bluuebunny/crossref_metadata_2025 huggingface dataset:
SUR LA SEPARATION DES ANOMALIES GRAVIMETRIQUES ET MAGNETIQUES*
Authors: BOTEZATU RADU
Published: April 2006

Abstract

One of the most important aspects in interpretation of gravity and magnetic anomalies, which generally represent the sum of many effects from the subsoil, is that these effects cannot be adequately separated into individual effective anomalies by means of known procedures. Moreover, the concept itself of anomaly separation must be expanded in a certain sense, because the addition of these effects in the mapped anomalies can arise from many structures situated in the subsoil in different relative positions to one another. A combined procedure is proposed, based on Fourier analysis utilising both periodic complex functions and nonperiodic ones, by use of which the number of individual effective anomalies cumulated in the mapped anomalies is obtained. In order to illustrate the use of this procedure and the advantages of obtaining more valuable informations on geological causes of anomalies, one example from Romania is given.
DIFFERENT METHODS OF TIME‐DEPTH CONVERSION WITH AND WITHOUT MIGRATION*
Authors: PATURET DANIEL
Published: April 2006

Abstract

The increasing use of velocity analysis programs in seismic processing in addition to direct application to normal move out corrections often makes it possible to study the variation of the average velocity versus time and distance. Usually, it is even feasible to compute interval velocities with a good accuracy.

We intend to use these velocities to convert the usual time domain displays to the depth domain.

An exact conversion requires the knowledge of all interval velocities and all the velocity interfaces.

However, when the dips are small, the conversion can be done without considering the dip of all these interfaces, and in some cases migration can even be completely neglected.

Three different programs will be described and their use discussed in view of the type of problems to which they are applied.

Practical examples of the various methods will be presented.
FIRST EXPERIENCE AND RESULTS WITH A NEW SEISMIC INTERPRETATION METHOD*
Authors: DOHR G.
Published: April 2006

Abstract

At the EAEG meeting in Venice (May 1969) Linsser presented a new method for the interpretation of seismic measurements (“Transformation of Seismic Data into Geological Models by Digital Template Analysis”). First results were presented during the S.E.G. convention in Calgary (September 1969). Meanwhile some experience concerning the possibility of the application of this method is available.

Starting from the fact that the computer is able to store several physical parameters (amplitude, wave‐length, direction of the seismic event, coincidence with a template and the beginning and the end of each reflection), the importance of these data for the investigation of special geophysical questions will be discussed.

This method has been tested with several examples on the following problems:

Proving and exact localisation of faults.

Investigation of velocities of interesting layers.

Additional information for the geological interpretation of seismic sections.

Picking of weak reflections in the “Pre‐Zechsteinian” in Northern Germany.

Finally the possibilities of further applications and developments are discussed.
A NEW METHOD OF CANCELLING MULTIPLE REFLECTIONS: “SOUSTON”*
Authors: MICHON D. , WLODARCZAK R. , MERLAND J.
Published: April 2006

Abstract

Cancelling of multiple reflections by CDP stacking is generally incomplete. When the order of coverage is low, an improvement may be obtained by the use of special filters (e.g. Schneider et al: Geophysics 1965; D'Hoeraene: Geophysics 1966). But when the order of coverage is high, the efficiency of these filters is less obvious. On the other hand, the higher the order of coverage, the higher the efficiency in the solution presented.

Schematically, our method includes three steps:

Firstly: Searching for the multiple reflections.

For this purpose Move Out corrections corresponding to the multiples are applied to each individual record, then CDP stacking is carried out.

Secondly: Cleaning up of the individual records.

“Decorrections” are firstly applied, then multiples are subtracted. For this, subtraction is weighed versus the energy of the multiples, that is to say versus the correlation between the original record and the pure multiple reflections.

Thirdly: Normal processing with the cleaned records which have been obtained.

