bluuebunny/crossref_metadata_2025_split huggingface dataset:
A COMPARISON OF TRANSIENT VOLTAGE DECAY CURVES OBTAINED WITH DIFFERENT ELECTRODE ARRAYS AND CONFIGURATIONS OVER A MINERALIZED ZONE
Authors: PHILLIPS W. J. , RICHARDS W. E. 
Published: April 2006

AbstractThe Huntec Mark 3 time domain equipment provides information from which the nature of the transient voltage decay curves may be determined. A zone of disseminated mineralization extending above a normal fault zone was selected for a comparative study of decay curves.In a preliminary study in which steel stakes were used as current electrodes it was observed that at some stations the decay curves obtained with reciprocal dipole‐dipole configurations were significantly different and that many decay curves initially increased from low values indicating the presence of a negative component.It was assumed that the decay curves were composite and were composed of the superposition of transient voltage components due to electromagnetic and polarization effects which could be described in terms of simple exponential functions. The initial negative component was thought to be an electromagnetic effect, but when the traverses were repeated using porous pots as current electrodes it was found that the initial negative component was usually completely eliminated, indicating that it was due to negative polarization arising from the steel stakes.When the effect of the current electrode polarization was completely eliminated identical decay curves were obtained with the reciprocal array configurations, but the remaining decay curve‐which is due to induced ground polarization—cannot be described by a single simple exponential function. It is also demonstrated that the initial negative component due to stake polarization may also be present in decay curves obtained with gradient arrays indicating that separation of the current and potential electrodes does not avoid this effect which may significantly modify the decay curve in certain circumstances up to three seconds after switch‐off.
SEPARATION OF TWO‐DIMENSIONAL RESIDUAL GRAVITY ANOMALIES USING LATERAL DENSITY VARIATION *
Authors: SINGH J. , GARDE S. C. 
Published: April 2006

AbstractA set of parabolae is defined to approximate the observed gravity profile for specifying the lateral density variation in a two dimensional causative body. This variation separates the “biased” as well as “unbiased” residual anomaly structure. The method may be directly incorporated into existing numerical and graphical techniques of interpretation for subsequent delineation of structural configuration. The simple relations derived by inspection of the observed gravity profile permit universal application of the technique and in particular for the determination of the size and shape of sub‐shelf structures. The suitability of the method in various problems of two dimensional interpretation has been indicated through an illustration of the Godavari Basin (India).
TRAITEMENT „A LA MAIN” ET SUR ORDINATEUR DES TRANSITOIRES EN POLARISATION PROVOQUEE *
Authors: BERTIN J. , LOEB J. 
Published: April 2006

SummaryDecomposition of IP discharge curve into a sum of exponentials yields more information than present practice does. Such a work was performed, as a beginning, by hand, plotting the ordinates on semi logarithimic paper. It gave, according to the cases, two or three exponentials, each with an amplitude Ai and the time constant τi.Three examples of ground prospection are given, the third of which shows an anomaly, unnoticed with present technique but obvious on a profile curve with A1/A2 in ordinates.A computer processing technique is described which yields directly the time constants τi by seeking a differential equation which the IP voltage satisfies; the order of the equation is set arbitrarily. On a practical prospection record with only two exponentials, the assumption that the differential equation does not depend on charge duration (everything else being equal) has been checked (the shape of the curves would depend on boundary conditions). The assumption roughly holds for charge durations from 5 seconds to 40 seconds. We hope that a larger number of exponentials will yield better results.SommaireLa décomposition de la courbe de décharge en une somme d'exponentielles permet d'en tirer plus d'information que la pratique actuelle. Un tel travail a été fait, pour commencer, à la main en portant les ordonnées sur papier logarithmique; il a fourni, suivant les cas, deux ou trois exponentielles avec, pour chacune d'elles, le coefficient Ai et la constante de temps τi.On donne trois exemples de prospection sur le terrain, dont le dernier comporte une anomalie, inaperçue avec la technique actuelle, mais bien mise en évidence en traçant la courbe de A1/A2 le long du profil.On donne ensuite une technique de calcul sur ordinateur des τi, en recherchant l'équation différentielle à laquelle obéit la tension ΔVPP enregistrée, l'ordre de cette équation étant fixé arbitrairement. On a cherchéà vérifier sur un enregistrement de terrain, avec. deux exponentielles seulement, l'hypothèse suivant laquelle l'équation différentielle ne dépend pas de la durée de charge (toutes choses égales d'ailleurs). L'hypothèse se trouve grossièrement vérifiée pour les durées de charge allant de 5 secondes à 40 secondes, et nous espérons qu'un plus grand nombre d'exponentielles donnera de meilleurs résultats.
MAGNETOTELLURIGS: PRINCIPLE AND OUTLINE OF THE RECORDING TECHNIQUE A CASE HISTORY *
Authors: WAESELYNCK M. 
Published: April 2006

