bluuebunny/crossref_metadata_2025_split huggingface dataset:
THE RADIATION OF ACOUSTIC WAVES FROM AN AIR‐GUN*
Authors: SAFAR M. H. 
Published: April 2006

AbstractA theoretical model is developed for predicting three important parameters of the pressure pulse radiated by an air‐gun, namely the rise time, the amplitude of the initial pulse, and the period of the bubble pulse. A knowledge of these three parameters is essential for the efficient design of air‐guns arrays.The prediction of the amplitude of the initial pulse is based on the assumption that the initial pulse is radiated by a spherical source with surface area equal to that of the air‐gun ports and not by a spherical source with initial volume equal to that of the air‐gun chamber, as has been assumed previously.A simple equation is obtained for predicting the period of the bubble pulsation, taking into account the effect of the air‐gun body, boundaries such as the sea‐surface and seabed and the presence of a number of identical air‐guns placed at the same depth and fired simultaneously.
A COMPARATIVE STUDY OF THE SHORTENING OPERATORS USED IN VARIOUS DATA PROCESSING TECHNIQUES IN GRAVITY INTERPRETATION*
Authors: AGARWAL B. N. P. , SINGH Jagdeo 
Published: April 2006

AbstractDiscrete 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.
ESTIMATION OF DEPTH TO CONDUCTORS BY THE USE OF ELECTROMAGNETIC TRANSIENTS*
Authors: LEE T. 
Published: April 2006

AbstractThe 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: ELLIOT C. L. , LAURITSEN E. 
Published: April 2006

AbstractThe 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.
ON THE MIGRATION OF REFLECTION TIME CONTOUR MAPS*
Authors: KLEYN A. H. 
Published: April 2006

AbstractAfter 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.
SHORT RANGE DISTANCE MEASUREMENT BY ELECTRO‐MAGNETIC PHASE COMPARISON TECHNIQUE*
Authors: GREENE J. R. 
Published: April 2006

AbstractThe contribution of electromagnetic phase comparison techniques to distance measuring is reviewed in an historical context. Special emphasis is accorded the new class of near infra‐red electro‐optic instruments suited to the measurement of distances under 2 km with an accuracy of a few millimetres. Principles of operation, sources of error, and practical applications are discussed.
QUELQUES CONSIDÉRATIONS SUR LA DIGITALISATION DES SIGNAUX MAGNÉTOTELLURIQUES — DESCRIPTION D'UN ÉQUIPEMENT*
Authors: ILICETO V. 
Published: April 2006

AbstractA magnetotelluric digital equipment has been designed and built for operation in geophysical and applied geophysical research. The five components of the natural electromagnetic field can be digitized in the frequency range 1/1000 Hz (magnetic sensor's response limits) to some tens Hz.The magnetotelluric (MT) data are directly digitized in the field and stored in a cassette tape recorder.
HORIZONTALLY STRATIFIED MEDIUM AS A REFERENCE LARGE SCALE SYSTEM*
Authors: SZARANIEC E. 
Published: April 2006

AbstractThe geophysical system of horizontal layers may be considered as a large scale system. A large class of systems is identified which are equivalent to the geophysical processes in horizontal layering. The property of hierarchical aggregation of layers forms a convenient framework within which the peculiarities of such systems can be studied. Geophysical processes in multilayered media may serve as a reference physical model for a specified class of large scale systems, in order to take advantage and to apply geophysical methods of data processing, system analysis, and parameter identification.Specified large scale systems are decomposable, like geophysical systems, by use of recursive schemes. A category particulary susceptible to such a decomposition is any polynomial which is stable inside the unit circle. Sophisticated geophysical concepts, such as anisotropy, equivalence, and disappearance, can be used to approximate or to simplify the system by vanishing or adjusting the decomposition coefficients.
EFFECT OF VERTICAL CONTACT ON WENNER RESISTIVITY SOUNDINGS*
Authors: KUMAR RAKESH , CHOWDARY M. V. RAMANAIAH 
Published: April 2006

