- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 25, Issue 1, 1977
Geophysical Prospecting - Volume 25, Issue 1, 1977
Volume 25, Issue 1, 1977
-
-
A COMPARATIVE STUDY OF THE SHORTENING OPERATORS USED IN VARIOUS DATA PROCESSING TECHNIQUES IN GRAVITY INTERPRETATION*
Authors B. N. P. AGARWAL and Jagdeo SINGHAbstractDiscrete Fourier transform analysis provides an infinite number of weight coefficients for filters like upward and downward continuation. For practical applicability, the lengths of such filters have been reduced to a manageable number by various shortening operators, viz. those by Peters, Martin, Mufti, v. Hann, Hamming, and the truncation operator. A comparative study for choosing an operator which approximates the theoretical filter response best has indicated that Martin's shortening operator and the truncation operator are best, respectively, for normalized and non‐normalized sets of weight coefficients.
-
-
-
ELECTROMAGNETIC DEPTH SOUNDING EXPERIMENT*
More LessAbstractAn electromagnetic frequency sounding experiment with a rigid horizontal transmitter coil carrying a stabilized oscillating current was carried out in South Tunesia.
The field data were interpreted in terms of the mutual impedance ratio in the horizontal coils system. Where the measurements were sufficiently accurate they generally could be interpreted to a high degree of fit. It is concluded that a vehicle‐mounted electromagnetic frequency sounding system is suitable for a fast survey.
-
-
-
ELECTROMAGNETIC COUPLING*
Authors J. C. WYNN and K. L. ZONGEAbstractElectromagnetic coupling in grounded electrical prospecting systems has been studied over four decades. Recently, advanced digital electronic systems have been developed which permit both magnitude and phase measurements over four frequency decades (10−2— 102 Hz). This development has stimulated theoretical studies demonstrating behavior of EM coupling for the more commonly‐used electrical prospecting arrays a wide frequency range. A comparison is made between field and theoretical results which demonstrates the validity of the fundamental assumptions involved. Additionally electromagnetic coupling is used successfully as a deep‐sounding technique in a highly‐conductive sedimentary basin environment.
-
-
-
AN EXAMPLE OF THE APPLICATION OF THE RULES OF COMPOSITION IN RESISTIVITY INTERPRETATION*
Authors S. A. G. MOHAMMED and G. M. HABBERJAMAbstractThe application of approximate rules, whereby apparent resistivity space sections for two dimensional structures can be composited from spaces derived for elementary features is extended to a complex example drawn from a field survey over a fluorite mineral vein.
A quantitative solution for the observed resistivity space is presented and the computational sequence involved in matching the observed space is given in detail.
The interpreted results are examined in relation to the known geology, supplemented by the results of excavation, and to model tests conducted using a tank analogue.
The example also illustrates how successive compositions can be employed in estimating the form of resistivity space in a relatively complex situation.
-
-
-
ESTIMATION OF DEPTH TO CONDUCTORS BY THE USE OF ELECTROMAGNETIC TRANSIENTS*
By T. LEEAbstractThe transient response of a layered structure to plane wave excitation can be considered to be composed of a series of waves and a ground wave. For the case of a half‐space of conductivity σ and permeability μ the maximum in the electric field is found at a depth z and time t when t=z2σμ/2.
This formula can be used to estimate the depth to a buried horizontal conductor with an accuracy that depends upon the resistive contrast at the conductor's surface.
The above ray type of solution can be converted to a solution composed of a number of modes by the use of a Poisson transform and the transformed solutions yield decay constants that are consistent with the previously reported results.
In the case of a finite source, the maximum in the electric field is strongly directed. The direction depends upon the geometry of the source and the air‐earth interface. Although the maximum varies with direction it can be shown that in some directions similar laws to that above are valid.
The depth to a conductor can be estimated from the early part of the transients when the ground wave is removed. The removal of the ground wave from the transient is facilitated by the use of an apparent conductivity formula.
Although these results were obtained under restrictive conditions they do provide some insight into the electrical transients that are encountered by studying more complex models.
-
-
-
INDUCED POLARIZATION RESPONSE OF A HORIZONTALLY MULTILAYERED EARTH WITH NO RESISTIVITY CONTRAST*
Authors C. L. ELLIOT and E. LAURITSENAbstractThe induced polarization response of a horizontally multilayered earth with no resistivity contrast can rapidly be calculated on a desk calculator or minicomputer for any electrode array. The formulation is a simple series summation of the products of weighting coefficients and the true induced polarization responses for each of the layers. The coefficients are directly derivable from the corresponding resistivity model. This series approach to IP formulation was originally described by Seigel but has not been treated extensively in the present‐day geophysical literature. This method can be applied to either time or frequency domain induced polarization measurements. Once the coefficients are known, apparent induced polarization response can readily be obtained by judicious substitution of known, suspected, or assumed values of the true induced polarization of each layer.
Basic formulation is presented for the IP potential coefficients (pole‐pole or two array) with no resistivity contrast between the layers. From these coefficients, response of any number of layers for any electrode array can be obtained by suitable differentiation. Some examples of Wenner array for a three‐layered earth and dipole‐dipole array for a four‐layered earth are used to illustrate the application.
