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- Volume 31, Issue 4, 1983
Geophysical Prospecting - Volume 31, Issue 4, 1983
Volume 31, Issue 4, 1983
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MIGRATION IN THE PRESENCE OF A RUGGED INTERFACE WITH HIGH VELOCITY CONTRAST*
Authors L. AMEELY, TH. KREY, F. MUHTADIE, H.‐F. RAU and H. RISTAbstractIn the western coal‐mining area of Ruhrkohle AG, reflection seismic prospecting for the Carboniferous coal measures is severely impaired by structures with halokinetic features. These structures make the interface between Mesozoic and Paleozoic layers, i.e., the top of Zechstein in general, very rugged. Unfortunately the velocity contrast at this interface is very high in that area, the ratio of velocities being 1.5 to 2.0. Therefore, migration and stacking become a problem.
Three types of migration are presented:
- 1 (f, x)‐time‐migration with vertical time‐to‐depth conversion as a second step.
- 2 Kirchhoff migration down to a level determined approximately by the highest points of the top of Paleozoics, i.e., 0.35 s, and Kirchhoff‐downward continuation for all times exceeding 0.35 s. Intermediate static corrections for these latter times with subsequent (f, k)‐time‐migration and final vertical time‐to‐depth conversion.
- 3 Direct depth migration in the (f, x)‐domain using three interval velocities.
In all cases an intermediate picking of the velocity interfaces is necessary. In case 2 this occurs at an earlier stage of the process than in case 1, and in case 3 at a still earlier stage.
The results of the second and third migration procedures are superior to those of the first. Possibilities for misinterpretation of faults are reduced considerably when the second or third migration procedure is applied.
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THEORETICAL AND EXPERIMENTAL APPROACHES TO THE GEOPHONE SPURIOUS FREQUENCY*
Authors SHI JUNG CHEN and CH. DALTONAbstractThe geophone spurious frequency is modeled as the resonance of the planar motion of a spider spring carrying a moving mass. An analytic solution is found using the Castigliano method by assuming that the spring arm is a single‐mode vibrating cantilever beam. The spring shape is found from this analysis. When the typical spring has a circumferentially varying cross‐section, the Castigliano method is no longer applicable. A dimensional analysis is used as an approximate method for general design. Based on the theoretical result, a rotational fixture and a translational fixture were designed for experimental purposes. A low‐noise fixture and a phase‐averaging technique provide the amplitude and spurious resonance in the frequency domain. Finally, a test is run by using a rotational fixture to compare with the approximate method of spurious frequency prediction. Very good agreement between prediction and experiment is found.
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THE AUTOBALANCER—AN AUTOMATIC SYSTEM FOR THE REDUCTION OF POWER‐LINE INTERFERENCE WITH SEISMIC SIGNALS*
By L. ENSINGAbstractSince the early years of seismic surveying, field engineers and observers have been faced with the appearance of interference at power‐line frequency (and harmonics) on seismic records: 60 Hz in the Americas, 50 Hz in Eurasia; and 16 2/3 Hz (the train supply frequency in Germany, Austria, and Switzerland) in part of Europe. Such interference is picked up from the ground surface through electric leakage between geophone leads and ground.
To minimize power‐line interference, the autobalancer automatically performs a balancing procedure by adjusting two potentiometers on each channel—essentially the same operation as manually performed by seismic operators for decades. The first 24‐channel system became operational in 1977.
The first‐generation design comprised two 256‐position electronic switching potentiometers. Over 4 years of experience with this system in all parts of the world has shown its value under a variety of field conditions. Improvements up to 30 dB were often found, sometimes even up to 40 dB. Balancing time was about 30 s.
A second‐generation design was field‐tested in 1981. Potentiometer stepping is now performed at three levels: coarse, medium, and fine. This leads to faster operation and better resolution: balancing time is about 10 s; improvements up to 60 dB are occasionally found.
A later modification gives a further reduction of the balancing time to 5 s in 60 Hz areas or 6 s in 50 Hz areas.
