- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 18, Issue 3, 1970
Geophysical Prospecting - Volume 18, Issue 3, 1970
Volume 18, Issue 3, 1970
-
-
A TWO‐LAYER GROUND IN THE FIELD OF AN INFINITELY LONG CABLE*
Authors V. BEZVODA and K. SEGETHAbstractWhen the fixed‐source electromagnetic method is applied the quantities measured are influenced by the resistivity of the whole environment. This occurs mostly in the cases of measurements in areas with prevailing high resistivities, and if a weathered layer with considerably lower resistivity than that of the original rock has been formed near the surface. For this reason it is important to be acquainted with the properties of the two‐layer ground with an overlying conductive layer.
In the present paper attention will be drawn to theoretical calculations connected with the field of the homogeneous ground and that of the two‐layer ground.
-
-
-
AN INVESTIGATION OF THE RELATIVE ACCURACY OF THE MOST COMMON NORMAL‐MOVEOUT EXPRESSIONS IN VELOCITY ANALYSES*
More LessAbstractThree common expressions for the normal moveout of recorded seismic events are investigated by numerical simulation procedures for accuracy in predicting the root‐mean‐squared (RMS) or mean, as the case may be, subsurface velocity function from seismic data. The principal investigation, for which detailed error curves are shown, was derived for a stochastic subsurface model composed of strata with thicknesses ranging up to 91.4 m (300 ft) and boundary velocity contrasts ranging up to 45.7 m/sec (150 ft/sec); there was a 95 percent chance of velocity increase with increased depth. The effects of changes in the basic statistical subsurface model are discussed.
The results appear to confirm the judiciousness of the choices of to and (x/z') as plotting parameters to be used with the respective percent errors in the three expressions, where are, respectively, the zero‐offset arrival time of, the offset distance of, and the mean‐squared velocity encountered by a seismic ray. Out of the three normal‐moveout expressions examined, the “straight‐raypath” expression with the RMS velocity substituted as its velocity term proved to be the most accurate in the determination of velocities.
-
-
-
APPLICATION OF REFRACTION SEISMICS TO SUBSALT TECTONIC PROBLEMS IN A DEEP SALTDOME BASIN*
More LessAbstractDespite the use of CDP and digital methods the Zechstein base is still the deepest horizon in the vast salt‐dome basin of Central Europe for which continuous information can be obtained by reflection seismics. Thus in North‐western Germany, in addition to reflection seismics, the refraction seismic method has been increasingly used for a reliable survey of deeper horizons.
The first part of the paper deals with the investigation of the various possibilities and limitations of refraction seismics with regard to the investigation of Pre‐Zechstein layers in a basin with a tectonically very complicated overburden. The recording techniques specially developed for continuous profiling of the desired refraction seismic arrivals and the data processing methods are described.
The main problems of interpretation are then discussed, in particular with regard to depth representation. The advantages and disadvantages of the various methods, e.g. Gardner's, Hales' and Wyrobek's, and of the wave‐front method, are compared. On account of the tectonically complicated overburden Thornburgh's wave‐front method proved to be the most useful.
In a further section the various possibilities for velocity determinations are mentioned, e.g. Wyrobek's determination of the overburden velocity, for which the wave‐front method automatically furnishes the necessary corrections to a deep datum.
Finally, some examples are given for the results obtained, including some incidental information on the deeper crust.
-
-
-
ETINCELEURS MULTIELECTRODES A HAUTE RESOLUTION*
Authors J. CASSAND and M. LAVERGNEAbstractHIGH RESOLUTION MULTIELECTRODES SPARKERS
Multielectrode sparkers can bring a considerable improvement of high resolution seismic. A 2 meter resolution and a penetration of about 150 meters can be obtained with 1000 joule sparker with several hundred electrodes. Several records are shown.
-
-
-
RELATIVE CONTRIBUTION TO SIGNAL BY GROUND ELEMENTS IN TWO‐COIL INDUCTION LOGGING SYSTEM*
Authors A. ROY and R. L. DHARAbstractA two‐coil induction logging system in an infinite homogeneous medium has been theoretically analysed for the relative contributions of various ground elements to the total signal at the receiver. Unlike an earlier approximate treatment by Doll (1949), the present analysis takes skin effect into account (while neglecting displacement currents). Contour diagrams depicting relative contributions from every ground point and diagrams for vertical and radial/lateral investigation characteristics are presented. The results differ significantly from Doll's in the following points:
- 1 The relative contribution is frequency and conductivity dependent;
- 2 It does not show symmetry with respect to the center of coil system;
- 3 It alternates between positive and negative values with distance;
- 4 Real and imaginary contributions for a given conductivity and frequency are different from each other.
The possibilities of a horizontal‐horizontal sonde is briefly examined and found to be inferior to the usual sonde because of its higher sensitivity to the mud column and the invaded zone.