AbstractThe allowance for the influence of a vertical contact is evaluated on Wenner resistivity sounding curves, which are graphically constructed on bilogarithmic paper over simple composite earth models consisting of a vertical contact separating two‐ or three layered earth on one side and a homogeneous medium on the other side. The error incurred in the graphical constructions is explored.Finally, the use of these graphically constructed sounding curves is shown in the interpretation of two Wenner field soundings measured in a complex geologic area.
THE CONCEPT OF APPARENT RESISTIVITY IN LATEROLOG 7*
Authors: ROY A. 
Published: April 2006

AbstractIt appears that Doll (1951) and N. N. (1958, 1969, 1972) on the one hand, and Roy (1975) and Roy and Apparao (1976) on the other, used different formulas for computing the apparent resistivity for the Laterolog 7 sonde. This would partly explain the contra‐dictory nature of LL7 results from these two groups of workers.The first group use a formula that relates the measured potential to a system of currents that are largely fictitious and non‐existent in the ground at the time of measurement. The second group, on the other hand, employ a formula that combines the observed signal with the currents that actually exist in the ground and produce that signal. We believe that the second procedure is the right one.
AN ULTRASONIC PROFILING INVESTIGATION ON SOME FRESH AND WEATHERED GRANITES OF HYDERABAD, INDIA*
Authors: RAGHAVA M. S. VIJAYA , JAWAHAR G. , SHERBAKOVA T. V. 
Published: April 2006

AbstractThe ultrasonic profiling method of measuring the compressional and shear wave velocities in cylindrical rock samples is extended to measurements in some weathered and fresh granite blocks collected from the Hyderabad (India) region. This possibility of the method provides a means of investigating the elastic properties of the less compact rocks, of which the near‐surface formations are particularly important.In this article the important parts of the ultrasonic profiling instrument developed are described and the relevant aspects of the seismic wave fields and identification of the individual waves in the wavetrain responses to longitudinal excitation are considered. Compressional, shear and surface (Rayleigh) wave velocities in some fresh and weathered granites are detailed. The compressional velocities range from 4.8 km/s to 5.5 km/s in fresh granites and lie between 1.1 km/s and 2.5 km/s in weathered granites. Young's modulus and Poisson's ratios computed from the measured velocities are also presented. An empirical relation of the form log E= 4.27 + 2.11 log Vp between Young's modulus E and compressional velocities Vp in the fresh granites studied is deduced. The versatility of the approach is thus demonstrated.
TRANSFORMATION OF DIPOLE TO SCHLUMBERGER SOUNDING CURVES BY MEANS OF DIGITAL LINEAR FILTERS*
Authors: KUMAR R. , DAS U. C. 
Published: April 2006

AbstractLinear relationship between dipole and Schlumberger sounding resistivities leads to the use of digital filters to transform the former to the latter. This transformation is of importance from the viewpoint that Schlumberger interpretational techniques and know‐how could then be applied to the pseudo‐Schlumberger field curve. Filters for this transformation are presented for the radial, perpendicular, and parallel (30°) dipole method. The characteristics of these filters are similar to the ones for transforming dipole data to the kernel and are favourable in that they do not amplify noise. A sampling interval of (In 10) /6 has been used in determining the filter yielding good accuracy. Like previous filters the present one is handy and fast in operation.
ENCODING TECHNIQUES FOR THE VIBROSEIS * SYSTEM **
Authors: BERNHARDT T. , PEACOCK J. H. 
Published: April 2006

AbstractThe use of certain codes for the Vibroseis system of exploration has been investigated. The codes considered are well‐known in communication theory. The effect of coding the Vibroseis input signal is the suppression of correlation noise and therefore an improvement of the signal to noise ratio.It is believed that, during the course of research, a new property of one of the codes used has been discovered.
CARTOGRAPHIE DES “PSEUDO‐RESISTIVITES APPARENTES’ PAR PROFILAGE TELLURIQUE‐TELLURIQUE ASSOCIE A LA MAGNETO‐TELLURIQUE *
Authors: VAN NGOC PHAM , BOYER D. , CHOUTEAU ET M. 
Published: April 2006

AbstractThe magneto‐telluric and telluric‐telluric profiling techniques are presented, and then a new method which combines these two techniques for mapping a parameter defined as the “apparent pseudo‐resistivity”.The results obtained for the uranium structures in the Cluff Lake district (Saskatchewan Canada) indicate the use of these different techniques in the detection of zones of conduction for surface zones and for deeper zones, and demonstrate the advantages of these methods over the usual electrical methods in problems of structural research.
SELF‐MATCHING DECONVOLUTION IN THE FREQUENCY DOMAIN *
Authors: DEREGOWSKI S. M. 
Published: April 2006

