bluuebunny/crossref_metadata_2025 huggingface dataset:
COMPARATIVE FIELD PERFORMANCE OF ELECTRODE ARRAYS IN TIME‐DOMAIN INDUCED POLARIZATION PROFILING *
Authors: ROY A. , JAIN SHIKHAR C.
Published: April 2006

Abstract

Five test profiles in time‐domain induced polarization measurements over conducting sulphide and graphite mineralisations were run with the two‐electrode, three‐electrode, modified unipole, pole‐dipole, dipole‐dipole, Schlumberger, and Wenner α and β configurations. The results show that, compared to the other electrode systems, the simplest two‐electrode array produces the largest anomalies with the smallest of spacings.
INLINE AND BROADSIDE EM DIPOLE DIPOLE PROFILING OVER A THIN VERTICAL INFINITELY CONDUCTING VEIN *
Authors: JAIN SHEEL C.
Published: April 2006

Abstract

Master diagrams for electromagnetic responses of nine dipole dipole systems are presented for various depths of a vertical infinitely conducting vein. The YY system gives the minimum anomaly for both the inline and broadside arrays. Among other inline systems, it is difficult to decide the clearcut superiority of one system over the other, whereas the XX system in broadside array gives maximum anomaly.
THE ESTIMATION OF SIGNAL SPECTRA AND RELATED QUANTITIES BY MEANS OF THE MULTIPLE COHERENCE FUNCTION *
Authors: WHITE R. E.
Published: April 2006

Abstract

A seismic trace recorded with suitable gain control can be treated as a stationary time series. Each trace, χj(t), from a set of traces, can be broken down into two stationary components: a signal sequence, αj(t) *s(t—τj), which correlates from trace to trace, and an incoherent noise sequence, nj(t), which does not correlate from trace to trace. The model for a seismic trace used in this paper is thus χj(t) =αj(t) * s(t—τj) +nj(t) where the signal wavelet αj(t), the lag (moveout) of the signal τj, and the noise sequence nj(t) can vary in any manner from trace to trace. Given this model, a method for estimating the power spectra of the signal and incoherent noise components on each trace is presented.

The method requires the calculation of the multiple coherence function γj(f) of each trace. γj(f) is the fraction of the power on traced at frequency f that can be predicted in a least‐square error sense from all other traces. It is related to the signal‐to‐noise power ratio ρj(f) by

image

where Kj(f) can be computed and is in general close to 1.0. The theory leading to this relation is given in an Appendix.

Particular attention is paid to the statistical distributions of all estimated quantities. The statistical behaviour of cross‐spectral and coherence estimates is complicated by the presence of bias as well as random deviations. Straightforward methods for removing this bias and setting up confidence limits, based on the principle of maximum likelihood and the Goodman distribution for the sample multiple coherence, are described.

Actual field records differ from the assumed model mainly in having more than one correctable component, components other than the required sequence of reflections being lumped together as correlated noise. When more than one correlatable component is present, the estimate for the signal power spectrum obtained by the multiple coherence method is approximately the sum of the power spectra of the correlatable components. A further practical drawback to estimating spectra from seismic data is the limited number of degrees of freedom available. Usually at least one second of stationary data on each trace is needed to estimate the signal spectrum with an accuracy of about 10%. Examples using synthetic data are presented to illustrate the method.
DÉCONVOLUTION RAPIDE *
Authors: BOIS P.
Published: April 2006

Abstract

During the last ten years data processing has enabled the geophysicist to seriously attack the problem of how to improve the quality of seismic sections. For instance new processing methods have been proposed for improving the resolving power of the seismic pulse. These methods are based on deconvolution and they require lengthy computations.

The method discussed here for pulse contraction is based on the use of Hadamard transforms. It is very fast but nevertheless the quality of the result stands comparison with that of conventional methods.
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

Abstract

The 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

Abstract

A 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

Summary

Decomposition 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.