The different steps of this method are illustrated with the help of theoretical examples. Its efficiency improvement in regard to normal CDP stacking is then demonstrated by means of true examples.
A METHOD FOR INTERPOLATION AND EXTRAPOLATION OF POTENTIAL FIELD DATA*
Authors: CORBYN J. A.
Published: April 2006

Abstract

The application of automatic methods to the processing of potential field survey results has led to the need for the development of special procedures for the interpolation and extrapolation of such data. A method is proposed which utilizes the mathematical properties of potential field data to achieve a minimum distortion solution to the interpolation/extrapolation problem. The proposed method is suitable for use with any data point distribution. Digital computer programs have been prepared using the method; results obtained through the use of the programs are presented.
A NOTE ON THE LINEAR FILTER METHOD OF INTERPRETING RESISTIVITY SOUNDING DATA*
Authors: KOEFOED O.
Published: April 2006

Abstract

In the linear filter method of interpreting resistivity sounding data, as developed by Ghosh (1971), it appears that the filter function in the x‐domain approaches an oscillating function for both large positive and large negative abscissa values. In the present note the reason for this oscillating behaviour is derived, and a possible practical application is indicated.
MESURES SISMIQUES PAR TRANSMISSION—APPLICATION AU GENIE CIVIL *
Authors: PORTE M. LA , LAKSHMANAN J. , LAVERGNE M. , WILLM C.
Published: April 2006

Abstract

The new process of automatic determination of seismic velocities by well to well measurements (Bois et al., 1971, Geophysical Prospecting 19, 42‐73) gives the possibility to increase the knowledge of oil reservoirs, by detecting large inhomogeneities between wells. It can also give useful informations on the mechanical properties of rocks in mining exploitation and civil engineering, by gallery to gallery measurements. An application of the method is given to the search for the proper location of an underground hydroelectric power plant, where the problem was to investigate the rock properties in a horizontal plane between two exploration galleries.
CROSS‐CORRELATION AS AN AID IN SIMULTANEOUS GRAVITY AND MAGNETIC ANALYSIS *
Authors: BOTEZATU R. , CALOTA C.
Published: April 2006

Abstract

The properties of cross‐correlation functions for the case of gravity and magnetic total field anomalies produced by three geometric models of geological bodies (point‐mass, horizontal line‐mass, and vertical semi‐infinite rectangular prism mass distributions) are studied. This study is carried out on four cases of cross‐correlation: the whole curve of anomalies, a single branch of anomalies, the branch between the two inflexion points and the branch between the apex point and the inflexion point. In all cases, the crosscorrelation function can serve as a good indicator to discriminate the so‐called genetically related anomalies from those produced by different geological bodies situated on the same vertical line; but the best results are obtained using the cross‐correlation of the branches between the apex point and the inflexion point of the two geophysical anomalies.

A practical procedure is developed in order to undertake such analysis. The tests in some cases of real gravity and magnetic anomalies mapped in Romania show the validity of this practical procedure.
MATHEMATICAL MODELS OF CONDUCTING ORE BODIES FOR DIRECT CURRENT ELECTRICAL PROSPECTING *
Authors: STEFĂNESCU S. , STEFĂNESCU D.
Published: April 2006

Abstract

The authors generalize a method expounded in a previous paper (1971, Geoph. Prosp. 18, 786‐799) to the case of a local conductivity σ(M) of the infinite medium satisfying the relation

image

where the Ri's are the distances from the point M to n fixed points Si (i= 1,. n), k is a positive real constant and Ci, Cii are constants ensuring the condition α > O.

The sub‐surface conductivity distributions (half‐spaces) complying with (1) provide a wide variety of conducting structures, which can fit quite successfully the rather complicated distributions of conductivity occurring in natural ore bodies.

An exact algebraic calculation of the apparent resistivity for these grounds, valid for any dc electrical prospecting devices (Wenner, Schlumberger, dipole, etc.) leads to a set of simultaneous linear equations, with a matrix which is invariant with respect to the position of the quadrupole being used. This greatly simplifies the numerical computation.

