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- Volume 33, Issue 3, 1985
Geophysical Prospecting - Volume 33, Issue 3, 1985
Volume 33, Issue 3, 1985
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INVERSION FROM FINITE OFFSET DATA AND THE SIMULTANEOUS RECONSTRUCTION OF THE VELOCITY AND DENSITY PROFILES*
More LessABSTRACTRecursive inversion algorithms are described which facilitate (a) a direct inversion of a one‐dimensional velocity distribution from finite offset data and (b) the related simultaneous reconstruction of a two‐parameter, say velocity‐density, medium. The suggested procedures are based on an extended format of the WKBJ‐Bremmer scattering model. The attained accuracy levels and the limitations are highlighted by computer simulations against synthetic data.
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ON THE IMPROVEMENT IN PENETRATIO ACHIEVED BY USING EXTENDED MARINE SOURCE ARRAYS*
By M. H. SAFARABSTRACTIn dealing with the problem of large amplitude multiple reflections arising from a hard water‐bottom, it has been found that the use of extended source array techniques resulted in a considerably better penetration than that obtained using either computer simulated long arrays or the conventional air‐gun array systems. The purpose of this paper is to use the concept of the array directivity factor in discussing the reason for the improvement in penetration achieved by using extended marine source arrays.
Examples are given showing that the low frequency power radiated within the so called “penetration window” can be increased by a factor of two by choosing the correct spacing of the point sources forming the extended array. It is concluded that to ensure that most of the low frequency energy is concentrated within the penetration window to achieve deep penetration, a source array with spacing comparable with the wavelength is required.
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EFFECT OF INCLINED ANISOTROPIC SUBSTRATUM ON MAGNETOTELLURIC RESPONSE*
More LessABSTRACTIn tectonically active regions electrical conductivity anisotropies are the dominating features. The importance of conductivity anisotropy in the interpretation of magnetotelluric data is well known. In the present study numerical results presented which show the effect of a substratum with inclined anisotropy on the magnetotelluric response. The pronounced change on the magnetotelluric response is found for the models in which the substratum underlies (i) conductive and (ii) resistive overburden.
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A METHOD TO DETERMINE THE VELOCITIES OF THE SEAFLOOR AND NEAR‐SURFACE SEDIMENTS*
Authors A. M. BERGE and B. BESKOWABSTRACTShotpoint gathers from conventional reflection seismic surveys contain both reflected and refracted waves. In this study shot records were processed and analyzed, and the data were modeled with reflected, refracted, and reflected‐refracted waves to fit the recorded data. The result is a detailed velocity model. The inverse problem for refracted waves was solved by using the Wiechert‐Herglotz inversion.
A 500‐km‐long 26‐fold reflection seismic line from the Barents Sea, north of Norway, has been investigated. The data show high velocities, multiple reflections, and various types of noise. To test the method a total of 34 shot gathers were analyzed along this line. The aim of the interpretation was to determine the velocity in the seafloor and the near‐surface sediments. It is possible to map the vertical as well as the lateral velocity distribution in detail. Depending on the length of the streamer and the velocity gradient in the sediments, the calculated depth varies between 300 and 500 m below the seafloor. These velocities were also compared to the stacking velocities obtained from the reflection seismic data to see how the velocities determined by different methods were related.
The velocity distribution in the sediments is one of the key factors in seismic interpretation. The technique discussed in this paper can contribute to velocity information both in the processing and interpretation of seismic data.
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AN INVESTIGATION OF THE SPECTRAL PROPERTIES OF PRIMARY REFLECTION COEFFICIENTS*
Authors A. T. WALDEN and J. W. J. HOSKENABSTRACTThis paper investigates the form of the nonwhiteness found in the reflection coefficients from a wide variety of rock sequences around the world. In all but one case densities are taken as constant due to the paucity of suitable density data. The reflection sequences are pseudo‐white only above a corner frequency, below which their power spectrum falls away according to a power law fβ, where β is between 0.5 and 1.5. This spectrum can be adequately modelled in practice very simply with an ARMA (1, 1) process which acts on a white innovation sequence. The corollary of this is that before wavelet estimation methods are applied (all of which‐except those based on synthetic seismograms—presuppose white reflection sequences) or deconvolution filters estimated, seismic traces should be filtered with the inverse of this process.
Interestingly, the estimated ARMA processes group themselves into two clearly differentiated categories, having very different indices of predictability (or, strictly, indices of linear determinism). The two categories apparently correlate precisely with two kinds of sedimentation: one which consists largely of sequences of rocks with repeating properties, called “repetitive” in this paper but perhaps loosely describable as “cyclic”, and the other which is randomly bedded with no apparent pattern of components. The former has indices of predictability which are two to four times as great as those of the latter. Another, probably related, property is that β for the repetitive sequences tends to be greater than that for non‐repetitive rock columns.
The observed power spectra are shown to be consistent with a simple model for the logarithm of acoustic impedance consisting of a mixture of processes where the distribution of (time) scale parameters is reciprocal.
Detailed effects of block‐averaging and sampling the logs are shown to depend on the type of sequence under examination.
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DC GEOELECTRIC SOUNDING TO DETERMINE WATER CONTENT IN THE SALT MINE ASSE (FRG)*
Authors W. KESSELS, I. FLENTGE and H. KOLDITZABSTRACTIn the course of investigations concerning disposal of radioactive waste, DC‐geoelectric soundings were performed in the salt mine Asse in 1982. The survey resulted in the determination of resistivities of various salt formations. A comparison of various resistivities obtained in salt formations in the mine shows that it is possible to estimate the content of free water in salt using Archie's equation.
The significant result of this survey is that salt formations in the mine can be divided into two categories according to their resistivities:
1. Salt with resistivity between 107 and 108Ωm (particularly Na2Sp and Na3β). The particularly pure halitic formations are to be found within this resistivity range.
2. Salt with resistivity between 103 and 105Ωm (Na2Tl, K2, transition salt). The salt formations with thin layers of anhydrite and a high content of hygroscopic salts are classified in the paper.
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COMBINED SOUNDING‐PROFILING RESISTIVITY MEASUREMENTS WITH THE THREE‐ELECTRODE ARRAYS*
Authors M. KAROUS and T. K. PERNUABSTRACTA combination of DC‐resistivity sounding and profiling measurements can be used to obtain the maximum information about distribution of resistivities in the earth. Combined sounding‐profiling measurements with the AMN, MNB arrays are considered. The resistivity data from such measurements can be presented as: (a) normal sounding curves, (b) combined sounding‐profiling curves, (c) profiling curves, (d) pseudo‐sections, or as transformations obtained by the so‐called gradient processing, to emphasize the influence of the target objects.
The examples chosen from numerical modeling and field tests show the efficiency of measurements with three‐electrode arrays to accurately locate thin conductors and contacts of lithological units of different resistivities. An interpreted cross‐section is compared with the results of other geophysical measurements (VLF‐R, dipole EM, Δ, IP) showing good correlation.
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SELF POTENTIAL AND CURRENT CHANNELLING*
By J. P. CULLABSTRACTAnomalies in self potential (SP) are usually explained in terms of geochemical activity. However, there are other sources of current which cause complications in the interpretation. In particular, DC telluric currents can be generated outside the survey region. Contrasts in resistivity cause current channelling and secondary voltages can be induced by leakage. The effect can be demonstrated using network theory and internal sources can be emphasized by subtraction. Joint surveys are required to demonstrate correlations with apparent resistivity.
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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)