Sommaire

La 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

Abstract

First, 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

Abstract

A 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

Abstract

The 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

Abstract

A 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

Abstract

With 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

Abstract

The 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

Abstract

Two‐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

Abstract

For 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

Abstract

For 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

Abstract

In 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

Abstract

In 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

Abstract

In 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

Abstract

A 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

Abstract

The 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

Abstract

An 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

Abstract

Electromagnetic 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

Abstract

The 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

Abstract

Optimum 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

Abstract

The 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.
AN INTERACTION MODEL FOR ELASTIC WAVE PROPAGATION IN TWO‐PHASE MEDIA*
Authors: KUSTER G. T.
Published: April 2006

Abstract

The effect of interactions among spherical inclusions on the effective propagation of long wavelength elastic waves in a two‐phase medium is investigated theoretically. Interactions are taken into account in an average sense via a generalized self‐consistent scheme. Bounds on the effective elastic moduli are obtained by overestimating and neglecting contributions of neighbouring inclusions. An intermediate model is proposed, which introduces the effect of interactions into the effective properties without requiring more information about the geometry of the medium than the relative volumes of the constituents.
TIME MIGRATION—SOME RAY THEORETICAL ASPECTS*
Authors: HUBRAL P.
Published: April 2006

Abstract

Using an elementary theory of migration one can consider a reflecting horizon as a continuum of scattering centres for seismic waves. Reflections arising at interfaces can thus be looked upon as the sum of energy scattered by interface points. The energy from one point is distributed among signals upon its reflection time surface. This surface is usually well approximated by a hyperboloid in the vicinity of its apex. Migration aims at focusing the scattered energy of each depth point into an image point upon the reflection time surface. To ensure a complete migration the image must be vertical above the depth point. This is difficult to achieve for subsurface interfaces which fall below laterally in‐homogeneous velocity media. Migration is hence frequently performed for these interfaces as well by the Kirchhoff summation method which systematically sums signals into the apex of the approximation hyperboloid even though the Kirchhoff integral is in this case not strictly valid. For a multilayered subsurface isovelocity layer model with interfaces of a generally curved nature this can only provide a complete migration for the uppermost interface. Still there are various advantages gained by having a process which sums signals consistently into the minimum of the reflection time surface. The position of the time surface minimum is the place where a ray from the depth point emerges vertically to the surface. The Kirchhoff migration, if applied to media with laterally inhomogeneous velocity, must necessarily be followed by a further time‐to‐depth migration if the true depth structure is to be recovered. Primary normal reflections and their respective migrated reflections have a complementary relationship to each other. Normal reflections relate to rays normal to the reflector and migrated reflections relate to rays normal to the free surface. Ray modeling is performed to indicate a new approach for simulating seismic reflections. Commonly occuring situations are investigated from which lessons can be learned which are of immediate value for those concerned with interpreting time migrated reflections. The concept of the ‘image ray’ is introduced.
CORRELATION BETWEEN SEISMIC REFLECTION AMPLITUDE AND WELL PRODUCTIVITY – A CASE STUDY *
Authors: Hun F.
Published: April 2006

Abstract

The results of specific processing of seismic sections on an actual oil field have proved that it is possible to find a link between a seismic parameter (amplitude of a reflecting horizon) and petrophysical characteristics of a reservoir whose changes modify well productivity.

A good correlation appears between the values of good oil productivity and the weakness of the amplitudes of one horizon close to the oil producing layers.
SIGNATURE AND AMPLITUDE OF LINEAR AIRGUN ARRAYS *
Authors: NOOTEBOOM J. J.
Published: April 2006

Summary

During the last few years many airgun arrays have been designed with the objective of generating a short signature of high amplitude. For linear arrays of non‐interacting airguns two rules have been derived that may help in the design or evaluation of airgun arrays. To achieve a short pressure pulse, the total available air volume has to be distributed over the individual guns in such a way that the tail of the signal, owing to the added bubble signals, becomes as flat as possible.

When we think of ordering airguns according to volume, this flat signal tail can be achieved by designing the volumes such that the difference in bubble times of two adjacent guns is proportional to their volume to the power 2/3.

The amplitude expected from a linear array of non‐interacting airguns is limited by the physical length of the array.

A graph of measured values tends to confirm this relation.

No relation has been found between the total volume of an array and its amplitude.