We also present some examples of cross sections for the real and apparent resistivity obtained by this method.
THE POOR MAN'S SEISMIC SOURCE: A COMPUTER GAME *
Authors: ALTERMAN Z. , CENSOR D. , GINZBURG A. , SCHOENBERG M.
Published: April 2006

Abstract

Basic requirements for a seismic reflection system using a moderate‐power controlled source are discussed. General considerations and computer modelling show promise for geophysical prospecting, especially at low penetration and very high noise level. For many applications, ranging from mineral and water prospecting, to civil and military engineering, and even archeology, a relatively cheap, small and portable device of this kind might be more convenient than conventional explosive sources or sophisticated controlled sources of the vibroseis type. The use of a simple, low frequency periodic signal facilitates rapid data processing procedures and suggests that the mechanical generator will be very simple and cheap.
THEORETICAL RESISTIVITY SOUNDING RESULTS OVER A TRANSITION LAYER MODEL *
Authors: NIWAS SRI , UPADHYAY S. K.
Published: April 2006

Abstract

An earth model with a transition layer (anisotropic inhomogeneous) is considered. The inhomogeneity in σv (vertical conductivity) of the transition layer is represented by a power law variation. Expressions for potential distribution in the upper layer, transition layer and bottom layer are obtained by solving appropriate differential equation for each layer. By utilizing the boundary conditions, expressions of apparent resistivity for Wenner and Schlumberger configurations are derived. Numerical analysis is performed for linear and quadratic variation of σv. The results are presented in the form of theoretical apparent resistivity curves for both configurations. Negative apparent resistivities are the interesting feature of this analysis.
AFTER‐STACK MULTICHANNEL FILTERS WITHOUT MIXING EFFECTS *
Authors: CASSANO E. , ROCCA F.
Published: April 2006

Abstract

Several types of multichannel filters have been introduced in the past with the purpose of rejecting, in a seismic section, coherent noise having a slope different from that of the signal.

These filters, generally, tend to introduce a certain amount of mixing and therefore the output trace shows increased horizontal coherence. This is due to the model on which these filters are based, since the hypothesis is posed that the reflectors are continuous. This may be dangerous since it could lead to mistaken interpretations, for example when small faults or breaks are made to disappear in the output section.

Other problems that could arise in the application of multichannel filters after‐stack are space‐aliasing and high‐pass filtering. The former occurs when coherent noise is rejected with apparent Velocity V and frequency fa=V/X, where X is the distance between traces. In this case, the signal also is distorted since it is rejected in the same frequency range. The high pass filtering effect occurs when the multichannel filter is designed to remove low coherent noise with high apparent velocity.

In the paper a family of multichannel filters is presented based on a model of the seismic section such that minimum mixing effects appear. The filters are designed to give good results even in the case of low frequency and high velocity coherent noise.

Some practical examples are shown.
DETAILED ELECTROMETRIC AND THERMOMETRIC OBSERVATIONS IN OFFSHORE AREAS. *
Authors: BOGOSLOVSKY V. A. , OGILVY A. A.
Published: April 2006

Abstract

The paper deals with the prerequisites of application, specific peculiarities and methods of electrometric and thermometric investigations aimed at the solution of certain engineering‐geological problems in offshore areas. Practical examples are supplied.
AN EXAMPLE OF THE APPLICATION OF AUTO‐CORRELATION AND SPECTRAL ANALYSIS IN MINING EXPLORATION *
Authors: SARMA D. D.
Published: April 2006

Abstract

This paper deals with a case study on the application of auto‐correlation and spectral analysis methods to mineral assay data taken along seven drives and two winzes concerning the McTaggart's (West) lode of Kolar gold fields. The general model conceived in this study is: measured (estimated) ore grade at jth location =true value + error at jth location. Employing an auto‐correlation function of the form C e‐ a |k| for signals, the respective contributions of signals and noise to the total logarithmic variances for the data along drives and winzes are analysed. Smoothed spectral density estimates and their 90% confidence limits are computed for the series of data.
APPARENT RESISTIVITY, ANISOTROPY AND STRIKE MEASUREMENTS*
Authors: HABBERJAM G. M.
Published: April 2006

Abstract

In the search for a descriptive system capable of accounting for the wide variations of earth resistance with orientation of electrode array, which can occur in field measurements, a modified measure of apparent resistivity, based on an anisotropic earth, is proposed.