AbstractIn a previous paper the author showed how, by computing an inverse filter in the frequency domain, an automatic compromise could be made between the conflicting requirements to spike a wavelet and to keep the attendant noise amplification within bounds. This paper extends the technique to take account of errors in the estimated shape of the wavelet defined to the deconvolution process. The drastic effects which such errors can have if they are ignored are demonstrated. A novel form of filter–called the “self‐matching filter”–is defined which allows the user to limit not only the noise amplification but also the sensitivity of the filter to random uncertainties in the estimated wavelet. This is achieved by whitening the spectrum only within automatically selected pass bands whilst suppressing other noise‐dominated or uncertainly defined frequency components. Conventional Wiener filtering is shown to be a special case of this more general filter, namely one in which the wavelet uncertainty is completely ignored. The type of phase spectrum which the output pulse should be designed to possess (e.g. zero phase or minimum phase) is briefly discussed.
TRANSIENT ELECTROMAGNETIC FIELD OF A POLARIZABLE HALF‐SPACE DUE TO VARIOUS CURRENT PULSES *
Authors: RATHOR B. S. 
Published: April 2006

AbstractIn this paper, the expressions for the induced voltage function V(T) and mutual impedance function Zm(T) have been derived for specific range of time T and specific values of induced polarization parameters a for ramp and saw‐tooth type of current pulses. The computed results for various cases are also presented. The low values of induced polarization parameter a represent the medium possessing membrane polarization, whereas high values exhibit electrode polarization medium. The method has practical applicability and is best suited for the interpretation of transient electromagnetic fields over a polarizable half‐space.
TRANSFORMATION OF SCHLUMBERGER APPARENT RESISTIVITY TO DIPOLE APPARENT RESISTIVITY OVER LAYERED EARTH BY THE APPLICATION OF DIGITAL LINEAR FILTERS *
Authors: KUMAR R. , DAS U. C. 
Published: April 2006

AbstractA simple extension of our previous work in which digital filters were developed to transform dipole resistivity measurements over layered earth to Schlumberger ones leads us to the development of filters for transforming the latter to the former. As in the previous work we use a sampling interval of 1/6 In 10 in designing the filters that are both accurate and fast in operation.
A THEOREM FOR DIRECT CURRENT REGIMES AND SOME OF ITS CONSEQUENCES *
Authors: ROY A. 
Published: April 2006

AbstractFor any direct current regime, the theorem
image
holds, where φp is the total measured or calculated potential at any point P, φ is the potential distribution known a priori, r is the distance between P and any volume element dV, the gradients are evaluated at the element, and the current sources and sinks have finite dimensions. Thus, each space element behaves as a dipole of moment (1/4π) ∇φdV and contributes its share of signal or potential accordingly. By suitable summation or integration, the contribution from any assigned portion of space to the total measured signal can be determined. Except for the chargeability factor m, the formula also establishes Seigel's initial postulate for the time domain induced polarisation theory.The contribution depends on the potential gradient, not the current density, and the integration extends over the entire space. Although an insulating target carries no current, it contributes a signal that is in general larger than normal by virtue of its higher potential gradient, and thus helps in creating an overall positive anomaly or resistivity high. On the other hand, an infinitely conducting target—even though it supports a larger amount of current than normal—contributes nothing to the measured signal as the potential gradient is zero everywhere inside. Thus, by contributing less than normal, a conducting target promotes the creation of what is usually a resistivity low. In all cases, the contributions from the space elements add up exactly to the measured or total calculated value.Some other consequences of the theorem are also discussed, especially in relation to a simple two‐layer earth. For instance, the contribution from the upper half‐space (air) turns out to be equal to that from the lower (real ground), for all observation points on the ground surface and for any ground configuration.
INDUCED POLARIZATION RESPONSE OF A POLARIZABLE INHOMOGENEOUS SPHERE *
Authors: RATHOR B. S. 
Published: April 2006