The graph also detects inefficient use of available array length of existing arrays.
RADAR DETECTION OF BOREHOLES IN ADVANCE OF MINING*
Authors: TARANTOLO P. J. , UNTERBERGER R. R.
Published: April 2006

Abstract

Boreholes present a potential hazard to mining in rock salt mines. The only rock salt mine in England was almost lost when they mined into an unknown borehole open to an aquifer. Rock salt is a good transmitter of VHF radar waves, so a good method of detecting boreholes is by using radar probing. This paper investigates theoretically the detection of a borehole in advance of mining using two different radar systems, Bravo II and Charlie II. Bravo II has a peak power of 20 kW at 230 MHz and Charlie II has 3 W at 440 MHz. The borehole detectability depends critically on the radar wave attenuation in salt, which is characterized by its loss tangent, tan δ. The backscattering cross section σ of the borehole is also important. These parameters are related in the radar signal‐to‐noise equation which determines the signal‐to‐noise ratio (S/N) for a given range in salt to the borehole. Theoretically, in salt with a tan δ of 10‐3, Charlie II can detect (S/N= 3 dB) a 12.7 cm borehole 225 m in advance of mining while Bravo II, using only 100 W, can detect the same borehole 680 m ahead. These values are with the antenna E field parallel to the axis of the borehole. Theory shows this antenna orientation to be optimum for maximum detection probability. For salt with a tan δ of 2 × 10‐5, the smallest value measured for salt, Bravo II can detect the borehole up to 15.3 km using maximum power of 20 kW.
SEISMIC FACIES ANALYSIS CONCEPTS *
Authors: ROKSANDIĆ M. M.
Published: April 2006

Abstract

Seismic facies analysis makes use of different seismic parameters in order to get other than structural information. A review is given of possibilities and usefulness of seismic facies analysis in oil exploration.

A seismic facies unit can be defined as a sedimentary unit which is different from adjacent units in its seismic characteristics. Parameters that should be taken into consideration in the seismic facies analysis are as follows: reflection amplitude, dominant reflection frequency, reflection polarity, interval velocity, reflection continuity, reflection configuration, abundance of reflections, geometry of seismic facies unit, and relationship with other units.

Interpretation of seismic facies data may be either direct or indirect. The purpose of the direct interpretation is to find out geological causes responsible for the seismic signature of a seismic facies unit. So, the direct interpretation may be aimed at predicting lithology, fluid content, porosity, relative age, overpressured shales, type of stratification, geometry of the geological body corresponding to the seismic facies unit and its geological setting. The indirect interpretation is intended to reach some conclusions on depositional processes and environments, sediment transport direction, and some aspects of geological evolution (transgression, regression, subsidence, uplift, erosion).

The results of the seismic facies analysis may be shown on seismic facies cross‐sections and seismic facies maps. Depending on the available seismic data and geological conditions in the area under consideration, the seismic facies maps may be of different types such as general seismic facies maps showing distribution of different seismic facies units, sand‐shale ratio maps, direction of cross‐bedding and paleo‐transport maps etc.

Several kinds of seismic facies units and their geological interpretation are discussed as examples of seismic facies analysis.
PROCEDES DE FILTRAGE ET DE FOCALISATION DE CHAMP POUR LA DETECTION ELECTROMAGNETIQUE DES CARRIERES SOUTERRAINES *
Authors: DUBUS J. P. , CLICQUE D. , BAUDET J. , GABILLARD R.
Published: April 2006

Abstract

The electromagnetic surface detection of underground quarries by classical methods becomes difficult when they are situated at depths greater than ten meters and when the thickness and conductivity of the superficial layers are irregular.

The problem is tackled in two stages: at first using successive approximations, characteristics of the miscellaneous layers of a stratified medium are identified, and the quarries are then detected by observation of the conductivity changes of one of the lower layers. Computer interpretation, however, is necessary.

The chances of detection of the quarries are considerably improved by a field localization method developed by us: computer interpretation is eliminated. The new aspect is an auxiliary transmitter which annuls the currents induced by the principal transmitter in the uppermost—generally more conductive—layers.