AbstractFirst, we review briefly the principle of the method, the computation of theoretical curves for a layered earth, and the recording technique in use in our surveys.The case history deals with an area covered with overthrust nappes (marls of Miocene age), which had slid on a Triassic sole, obscuring the geological picture.The magnetotelluric survey followed those of gravity and aeromagnetics and preceded the seismic one from North to South, it displayed a shallow and gently dipping basin, a major fault system, and a deep basin with a thick resistive layer, often underlying a conducting one.The seismics, and later the drilling of a well East of the profile, confirmed these features; in particular, the thick resistive layer was revealed to be Jurassic; only its thickness had been slightly overestimated. This fact lead the people in charge of the operations to ask for a reinterpretation synthesis of magnetotellurics, seismics and gravity, the results of which are also presented.
GEOPHYSICAL PROSPECTING FOR BARITE *
Authors: BHATTACHARYA B. B. , JAIN SHIKHAR C. , MALLICK K. 
Published: April 2006

AbstractA case history is presented for the exploration of barite with the electrical resistivity method. Altogether eight locations were recommended out of which six correspond to barite bodies. In the present case the gravity method, which is commonly used for the location of barite because of its high density contrast with the host rocks, has failed, perhaps due to small size of the ore bodies.
TRANSIENT EM RESPONSE OF A LARGE LOOP ON A LAYERED GROUND *
Authors: Lee T. , Lewis R. 
Published: April 2006

AbstractThe voltage induced in a horizontal loop on a layered ground has been calculated for the case where the loop is excited by a step current and measurements are made during the off‐cycle.The expressions derived for a uniform ground show that for large time t the induced voltage E(t) is approximately given by E(t)≃— (Ibαμ/20t) (σμ2/t)3/2 where σ is the conductivity of the ground, μ the permeability, b the loop radius, and I the amplitude of the current step. For small times the corresponding result is E(t)≃—Ibμ/2t.When the ground is composed of a number of layers a numerical procedure for calculating the induced voltage is described.The calculated responses of various multilayered structures show that at short times the induced voltage is asymptotic to that produced in the case of a uniform ground of conductivity equal to the top layer.Interference effects in the top layer can lead to anomalous decay curves which may result in the underestimation of the conductivity of a buried layer.
AN ANALYSIS OF STACKING, RMS, AVERAGE, AND INTERVAL VELOCITIES OVER A HORIZONTALLY LAYERED GROUND *
Authors: AL‐CHALABI M. 
Published: April 2006

AbstractA correct derivation of rms, average and interval velocities from one another and from common depth point stacking velocities requires a clear understanding of the relationships between these velocities. We relate the average velocity to the rms velocity through a “heterogeneity factor” which is a quantity that gives a measure of the degree of velocity heterogeneity in the ground. The interval velocity is a quantity which varies according to the method of its derivation. The difference between rms and stacking velocities depends on the heterogeneity factor and on the length of the spread. Unless allowed for, this difference can reverse the advantages of long spreads and cause large errors in interval velocity determinations. It may be removed through a number of techniques. The accuracy of stacking velocities in the presence of random “noise” is independent of the heterogeneity factor. Relevant expressions can be broken down into simple formulae which give the accuracy quickly and with good precision.
COMPUTER PROCESSING OF AIRBORNE ELECTROMAGNETIC DATA *
Authors: PALACKY G. J. , WEST G. F. 
Published: April 2006

AbstractWith the increasing number of channels in AEM systems, computer data handling is becoming a necessity. The experience gained in processing of seismic and aeromagnetic data cannot be applied directly to low‐frequency (100‐5000 Hz) AEM methods. A novel scheme has been designed for AEM data processing and tested on 2900 km (1800 miles) of Input surveys.In the first step, the digital flight tapes are merged with digitized flight path recovery to form the primary data set. The validity of the raw data is controlled by the computer, but the interpreter has an option of checking them in perspective plots of channel amplitudes. The primary data set is reduced by processors which determine the location and type of anomalies and discard noise. Unlike the widely used deconvolution, the sequential processor determines first the anomaly location and then estimates parameters, such as peak amplitude, base width, and excess area, which are used as acceptance criteria. Interpretation parameters, such as σt, conductor depth, and dip are estimated by comparison with quantitative models. The recorded channel amplitudes are plotted together with the selected interpretation parameters in a profile form. The secondary data set which includes only the interpretation parameters for selected anomalies is graphically displayed as a schematic map of apparent σt. Elongated features are traced by a fan strike recognition routine and a trend map is automatically compiled and plotted. Disk storage is essential for second pass processing during which parts of the primary data set are searched for undiscovered anomalies matching the analyzed trend. The suggested procedure for AEM data processing is demonstrated on an Input MK V survey, Southern Indian Lake, Manitoba.
THE QUALITY OF SOME TWO‐DIMENSIONAL FILTERS IN SEPARATING REGIONAL AND LOCAL GRAVITY ANOMALIES *
Authors: APELL B. A. N. C. 
Published: April 2006

