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- Volume 31, Issue 6, 1983
Geophysical Prospecting - Volume 31, Issue 6, 1983
Volume 31, Issue 6, 1983
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TUTORIAL: RESEARCH, APPLICATIONS, AND PUBLICATIONS IN ELECTRICAL AND ELECTROMAGNETIC METHODS*
More LessAbstractHow do papers on electrical and electromagnetic methods published in Geophysics and Geophysical Prospecting reflect the actual field application of the various techniques? Our study based on 10 volumes of the journals (1973–1982) suggests a rather good agreement. EM techniques are the most important both in the field and in print. However, an imbalance was detected in the subject matter. Too many published papers deal with theory, and too few with new instrumentation and techniques, case histories, and physical properties of rocks. Papers presented at annual meetings of the EAEG and SEG give a more even‐handed and accurate view of geophysical activities. Unfortunately, many of our colleagues around the world are not able to attend the annual meetings regularly and depend on journals for their information. Practicing and research geophysicists should submit more papers on subjects that at present are not adequately covered.
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THE WAVEFIELD GENERATED BY TWO VERTICAL VIBRATORS IN PHASE AND IN COUNTERPHASE*
More LessAbstractA theoretical study is presented of the effect of source/receiver patterns with fixed phase difference on the radiation/receiving characteristics of the single elements of the pattern in linear elastic media. The radiation characteristic of two vertical vibrators in phase and in counterphase is investigated in detail.
By introducing source/receiver patterns the characteristics become dependent on dimension of the pattern, frequency and angle of observation in the horizontal plane. The types of waves generated by source patterns are, independent of phase differences, equal to those generated by single sources. With two vertical vibrators in phase radiation of P‐waves can be improved relative to that of SV‐waves. With two vibrators in counterphase radiation of SV‐waves in vertical directions is enhanced relative to that of P‐waves; this may be a valuable method of SV‐shear wave generation in exploration seismics. SV‐wave amplitudes larger than P‐wave amplitudes can be expected at angles larger than 5°–10° with the vertical.
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NON‐NORMAL INCIDENCE INVERSION: EXISTENCE OF SOLUTION*
Authors MOSTAFA SHIVA and J. M. MENDELAbstractAn inverse problem is one in which the parameters of a model are determined from measured seismic data. Important to the solution of inverse problems is the issue of whether or not a solution exists. In this paper we show, in a constructive manner, that a solution does exist to the specific inverse problem of determining the parameters of a horizontally stratified, lossless, isotropic and homogeneous layered system that is excited by a non‐normal incidence (NNI) plane wave. Mode conversion between P‐ and S‐waves is included.
We develop a seven‐step layer‐recursive procedure for determining all of the parameters for layer j. These parameters are P‐wave and S‐wave velocities and angles of incidence, density, thickness, traveltimes, and reflection‐ and transmission‐coefficient matrices. Downward continuation of data from the top of one layer to the top of the next lower layer is an important step in our procedure, just as it is in normal incidence (NI) inversion. We show that, in order to compute all parameters of layer j, we need to (and can) compute some parameters for layer j+ 1. This is a non‐causal phenomenon that seems to be necessary in NNI inversion but is not present in NI inversion.
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AUTOMATIC PICKING OF FIRST ARRIVALS AND PARAMETERIZATION OF TRAVELTIME CURVES*
Authors B. GELCHINSKY and V. SHTIVELMANAbstractOne of the methods for studying the near‐surface low‐velocity zone and for subsequent determination of static corrections is the technique of employing first arrivals. During the past few years several computer algorithms, based on some simplifying assumptions, have been suggested for automatic determination of first arrivals.
This paper suggests a new method for automatic picking of first arrivals, which works under quite general assumptions concerning the character of the data. The method is based on the use of correlation properties of signals and the application of a statistical criterion for the estimation of first arrivals time. A number of dimensionless parameters is used in the algorithm making it possible to regulate the level of reliability and the resolution of the picking procedure.
The second stage of the algorithm is the parameterization of the traveltime curve, that is a division of the previously obtained t—x curve into separate rectilinear segments. The suggested algorithm of parameterization is based on an heuristic use of some properties of maximum likelihood estimates. This permits location of the breakpoints of the t—x curve and the estimation of the parameters of each rectilinear segment.
A computer program has been written based on the picking and parameterization algorithm. This program has been tested on a large amount of field data and the results show that it works at least as well as the hand procedure.
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SIMULTANEOUS INTERACTIVE MAGNETIC AND GRAVITY INVERSION*
Authors V. MENICHETTI and A. GUILLENAbstractGravity and magnetic anomalies may have the same source but it is always difficult to achieve correlated solutions if interpretations are carried out separately. Therefore it is useful to invert both anomalous gravity and magnetic profiles at the same time, so that the solution mav be more constrained.
Existing inversion techniques do not lend themselves equally to this kind of extension, since the parameters—such as density and susceptibility contrasts—should not be related. The algorithms more easily adaptable to simultaneous inversion are those which look for the shape and the position of anomalous bodies with constant unknown density or susceptibility contrasts.
In this study, we use the generalized inverse method in the 2 1/2 D case. Parameters are the coordinates of the vertices of the polygonal cross‐sections of the anomalous bodies and both density and susceptibility contrasts of each body. The two types of profile to be processed must be superposable but may have different lengths, spacings, and coordinate origins. Both synthetic examples and field data from geologically known areas have been processed, and it appears that the simultaneous inversion technique may provide an important additional degree of control in the interactive interpretation process.
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ELECTROMAGNETIC FIELDS DUE TO A THIN RESISTIVE LAYER*
More LessAbstractUsing approximate boundary conditions, expressions for electromagnetic fields have been derived for a thin, highly resistive layer lying between two homogeneous layers excited by an electric dipole grounded on the surface of the earth. The variations of the fields with the parameter T/T1 (ratio of the transverse resistance of the thin layer to the transverse resistance of the first layer) were studied in relation to frequency, time, the normalized separation source—receiver, and the angle between the source and the radius to the observation point. For a value of h2/h1 (ratio of thickness of second layer to the thickness of the first layer) approximately equal to 0.2, the general three‐layer medium case gives the same results as this approach. It was found that the electric fields have a very strong dependence on the parameter T (transverse resistance) which characterizes the thin, highly resistive layer. However, the magnetic fields depend only very weakly on this parameter.
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VLF GROUND SURVEYS, A POWERFUL TOOL FOR THE STUDY OF SHALLOW TWO‐DIMENSIONAL STRUCTURES*
Authors G. FISCHER, B. V. LE QUANG and I. MÜLLERAbstractThe suitability of VLF ground surveys in the investigation of shallow two‐dimensional structures is analyzed. For such structures the polar formalism is derived, necessary in practice since the transmitters are generally not in the structural strike or profile. A simple vertical dike is considered to demonstrate the striking anisotropy which can be expected over such a structure, in particular the high apparent resistivity along the direction of a well‐conducting dike and the low resistivity across it. The theory is then confronted with the practical example of an asymmetrical vertical dike resulting from a strike‐slip fault. Modelling of the survey results is very successful and yields good confirmation of the polar behaviour. VLF ground surveys thus provide a quick and powerful tool for the study of geological accidents within about 100 m of the surface.
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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)