AbstractExpressions for the I. P. activity coefficient for an inhomogeneous sphere with the conductivity increasing/decreasing from the core towards the periphery according to a power law have been derived.In certain geometrical situations of the electrode configurations a negative I. P. signal is obtained. The negative I. P. may be understood as due to modifications in the discharge current from the polarized medium during the discharge process.The results of the investigation may be useful for more accurate interpretation of I. P. anomalies due to isometric inhomogeneous bodies.
MAGNETO‐TELLURIC EXPERIMENTS IN NORTHERN FINLAND *
Authors: BENDERITTER Y. , HÉRISSON C. , KORHONEN H. , PERNU T. 
Published: April 2006

AbstractMagneto‐telluric experiments were performed in August 1974 in the Muhos Jotnian sedimentary formation and the Koillismaa Precambrian basement complex by the Centre de Recherches Géophysiques, Garchy, and the Department of Geophysics, University of Oulu, employing the “Résistivimètre magnéto‐tellurique”, an instrument which permits apparent resistivity values to be observed at eight pre‐selected frequencies between 8–1700 Hz.Preliminary soundings were made on the Muhos formation and on its surroundings, after which a magneto‐telluric profile was measured involving eleven sites on a line 13 km long extending across the formation. Some further soundings were made at certain sites of known stratigraphy. The resistivity values in the sedimentary formation were found to vary between 10 ωm and 200 ωm, while those in the basement were markedly higher. This resistivity contrast suggests that the method seems to be suitable for delimiting the sedimentary formation.Soundings made on the basement at Koillismaa show apparent resistivity values in the range 1000–20000 ωm. The interpretation of these soundings was in agreement with other, independently obtained geophysical results.
ON GETTING REFLECTION COEFFICIENTS FROM WAVES *
Authors: HUBRAL P. 
Published: April 2006

AbstractA horizontally layered non‐absorptive system of homogeneous layers may be specified by giving the reflection coefficients at each interface. Provided the layers have equal vertical travel time and a perfect reflector as a free surface, the reflection coefficients are generally reconstructed from the reflected pulses by way of solving simultaneous equations of the Toeplitz matrix form with the Levinson recursion method. There exists an alternative approach to solving this problem which by simple reasoning immediately turns out the (Levinson) recursion scheme. The method is based on formulas that relate to solving the forward problem. It resembles Kunetz's (1962) original inverse solution in as much as the computation of the reflection coefficients is based on the idea of separating the contribution of a primary from the sum of all multiples.
FINITE DIFFERENCE AND WAVE NUMBER MIGRATION *
Authors: HOOD P. 
Published: April 2006

AbstractFinite difference migration has been developed and popularized by J. F. Claerbout of Stanford University and is now widely used in seismic processing. For most sections finite difference migration gives results comparable to those obtained by conventional Kirchhoff migration and, where events are not dipping too much, a cleaner appearance is often apparent. However, there are two practical limitations to the method, and these occur in regions of very steep dip and where there is a large variation of the velocity in the lateral direction.It is possible to develop successively more accurate equations to deal with the steep dip problem, but above third order these schemes become prohibitively expensive to implement. The finite difference method itself introduces errors and so imposes further limitations on the angle of dip. For the effective treatment of steeply dipping beds there appears to be no method available in the time domain which does not suffer from dispersion inaccuracies. However, by developing wavenumber migration, an exact one‐way wave equation can be used, and this eliminates any error except that caused by finite sampling.The other difficulty with wave migration is the correct migration in regions with lateral velocity variation. A number of approaches are possible of which three are discussed here. The first uses an exact theory, the second is based on the deviation from a depth stratified model, and the third uses a transformation to a depth co‐ordinate system. All methods are discussed with their advantages and limitations. Finally, some examples are shown of wave migration applied to synthetic and real data.
A SPATIAL ANALYSIS OF UPWARD CONTINUATION OF POTENTIAL FIELD DATA *
Authors: TSAY L. J. 
Published: April 2006

AbstractA spatial analysis of both continuous and discrete operators for upward continuation help us realize the problems and limitations which have been encountered before (Henderson 1960, Kontis 1971) but remained unsolved in practical application of upward continuation computation due to the finite length of data and operator in spatial domain. Various numerical examples show that an improvement of accuracy of continuation computations can be achieved through proper sampling and sufficient length of data.