AbstractThe separation of regional from local gravity anomalies by means of the application of two‐dimensional linear filters is analyzed. It was found that optimization of the filter in the least squares sense leads to filters that produce strong localized concentrations of the error, which may erroneously be interpreted as anomalies. For this reason the maximum absolute value of the error is a more important criterion for the quality of the filter than the root mean square error. This maximum absolute error is minimized by the minimax filter. Intermediate filters are derived which give a transition zone which comes appreciably closer to that of the optimal filter at only a small price in terms of increase of the maximum absolute error.
THE INTERPRETATION OF INDUCED‐POLARIZATION SOUNDING CURVES IN THE FREQUENCY DOMAIN *
Authors: BARKER R. D. 
Published: April 2006

AbstractTwo‐layer type curves of apparent frequency effect for the Wenner configuration are presented. The formulation is based on the normal definition of frequency effect in terms of resistivities measured at different frequencies plus the definition of apparent resistivity over two horizontal layers as a function of first and second layer resistivities. The use of these type curves in the interpretation of multilayer apparent frequency‐effect curves is described and some field examples are given.
CANCELLING MULTIPLE REFLECTIONS BY FEEDBACK *
Authors: KOEHLER K. , KREY TH. , WACHHOLZ H. 
Published: April 2006

AbstractFor horizontal layering and plane horizontal impulsive wavefronts it is theoretically possible to get rid of multiple reflections by a feedback procedure which can easily be derived using raypath philosophy. To reduce the increase of noise inherent in the method the precise theoretical formulae are altered in such a manner that a practical application becomes possible. For this purpose the autocorrelation will be used.Application of the new process to a CRP section where the shot geophone distances in the field had not been long enough to attenuate multiple reflections effectively gave favourable results.
A NEW APPROXIMATE METHOD FOR DIRECTLY INTERPRETING GRAVITY ANOMALY PROFILES CAUSED BY SURFACE GEOLOGIC STRUCTURES*
Authors: FOURNIER K. P. , KRUPICKA S. F. 
Published: April 2006

AbstractFor outcropping bodies an approximate direct interpretation of the associated gravity anomaly is generally obtained with the flat plate formula. Results can be significantly improved if the causative body is approximated by a bell shape instead of a flat plate. A set of parameter curves allows the conversion to depth data. The validity of the method is borne out by synthetic models and by field examples in a Nevada valley with Tertiary and in the Los Angeles Basin. The method provides structural definition more accurate than can be obtained with the flat plate formula, particularly in the case of narrow anomalies.
A COMBINATION OF GRAVITY AND SEISMIC REFRACTION MEASUREMENTS, APPLIED TO GROUNDWATER EXPLORATIONS NEAR TALTAL, PROVINCE OF ANTOFAGASTA, CHILE*
Authors: van OVERMEEREN R. A. 
Published: April 2006

AbstractIn northern Chile, which is part of the Atacama Desert, groundwater supply and storage are controlled by deep structural phenomena. Several geophysical exploration techniques were used to determine the structure and depth of the basement of a broad valley filled with unconsolidated alluvial deposits. The gravity method was applied to obtain a general picture of the basement. This was followed by seismic refraction measurements along two traverse lines which appeared to be of major interest. Additional experiments with the magnetic and the telluric methods supplemented the geophysical investigations.In this paper the results of the different geophysical methods are presented, compared, and evaluated with respect to the usefulness in arid regions.It is concluded that a complementary use of the gravity method and the seismic refraction method is a very fruitful and efficient approach for the geophysical reconnaissance of water‐bearing basins in flat and arid zones like the Atacama Desert.
SOME DETAILED INVESTIGATIONS ON INTERFACE WAVES*
Authors: GUPTA S. K. 
Published: April 2006

AbstractIn continuation of our earlier work on interface waves (Gupta and Martel 1971) we have extended our studies by using a plexiglas‐steel combination in addition to plexiglas‐brass. Two other emitter positions were also considered: emitter on plexiglas and emitter on the metal edge, a short distance away from the interface. In both cases the presence of an interface wave could be detected from the seismogram. A curved interface between plexiglas and brass was also studied and it was found that the curvature introduces a change in the phase velocity of the interface wave in the same way as a curved surface does for the Rayleigh wave.
A NUMERICAL COMPUTATION PROCEDURE FOR THE DIRECT INTERPRETATION OF GEOELECTRICAL SOUNDINGS*
Authors: PATELLA D. 
Published: April 2006