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AUTOMATIC THREE‐LAYER, THREE‐DIMENSIONAL DECONVOLUTION OF THE PAYS DE BRAY ANTICLINE*
Authors E. ALESSANDRELLO, M. BICHARA and J. LAKSHMANANAbstractGravity data have been transformed into a three‐layer, three‐dimensional model by using an automatic procedure based on linear filtering. The Bouguer anomaly is first transformed by linear filtering into density variations located between two planes 1100 and 2500 m deep. These densities are then transformed into thicknesses with a constant density contrast of 0.4 g/cm3 with two geological constraints for the second and third interface:
—minimum at 2500 m depth;
—maximum below a variable limit given by geology.
This gives the contact between the second and third layer. Differences between measured and computed gravity are then applied by a similar procedure to a layer located between depths of 0 and 500 m, giving the contact between the first and second layer. Interesting secondary anticlines and transverse faults are shown by various structural maps.
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AIRBORNE RESISTIVITY MAPPING USING A MULTIFREQUENCY ELECTROMAGNETIC SYSTEM*
More LessAbstractAirborne electromagnetic methods are most commonly used in mineral exploration. However, new developments, such as multifrequency capability and digital on‐board field recording, as well as improvements in instrumentation resulting in high signal‐to‐noise ratios in recorded data, have made their application in geological mapping possible.
A three‐frequency airborne EM survey carried out over an area northwest of Timmins, Ontario, was interpreted in terms of thickness and resistivity of the layers of a two‐layer earth section. Since both in‐phase and quadrature components are measured, this provides six independent parameters at each point in space. Based on prior geological information and a preliminary interpretation of the field records, two two‐layer models of the subsurface seemed to be appropriate for most of the survey area. An automatic computerized interpretation procedure was devised to interpret the field data at each point in terms of thickness and resistivity parameters of those two models. When the geology is more complex, the data do not fit the models and no interpretations are made. Two maps illustrating the variation of the resistivity and the thicknesses of the layers were constructed from the interpreted data. These maps agree with the known geological information about the distribution of glacial clay in the area. Areas where the layered models do not fit are known to be areas where the geology is complex with a large number of dykes and other lateral inhomogeneities. The study shows that multifrequency airborne EM surveys can be very useful in geological mapping over inaccessible terrain and can significantly help the mapping geologist where outcrops are scarce.
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TRANSFORMATION OF RESISTIVITY SOUNDING MEASUREMENTS OBTAINED IN ONE ELECTRODE CONFIGURATION TO ANOTHER CONFIGURATION BY MEANS OF DIGITAL LINEAR FILTERING*
More LessAbstractThe technique of digital linear filtering is used for transformation of apparent resistivity data from one electrode configuration into another. Usually filter spectra are determined via the discrete Fourier transforms of input and output functions: the filter characteristic is the quotient of the spectra of the output function and input function.
In this paper, the transformation of the apparent resistivities is presented for four electrode configurations (Wenner, the two‐electrode, Schlumberger, and dipole configurations). In our method, there is no need to use the discrete Fourier transform of the input and output functions in order to determine the filter spectrum for converting apparent resistivity in one electrode configuration to any other configuration. Sine responses for determination of the derivative of apparent resistivities are given in analytical form.
If the filter spectrum for converting the apparent resistivity to the resistivity transform for one electrode configuration is known, the filter spectra for transforming the apparent resistivity to the resistivity transform for any electrode configurations can be calculated by using newly derived expressions.
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THE INTEGRAL‐FINITE‐DIFFERENCE METHOD FOR CALCULATING TRANSIENT ELECTROMAGNETIC FIELDS IN A HORIZONTALLY STRATIFIED MEDIUM*
More LessAbstractA finite‐difference solution and an integral algorithm are developed for computing time‐domain electromagnetic fields generated by an arbitrary source located in horizontally stratified earth.
The finite‐difference problem is first solved for the kernel of an integral Bessel transform of the desired field and then an inverse transformation is performed either by numerical integration or by linear filtering. A description of the appropriate algorithm as well as numerical results are presented for two cases: an axially symmetric source (a vertical magnetic dipole or a circular loop) placed on the surface of a horizontally layered section overlying either an insulating or an ideally conducting basement.
In comparison with the conventional methods used for computing transient electromagnetic fields, the integral‐finite‐difference method is more universally applicable.
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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)