This measure requires the sampling of earth resistance in, at least, three orientations. A practical system, the crossed square array, which effectively samples in four, is considered in detail. A system of reduction of field observations yields, besides the apparent resistivity, apparent measures of anisotropy and strike.

Field trials show that the anisotropic analysis not only accounts for the major part of the observed orientational variations in resistance, but also yields sounding curves of resistivity, anisotropy and strike, which vary systematically with electrode spacing.

The effects and limitations of applying such techniques in regions of pronounced two dimensional structure are examined.

Finally, to avoid unnecessary labour in deploying the crossed square sounding technique at sites where it is not required, a modified single square technique is proposed, which will normally serve but will also alert the observer when crossing is required.
COMPUTATION OF INTERVAL VELOCITIES FROM COMMON REFLECTION POINT MOVEOUT TIMES FOR n LAYERS WITH ARBITRARY DIPS AND CURVATURES IN THREE DIMENSIONS WHEN ASSUMING SMALL SHOT‐GEOPHONE DISTANCES*
Authors: KREY TH.
Published: April 2006

Abstract

It is well known that interval velocities can be determined from common‐reflection‐point moveout times. However, the mathematics becomes complicated in the general case of n homogeneous layers with curved interfaces dipping in three dimensions.

In this paper the problem is solved by mathematical induction using the second power terms only of the Taylor series which represents the moveout time as a function of the coordinate differences between shot and geophone points. Moreover, the zero‐offset reflection times of the nth interface in a certain area surrounding the point of interest have to be known. The n—I upper interfaces and interval velocities are known too on account of the mathematical induction method applied. Thus, the zero‐offset reflection raypath of the nth interface can be supposed to be known down to the intersection with the (n—1)th interface.

The method applied consists mainly in transforming the second power terms of the moveout time from one interface to the next one. This is accomplished by matrix algebra.

Some special cases are discussed as e.g. uniform strike and small curvatures.
A COMPARISON BETWEEN WIENER FILTERING, KALMAN FILTERING, AND DETERMINISTIC LEAST SQUARES ESTIMATION*
Authors: BERKHOUT A. J. , ZAANEN P. R.
Published: April 2006

Abstract

The least squares estimation procedures used in different disciplines can be classified in four categories:

Wiener filtering,

b. Autoregressive estimation,

c. Kalman filtering,

d. Recursive least squares estimation.

The recursive least squares estimator is the time average form of the Kalman filter. Likewise, the autoregressive estimator is the time average form of the Wiener filter. Both the Kalman and the Wiener filters use ensemble averages and can basically be constructed without having a particular measurement realisation available.

It follows that seismic deconvolution should be based either on autoregression theory or on recursive least squares estimation theory rather than on the normally used Wiener or Kalman theory. A consequence of this change is the need to apply significance tests on the filter coefficients.

The recursive least squares estimation theory is particularly suitable for solving the time variant deconvolution problem.
CALCUL DES COURBES DE, SONDAGES ÉLECTRIQUES À L'AIDE DES FONCTIONS D'ÉCHANTILLONNAGE*
Authors: BARANOV W.
Published: April 2006

Abstract

Recently I had an opportunity to apply the theory of sampling functions to the functional transformations of gravity and magnetic anomalies (Baranov 1975). It would be interesting to examine if there is a possibility to use these functions in the electrical prospecting. It seems that the answer might be affirmative.