The theoretical and experimental results show that the probability of detection of the quarries by this method are four times as high as by the classical one.
HARMONIC INVERSION OF AN AEROMAGNETIC ANOMALY FROM A REMANENTLY MAGNETIZED PRECAMBRIAN SCHIST BELT (KARNATAKA, INDIA) *
Authors: MISHRA D. C.
Published: April 2006

Abstract

The conventional methods of magnetic interpretation consider in general the induced magnetization only. However, in a computer oriented inverse method, the direction of magnetization can be taken as arbitrary. Following this procedure, this study aims to present a realistic interpretation of an aeromagnetic anomaly from Chitaldrug Schist belt (Mysore, India) assuming a three‐dimensional configuration of the causative source possessing remanent magnetization. It further provides the direction of remanent magnetization of the source rock as obtained from the analysis of the magnetic anomaly. The total intensity map obtained delineates the surface and sub‐surface structures in the region and provides informations regarding the characteristics of primordial crust.
ATTENUATION OF COHERENT NOISE IN MARINE SEISMIC EXPLORATION USING VERY LONG ARRAYS *
Authors: URSIN B.
Published: April 2006

Abstract

The use of arrays to separate primary reflections from unwanted coherent seismic events is common practice in land seismic surveys. Very long source and receiver arrays have been used recently to reduce the effects of waterbottom multiples on marine seismic data.

The source array consists of five uniformly spaced identical subarrays, each with five different airguns, where the distance between the subarrays may vary from 20 m56 m. The volume of each subarray is 10.3 1 (630 cu.in.) which gives a total volume of the array of 51.5 1 (3150 cu.in.) operated at a pressure of 14 MPa (2000 psi). In order to have a flexible receiver system it was decided to implement the extended receiver array in data processing by computing a weighted sum of two to five traces. The hydrophone cable consists of fifty‐four channels with a group length of 50 m.

Data shot with the superlong airgun array are processed by a combination of standard techniques and special procedures. In particular, the quality of the stack section is improved by using a weighted stack. The stack weights are computed by a program which takes into account the primary‐to‐multiple ratio.

Comparisons with conventional data show significant improvements in data quality obtained by using the superlong airgun array. Examples show that the waterbottom multiples have been strongly attenuated and the deep seismic events have been enhanced.

The combined array response function for dipping events is given in an appendix.
A FREQUENCY DOMAIN APPROACH TO TWO‐DIMENSIONAL MIGRATION *
Authors: BOLONDI G. , ROCCA F. , SAVELLI S.
Published: April 2006

Abstract

The problem of the propagation of acoustic waves in a two‐dimensional layered medium can be easily solved in the frequency domain if the Dix approximation is used, i.e. when only the primary reflections are considered. The migrated data at a depth z are obtained by convolving the time section with a proper two‐dimensional operator dependent on z. The same result can be obtained by multiplying their two‐dimensional spectra and summing for all the values of the temporal frequency.

The aspect of the operator in the time‐space domain has the classic hyperbolic structure together with the prescribed temporal and spatial decay.

The main advantages of the frequency domain approach consist in the noticeable computer time savings and in the better approximation. On the other hand lateral velocity variations are very difficult to be taken into account. This can be done if a space variant filter is used in the time‐space domain.

To reduce computer time, this filter has to be recursive; the problem has been solved by Claerbout by transforming the hyperbolic partial differential equation into a parabolic one, and using the latter to generate the recursion operator.

In the presentation a method is given for the generation of recursive filters with a better phase characteristics that have a pulse response with the requested hyperbolic shape instead of the parabocli one. This allows a better migration of steeper dips.
EQUATIONS D'ONDE ET MODELES *
Authors: HEMON CH.
Published: April 2006

Abstract

The method of finite differences is applied to the computation of multi‐dimensional synthetic seismograms. This paper gives a study of the mathematical and numerical formulations of the problem, the boundary conditions, the convergence conditions and how to simulate the source in both one solid or a liquid. It is shown that the numerical formulation chosen is valid both for direct and inverse problems (i.e. for modeling and migration). This formulation makes it possible to use the normal incidence reflection coefficients for P and S waves, whether they travel horizontally or vertically. The examples shown have been chosen on purpose in order to be easily interpreted. They do not give a full idea of the possibilities of the algorithm which allows to consider non‐planar interfaces, except close to the vertical axis.
A “PROCESSING DENSITY’TO CALCULATE MARINE BOUGUER GRAVITY FREE OF TOPOGRAPHIC VARIATIONS IN CASE OF UNKNOWN BOTTOM DENSITY *
Authors: MALZAC J. , ROUSSEAU A.
Published: April 2006