-
-
-
DETECTION OF SOLUTION CHANNELS IN LIMESTONE BY ELECTRICAL RESISTIVITY METHOD*
Authors N. P. DUTTA, R. N. BOSE and B. C. SAIKIAAbstractThe results of geophysical surveys carried out in the Kopili Hydel Project site in the United North Cachar and Mikir Hills district, Assam, to locate and trace underground caverns in limestone are presented. Electrical resistivity observations over known limestone caverns showed significant anomalies (high resistivity). In the course of the systematic electrical resistivity survey conducted at the eastern fringe of the Umrong basin to study the competency of the proposed reservoir, a number of indications of possible caverns in limestone were obtained and a few of them have been verified by drilling. An analysis of the electrical resistivity data also suggests that the deep seated caverns are aligned along an axis trending NNW‐SSE, which may represent a zone of weakness– presumably a fault–that has influenced the formation of the caverns in preferred direction. Drilling over a few of the geophysical indications have corroborated the presence of a fault along the eastern fringe of the Umrong basin.
-
-
-
NUMERICAL INTERPRETATION OF RESISTIVITY SOUNDINGS OVER HORIZONTAL BEDS*
More LessAbstractThe digital computer technique described for interpreting resistivity soundings over a horizontally stratified earth requires two steps. First, the kernel function is evaluated numerically from the inverse Hankel transform of the observed apparent resistivity curve. Special attention is given to the inversion of resistivity data recorded over a section with a resistant basement. The second step consists in the least‐squares estimation of layer resistivities and thicknesses from the kernel function. For the case of S or T‐equivalent beds only one layer‐parameter can be obtained, either the longitudinal conductance, or the transverse resistance respectively.
Two examples given in the paper show that a wide tolerance is permitted for Choosing the starting values of the layering parameters in the successive approximation procedure. Another important feature for practical applications is good convergence of the iterations. The method is probably best suited for interpreting profiles of electrical soundings with the purpose of mapping approximately horizontal interfaces at depth.
-
-
-
METHODE DE FILTRAGE DU FANTOME: APPLICATION A DES CAS REELS*
Authors J. P. FAIL and P. C. LAYOTTEAbstractAn optical process is used first to study the effects of the ghost on seismic records. Two different techniques are described in the present paper to reject the ghost energy. The first process uses a vertical stacking of delayed charges sequentially fired. The second method is achieved by means of addition of two records from different shot‐depths after filtering. These two processing methods are applied to field examples.
-
-
-
BOOK REVIEWS
Book reviewed in this article:
J. R. Tarrant, Computers in Geography, GeoAbstracts, School of Environmental Sciences
F. W. Dumming: Geophysical Exploration
-
Volumes & issues
-
Volume 72 (2023 - 2024)
-
Volume 71 (2022 - 2023)
-
Volume 70 (2021 - 2022)
-
Volume 69 (2021)
-
Volume 68 (2020)
-
Volume 67 (2019)
-
Volume 66 (2018)
-
Volume 65 (2017)
-
Volume 64 (2015 - 2016)
-
Volume 63 (2015)
-
Volume 62 (2014)
-
Volume 61 (2013)
-
Volume 60 (2012)
-
Volume 59 (2011)
-
Volume 58 (2010)
-
Volume 57 (2009)
-
Volume 56 (2008)
-
Volume 55 (2007)
-
Volume 54 (2006)
-
Volume 53 (2005)
-
Volume 52 (2004)
-
Volume 51 (2003)
-
Volume 50 (2002)
-
Volume 49 (2001)
-
Volume 48 (2000)
-
Volume 47 (1999)
-
Volume 46 (1998)
-
Volume 45 (1997)
-
Volume 44 (1996)
-
Volume 43 (1995)
-
Volume 42 (1994)
-
Volume 41 (1993)
-
Volume 40 (1992)
-
Volume 39 (1991)
-
Volume 38 (1990)
-
Volume 37 (1989)
-
Volume 36 (1988)
-
Volume 35 (1987)
-
Volume 34 (1986)
-
Volume 33 (1985)
-
Volume 32 (1984)
-
Volume 31 (1983)
-
Volume 30 (1982)
-
Volume 29 (1981)
-
Volume 28 (1980)
-
Volume 27 (1979)
-
Volume 26 (1978)
-
Volume 25 (1977)
-
Volume 24 (1976)
-
Volume 23 (1975)
-
Volume 22 (1974)
-
Volume 21 (1973)
-
Volume 20 (1972)
-
Volume 19 (1971)
-
Volume 18 (1970)
-
Volume 17 (1969)
-
Volume 16 (1968)
-
Volume 15 (1967)
-
Volume 14 (1966)
-
Volume 13 (1965)
-
Volume 12 (1964)
-
Volume 11 (1963)
-
Volume 10 (1962)
-
Volume 9 (1961)
-
Volume 8 (1960)
-
Volume 7 (1959)
-
Volume 6 (1958)
-
Volume 5 (1957)
-
Volume 4 (1956)
-
Volume 3 (1955)
-
Volume 2 (1954)
-
Volume 1 (1953)