This paper is an attempt to compute the master curves for the electrical vertical sounding with the aid of sampling functions.
PROGRESS IN THE DIRECT INTERPRETATION OF RESISTIVITY SOUNDINGS: AN ALGORITHM*
Authors: KOEFOED O.
Published: April 2006

Abstract

An algorithm is presented for the direct interpretation of resistivity sounding data. The algorithm is based on the method of successive reductions to lower boundary plane of the resistivity transform function. A novel aspect of the algorithm is that error limits are assigned to the initial values of the resistivity transform, and these error limits are carried through in all the subsequent computations. The width of the error range is then used as the basis for assigning weight factors in the final computation of thicknesses and resistivities of the layers. The errors in the resistivity transform derived from the solution given by the algorithm are usually not more than twice as large as those in the original data.
ELECTROMAGNETIC RESPONSE OF A BROKEN ORE‐VEIN (MODEL STUDIES)*
Authors: PODDAR M. , LAHIRI A. K. , DEB B. K.
Published: April 2006

Abstract

A broken ore‐vein is very common in nature. A small scale model of such a vein has been made by placing two thin metal sheets edge to edge with an air gap between them. The electromagnetic response of this model in a uniform oscillating magnetic field has been measured. The results show that (a) the response of the whole body is not the sum of responses of the constituent parts, (b) the response of the upper part is dominant by virtue of its proximity to the receiving coil and (c) the lower part has the effect of making the upper part appear nearer to the surface and poorer in conductance than it really is.
COMBINED REFRACTION SEISMIC AND GRAVIMETRIC INVESTIGATIONS IN A PALEOGEOGRAPHICAL AND GEOLOGICAL RESEARCH PROGRAM*
Authors: VANDENBERGHE J.
Published: April 2006

Abstract

Refraction seismic and gravity investigations were executed to solve geological and hydrographical problems in recent strata: These loose sediments necessitated a special arrangement for the energy source in the seismic measurements, By the use of a falling weight the refracted wave could be registered from a depth of more than 30 m. Quaternary fluviatile sands are characterized by lower velocities than the Tertiary sediments. Even a marked velocity difference between different Tertiary formations could be observed. Sand‐filled valleys in Tertiary clays give relatively important variations in the gravity field caused by the small differences of densities. Thus buried channels in a clay subsoil can be found by accurate gravity measurements.
HYDROGEOPHYSICAL PROPERTIES OF PARTS OF THE BRITISH TRIAS*
Authors: WORTHINGTON P. F.
Published: April 2006

Abstract

Laboratory studies of Bunter Sandstone specimens from Northwest Lancashire, Shropshire, West Cumberland, and the Vale of Clwyd have indicated that the parameters effective porosity, intergranular permeability, compressional wave velocity, formation resistivity factor, and effective matrix resistivity have significantly different distributions in each of these four regions. Regression analyses have shown that bivariate and trivariate expressions for the prediction of the two hydrological parameters from petrophysical data vary from region to region. It is concluded that, in quantitative geophysical investigations of these formations, each area must be investigated independently.

For all four aquifers, and for both horizontally and vertically oriented specimens, effective porosity can be most reliably and readily estimated through a bivariate relationship involving formation resistivity factor. On the other hand, the best estimate of intergranular permeability from geophysical data is obtained through a trivariate expression involving both formation resistivity factor and effective matrix resistivity. The use of hydrogeophysical relationships to estimate hydrological parameters in situ is illustrated by reference to field examples.
A NEW APPROACH TO VIBROSEIS DECONVOLUTION*
Authors: LINES L. R. , CLAYTON R. W.
Published: April 2006

Abstract

A method is proposed to obviate the shortcomings of conventional deconvolution approaches applied to vibroseis data. The vibroseis wavelet reduces the time domain resolution of the earth's impulse response by restricting its passband. The spectrum of the wavelet is assumed to be a “low quefrency”phenomenon, and hence it can be estimated by low cut cepstral filtering. The wavelet's amplitude spectrum can then be removed by spectral division. By using an approach which is consistent with the principle of maximum entropy, the undetermined portions of the seismogram's Fourier transform can be filled in by autoregressive prediction. The process of initially deconvolving in a restricted passband reduces the enhancement of noise contaminated parts of the spectrum, and the spectral extension scheme increases the time domain resolution of the process.
VIBROSEIS SIGNALS WITH PRESCRIBED POWER SPECTRUM*
Authors: RIETSCH E.
Published: April 2006

Abstract

For a swept frequency signal with constant envelope the power spectral density is approximately inversely proportional to the signal's rate of frequency change. Hence, by means of this relation, the phase function which describes the sweep's frequency variation may be derived from a predefined power spectrum. This is in contrast to the present use where the power spectrum is the result of a rather arbitrarily chosen phase function.