Abstract

In marine gravity interpretation, the density required for the Bouguer correction is not easily measured, and, therefore, one often uses either standard values or extrapolates density from a station or a profile to an entire survey. If the real density varies, this leads to inconsistencies. In this paper a method is proposed to eliminate as much as possible the gravimetrical effects of real density variations. The principle is to apply Nettleton's method to the whole space, but also to use a continuously variable density in the Bouguer gravity computation. It is possible to avoid affecting real gravity anomalies by using stations situated on suitable area elements the optimal size of which is determined by the program. The density that is obtained is called “processing density’ because it is not a real one, but rather the resultant of the densities affect on each station. Use of the method is demonstrated on an example from the southeastern part of Bay of Biscay.
COHERENCY WEIGHTING—AN EFFECTIVE APPROACH TO THE SUPPRESSION OF LONG LEG MULTIPLES*
Authors: BUTTKUS B.
Published: April 2006

Abstract

Long leg multiples can be suppressed by a method which provides an alternative to weighted common‐depth‐point stacking and multichannel stacking filtering. The suppression is achieved by coherency weighting whereby the time‐dependent weighting factor decreases as the semblance of the multiple reflections increases.

The algorithm of the method is described. Its efficiency is discussed in relation to the input data and results of its application to marine seismic data are presented.

For practical application, the stacking velocity of the multiples has to be known. As the process is based on stacking velocities, different types of multiples can be handled, for instance water‐bottom multiples or internal multiples. The parameter analysis shows that the degree of multiple suppression can easily be controlled by adapting the parameters of the procedure to the field conditions.

During the suppression of multiples, the primaries are saved according to the moveout differences between the two. The non‐linear behaviour of the process causes signal suppression and distortion effects, which have to be corrected by AGC normalization and low‐pass filtering.

Among the various applications available, only the suppression of long leg water‐bottom multiples is treated here. The results show that their suppression on the basis of moveout differences is efficient even when standard length streamers are used in regions with water depth of up to 1500 m and more, if the stacking velocity of the primaries is about 10 to 20% higher than that of the multiples. Even if those parts of the primaries which are masked by the multiples are suppressed in the individual common‐depth‐point gathers by the procedure, the remaining primaries in the AGV stacked section are largely uncovered by the multiple suppression.
TIME‐VARYING PREDICTION FILTERING BY MEANS OF UPDATING*
Authors: RISTOW D. , KOSBAHN B.
Published: April 2006

Abstract

In contrast to the conventional deconvolution technique (Wiener‐Levinson), the spike‐, predictive‐, and gap‐deconvolution is realized with the help of an adaptive updating technique of the prediction operator. As the prediction operator will be updated from sample to sample, this procedure can be used for time variant deconvolution. Updating formulae discussed are the adaptive updating formula and the sequential algorithm for the sequential estimation technique. This updating technique is illustrated using both synthetic and real seismic data.
MINIMUM ENTROPY DECONVOLUTION WITH AN EXPONENTIAL TRANSFORMATION*
Authors: OOE M. , ULRYCH T. J.
Published: April 2006

Abstract

The Minimum Entropy Deconvolution (MED) technique of Wiggins (1977) represents a breakthrough in deconvolution and will undoubtedly find wide application in many fields. MED does not require any phase assumptions about the disturbing function and seeks a deconvolved output which consists of the smallest number of large spikes consistent with the input data.

The efficiency of MED is much improved when an exponential transformation is incorporated into the algorithm. This is particularly true when the input traces contain additive noise. In this case the noise suppression characteristics of MED are considerably enhanced by the transformation and the identification of smaller spikes is improved. This paper also presents a kurtosis criterion of simplicity rather than the varimax norm introduced by Wiggins. It appears that for a multiple trace input the kurtosis measure leads to improved results.