AbstractIn this paper a numerical method of direct interpretation of geoelectrical soundings is described. It has similarities with already existing direct methods, but owing to its simplicity and, in particular, to the possibility of applying it also without digital computers, it proves useful mostly in the field, where very often an accurate method for the interpretation of multi‐layer curves is required. The direct interpretation system splits up into three steps: i) the evaluation of the resistivity transform after application of Hankel's inversion theorem; ii) the determination of the layer distribution after application of Koefoed's recurrent procedure; iii) the control of the solution. Each step is considered and the practical procedures suggested. Finally two field examples are presented and discussed.
AN INTERACTIVE COMPUTER/GRAPHIC‐DISPLAY‐TERMINAL SYSTEM FOR INTERPRETATION OF RESISTIVITY SOUNDINGS*
Authors: JOHANSEN H. K. 
Published: April 2006

AbstractA fast computer‐procedure giving the apparent resistivity curve as well as the partial derivatives with respect to the layer‐parameters is presented. It is based on the linear filter method developed by D. P. Ghosh in 1971.The sampling frequency is 10 points per decade, and 3 decades are covered. The maximum relative error is less than 10−3, and in most cases orders of magnitude smaller. The computation time on a CDC 6400 for one curve given in 30 points ranges linearly from .17s for a two‐layer case to .36s for a ten‐layer case.The procedure is used to plot master curves interactively on a graphic display terminal (Tektronix 4010) connected to the CDC 6400. By trial‐and‐error adjustments a set of layer‐parameters is found, giving essentially the measured curve.
THE DETERMINATION OF DIGITAL WIENER FILTERS FROM AN ILL‐CONDITIONED SYSTEM OF NORMAL EQUATIONS*
Authors: TREITEL S. , WANG R. J. 
Published: April 2006

AbstractThe system of normal equations associated with the discrete Wiener filter is sometimes ill‐conditioned. The purpose of this paper is to show that in such cases the solutions obtained vary drastically with the particular choice of an algorithm and of the computer used for its implementation.A review of the basic mathematical theory behind an ill‐conditioned matrix is first presented. Numerical examples are then given to illustrate that the solutions of the normal equations are sensitive to the word length of a given computer. Finally, two possible remedies are described: (1) The well‐known method of prewhitening and (2) the use of the conjugate‐gradient algorithm for solving the normal equations.
ELECTROMAGNETIC DEPTH SOUNDING EXPERIMENT*
Authors: BIEWINGA D. T. 
Published: April 2006

AbstractAn 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: WYNN J. C. , ZONGE K. L. 
Published: April 2006

AbstractElectromagnetic 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.
METHODS FOR CONTOURING IRREGULARLY SPACED DATA*
Authors: BOLONDI G. , ROCCA F. , ZANOLETTI S. 
Published: April 2006

AbstractThe 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.
THE PERFORMANCE OF OPTIMUM STACKING FILTERS IN SUPPRESSING UNCORRELATED NOISE*
Authors: WHITE R. E. 
Published: April 2006

AbstractOptimum 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.
ADAPTIVE PREDICTIVE DECONVOLUTION OF SEISMIC DATA*
Authors: WANG R. J. 
Published: April 2006

AbstractThe Wiener prediction filter has been an effective tool for accomplishing dereverberation when the input data are stationary. For non‐stationary data, however, the performance of the Wiener filter is often unsatisfactory. This is not surprising since it is derived under the stationarity assumption.Dereverberation of nonstationary seismic data is here accomplished with a difference equation model having time‐varying coefficients. These time‐varying coefficients are in turn expanded in terms of orthogonal functions. The kernels of these orthogonal functions are then determined according to the adaptive algorithm of Nagumo and Noda.It is demonstrated that the present adaptive predictive deconvolution method, which combines the time‐varying difference equation model with the adaptive method of Nagumo and Noda, is a powerful tool for removing both the long‐ and short‐period reverberations. Several examples using both synthetic and field data illustrate the application of adaptive predictive deconvolution. The results of applying the Wiener prediction filter and the adaptive predictive deconvolution on nonstationary data indicate that the adaptive method is much more effective in removing multiples. Furthermore, the criteria for selecting various input parameters are discussed. It has been found that the output trace from the adaptive predictive deconvolution is rather sensitive to some input parameters, and that the prediction distance is by far the most influential parameter.