. Then the boundary‐element method is used to solve the integral equation and to obtain the MT field and its normal derivative on the terrain. From these values, the apparent resistivity can be calculated.
Compared with the finite‐element method, the boundary‐element method is simpler in element division and the initial data preparation. The configuration of a terrain divided by the boundary‐element method is more consistent with the practical terrain. The method proposed in this paper can be run on a microcomputer, so that it can be used in the field.
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Elimination of higher modes in dispersive in‐seam multimode Love waves[Link]
By Xiao‐Ping LiA deterministic pure phase shift filter (PPSF) is developed to extract the fundamental mode from multimode surface Love waves. Because of different phase velocities of modes and hence different phase traveltimes for a fixed travel distance between source and receiver, the deterministic PPSF can be computed, provided that the dispersion relation of the medium is estimated from the existing transmission data. The process consists of (a) applying the deterministic PPSF to the multimode wave (this step of the filtering process results in a time series in which the amplitudes of the fundamental mode appear at acausal times and the amplitudes of higher modes appear at causal times); (b) setting amplitude values equal to zero for positive times; (c) applying the inverse PPSF to the filtered signal. By using such a deterministic PPSF process, the higher modes almost disappear. The method is applied to synthetic multimode data computed by the normal‐mode summation method.
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3D FD resistivity modelling and sensitivity analyses applied to a highly resistive phonolitic body[Link]
Authors K. Spitzer and H.‐J. KümpelA highly resistive phonolitic body near Sainerholz / Westerwald in Germany has been investigated for geological mapping using vertical electrical soundings in a Schlumberger configuration. Because of its explicitly three‐dimensional shape, conventional 1D and 2D interpretation techniques are not applicable. Therefore, a new 3D finite‐difference forward modelling algorithm has been applied to acquire information about its subsurface structure and to explain the observed data. This investigation focuses on two exposed soundings: one located near the centre of the body and the other close to its rim. For the interpretation, data from electromagnetic measurements on the lateral extension of the body are additionally taken into account as well as geological a priori information. A possible 3D conductivity model is presented and evidence for its validity is discussed using model studies and sensitivity analyses. The latter are carried out using a newly developed 3D FD sensitivity modelling code with which the total subsurface response can be decomposed. This permits the determination of the resolution of model parameters, indicating the contribution of different parts of the model to the overall response. The results emphasize the feasibility of 3D forward modelling in practice.
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A novel mobile multipole system (MUCEP) for shallow (0–3 m) geoelectrical investigation: the ‘Vol‐de‐canards’ array[Link]
Authors C. Panissod, M. Dabas, A. Jolivet and A. TabbaghThe aim of this paper is to point out the advantages of multipoles for the exploration of the very near subsurface (0–3 m) by continuous profiling. We propose a new geometry with eight poles for a MUltipole Continuous Electrical Profiling (MUCEP) measuring system, where the array has a V‐shape and is thus called ‘Vol‐de‐canards’. A series of criteria including 3D numerical simulations are performed (direct and inverse modelling) to determine the optimal geometry and to compare its performance (in terms of depth of investigation and resolution of the geometry of the targets) with the other arrays (quadrupoles or rectangular‐type multipoles). This multipole was built together with a real‐time acquisition system. The multidepth maps obtained confirm the characteristics predicted by numerical simulations.
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A generalized concept of resultant gradient to interpret potential field maps[Link]
Authors R.K. Shaw and B.N.P. AgarwalThe existing concept of the gradient of the potential field anomaly over a 3D source has been generalized. An observed anomaly is modified through a filter based on an assumed source geometry. The first‐order derivatives of this modified anomaly in three mutually orthogonal directions form the components of a vector termed the resultant gradient. The gravity anomaly over a point mass, a vertical line mass and the gravity/magnetic anomaly reduced‐to‐pole over a bottomless right rectangular prism have been suitably modified to yield a specific shape for the amplitude of the resultant gradient in order to decipher the depth of the source centroid/corner. The applicability of the proposed technique is demonstrated by the analyses of a simulated example over a composite source and a real example from published literature with drill‐hole information.
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Topographic accessibility and the tectonic interpretation of gravity data[Link]
Authors A. J. Daniel and P. StylesDuring recent gravity surveys in Patagonia, we were unable to acquire data on the North Patagonian Icecap, which is completely inaccessible for land surveying. The Icecap is clearly of tectonic significance, but, because of its tectonic history, it is uplifted and rugged, and consequently inaccessible. Therefore the distribution of geophysical observations which can be acquired in a rugged field area is dependent on the tectonic history. This suggests that a tectonic history which is reconstructed from geophysical data may be systematically biased.
We use models of local and flexural isostasy to estimate the gravity errors associated with interpolation across inaccessible topography. The gravity error is largest for Pratt isostasy, where the mass deficit which supports the topography is at relatively shallow depths. The gravity error is least for flexural isostasy, because in this case the inaccessible topography is supported regionally by a mass deficit which extends beyond the inaccessible region.
An analytical flexural interpolation scheme is proposed for interpolation across data gaps associated with inaccessible topography. Flexural theory and Gauss’s theorem are used to predict the gravity anomaly due to the mass excess of the inaccessible topography. We apply this scheme to the North Patagonian Icecap, to demonstrate that flexural interpolation predicts a relative gravity low at the site of the Icecap, which would not be predicted by purely geometrical interpolation schemes.
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One‐dimensional magnetotelluric inversion using an adaptation of Zohdy's resistivity method[Link]
Authors B.A. Hobbs and C.C. DumitrescuAn iterative refinement method for determining a layered resistivity model from a Schlumberger or Wenner sounding curve is adapted to determine a layered resistivity model by using apparent resistivity and phase derived from the magnetotelluric impedance. Magnetotelluric observations presented as a function of period are first converted to an approximate resistivity–depth profile using Schmucker's transformation and this is used to construct an initial guess (starting) model. A two‐stage procedure is then invoked. Keeping resistivities constant, layer boundaries are first adjusted to give a minimum misfit between measured data and responses and this is followed by resistivity adjustments with fixed layer boundaries to reduce the misfit further. The method is illustrated by application to some synthetic data both exact and with added noise, to a real field data set and to some magnetotelluric profile data obtained in a survey over the Carnmenellis granites in south Cornwall. The method is validated by recovering conductivity models from the exact and noisy 1D synthetic data. For complicated three‐dimensional data at a single site and along a profile of stations, the method is shown to produce acceptable solutions which may be used as starting models in further two‐ or three‐dimensional studies.
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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)