The results of this technique are valid for many natural situations of modest resistivity contrast. However, they definitely cannot be used if there are highly contrasting resistivity layers present. Such an approach is conceptually simple and is useful for survey planning, checking or setting the “depth‐of‐penetration”of a given array. For field induced polarization data that fits reasonably well to the no‐resistivity‐contrast model, this simple approach facilitates quantitative interpretation.
-
-
-
METHODS FOR CONTOURING IRREGULARLY SPACED DATA*
Authors G. BOLONDI, F. ROCCA and S. ZANOLETTIAbstractThe sampling theorem in two dimensions univocally defines a surface, provided that its values are known at points disposed on a regular lattice. If the data are irregularly spaced, the usual procedure is first to interpolate the surface on a regular grid and then to contour the interpolated data: however, the resulting surface will not necessarily assume the prescribed values on the irregular grid.
One way to obtain this result is to introduce a transformation of the coordinates such that all the original data points are transferred into part of the nodes of a regular grid. The surface is then interpolated in the points correspondent to the other crosspoints of the regular grid; the contour lines are determined in the transformed plane and then, using the inverse coordinate transformation, are transferred back to the original plane where they will certainly be congruent with the original data points.
Nonetheless, the resulting surface is very sensitive to the interpolation method used: two algorithms for that are analyzed. The first (harmonization) corresponds to the determination of the potential of an electrical field whose contour conditions are those defined by the data points. The second method consists in two dimensional statistical estimation (krigeing); in particular, the effects of different choices for the data auto‐covariance function are discussed.
The solutions are compared and some practical results are shown.
-
-
-
ON TRACING SEISMIC RAYS WITH SPECIFIED END POINTS IN LAYERS OF CONSTANT VELOCITY AND PLANE INTERFACES*
By R. CHANDERAbstractA polygonal ray path connects the seismic source and detector positions when the intervening medium consists solely of constant velocity layers with plane interfaces which may have arbitrary orientation. The coordinates of the ray vertices satisfy a system of coupled equations resulting from the requirement that Fermat's principle be satisfied along the ray path. Solving the system of equations is equivalent to tracing the ray numerically. A notable feature of this approach is that a ray which is critically refracted over a segment of its path requires no special handling.
-
-
-
ON THE MIGRATION OF REFLECTION TIME CONTOUR MAPS*
By A. H. KLEYNAbstractAfter the sampling of a reflection time contour map, i.e. after times and time gradients at the grid points of a square sampling grid have been determined, its conversion into true depth contours can be performed by normal incidence ray tracing.
At each grid point the spatial orientation of the ray is uniquely defined by a corresponding time gradient vector, whereas its continuation into the subsurface is controlled by Snell's law. For arbitrarily orientated velocity interfaces the 3 – D ray tracing problem can systematically be solved with the aid of vector algebra, by expressing Snell's law as an equation of vector cross products. This allows to set up a computer algorithm for migration of contour maps.
Reliable sampling of reflection time contour maps in the presence of faults is essential for the realization of a practical map migration system. A possible solution of the relevant sampling problem requires a special map editing and digitization procedure.
Lateral migration shifts cause a translation and distortion of the original sampling grid. On the transformed grid the true positions of faults can be related to their apparent ones on the reflection time contour map.
Errors in the time domain correlations or an incorrect velocity distribution or a combination of both these effects may cause migration failures due to total reflection and time deficiencies, or give rise to an anomalous distortion of grid cells, the latter signifying a violation of the maximum convexity condition.
Emphasis is placed upon the significance of map migration as an interpretive tool for solving time to depth conversion problems in the presence of severely faulted or salt intruded overburdens.
-
-
-
ATTENUATION DES MULTIPLES EN SISMIQUE MARINE PAR ANTIMOYENNE*
By A. LAMERAbstractA satisfactory attenuation of the multiples in marine seismic may be obtained by the application of the principle of “Antiaveraging”.
This principle in a first step consists in getting the model of the organized noise, which one tries to eliminate by using an averaging method, and in a second step to subtract that model from the initial information.
Obviously the elimination of the model should not simultaneously cause the elimination of useful signals.
The model may be obtained if the considered organized noise keeps a constant shape or if its time‐space deformation is known. Besides one has to assume the time‐distance curve of the organized noise can be determined. Thus noise arrivals may be detected on the records.
The “antiaveraging” is very often efficient when organized noises are stronger than signals or when a signal, once identified, exploited and then considered as an organized noise, can be attenuated in order to make the detection of the other signals easier.
-
-
-
THE PERFORMANCE OF OPTIMUM STACKING FILTERS IN SUPPRESSING UNCORRELATED NOISE*
By R. E. WHITEAbstractOptimum stacking filters based on estimates of trace signal‐to‐uncorrelated noise ratios are assessed and compared in performance with conventional straight stacking. It is shown that for the trace durations and signal bandwidths normally encountered in seismic reflection data the errors in estimating signal/noise ratios largely counteract the theoretical advantages of the optimum filter. The more specific the filter (e.g. the more frequency components included in its design) the more this is true. Even for a simple weighted stack independent of frequency, the performance is likely to be better than a straight (equal weights) stack only for relatively high signal/noise ratios, when the performance is not critical anyway.
-
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)