A numerical algorithm based on the relation between power spectrum and phase function was found to lead to sweeps whose power spectra matched the prescribed ones rather closely.
INVERSION OF REFLECTION DATA *
Authors: NILSEN A. , GJEVIK B.
Published: April 2006

Abstract

An iterative inversion method which also takes into account wave energy absorption is described. This method allows estimates of the acoustic impedance in sedimentary layers with nearly plane and parallel stratification. A set of reflection data has been inverted and an impedance model was obtained which correlates well with the essential features of the borehole velocity log.
A METHOD OF WEATHERING CORRECTION *
Authors: GENDZWILL D. J.
Published: April 2006

Abstract

In western Canada, the near surface weathered zone has variable velocity and is underlain by more uniform subweathering material. The uphole time is often used to compensate for the weathered zone, but there are limitations to its use.

A method is proposed using the refraction delay time which is exactly proportional to the weathering correction time for several commonly assumed weathering velocity functions. The method does not require accurate determination of velocities or weathering depth nor even the depth of shot, yet is theoretically exact. A strip off timetris used to replace the weathered zone mathematically with high velocity material wheretr= zD/F, whereDis the delay time andFthe Blondeau factor.

An algorithm using first break times is proposed that is amenable to computer use for determiningDfor each geophone point.

Fmight be constant in some areas but is more likely a variable to be solved in an automatic static program. Thus, structure and residual statics can be handled at once.
RESISTIVITY MODELLING FOR ARBITRARILY SHAPED TWO‐DIMENSIONAL STRUCTURES*
Authors: DEY A. , MORRISON H. F.
Published: April 2006

Abstract

A numerical technique is developed to solve the three‐dimensional potential distribution about a point source of current located in or on the surface of a half‐space containing arbitrary two‐dimensional conductivity distribution. Finite difference equations are obtained for Poisson's equations by using point‐ as well as area‐discretization of the subsurface. Potential distributions at all points in the set defining the half‐space are simultaneously obtained for multiple point sources of current injection. The solution is obtained with direct explicit matrix inversion techniques. An empirical mixed boundary condition is used at the “infinitely distant” edges of the lower half‐space. Accurate solutions using area‐discretization method are obtained with significantly less attendant computational costs than with the relaxation, finite‐element, or network solution techniques for models of comparable dimensions.
THE BLIND‐ZONE PROBLEM IN MULTIPLE REFRACTION‐LAYER OVERBURDEN SITUATIONS*
Authors: RAGHAVA M. S. VIJAYA , KUMAR G. NANDA
Published: April 2006

Abstract

The blind zone problem is examined with a view to provide a simple means of solving the problem in the general case of arbitrary number of refraction layers constituting the overburden to the masked layers. Considering the first arrival refraction interval corresponding to the masked layer which reduces to zero under blind zone conditions, a method to solve the problem is presented. Derivation of the critical distance expression is also included and convenient solutions to compute the thicknesses of the blind zone and its immediate overlayer are worked out. Based on test calculations on some known instances, the efficacy of the method is illustrated. A Fortran program for use when large numbers of refraction layers are involved under the overburden or when more than one masked layer is encountered is available from the authors.
MULTIPLE REFLECTIONS AND HEAD WAVES IN THE GULF OF SUEZ*
Authors: DOMENICO S. N.
Published: April 2006

Abstract

Inferior reflection quality in the Gulf of Suez at the target depth interval is attributable in part to surficial multiple reflections. An excellent example of the latter is observed on a typical seismic line in the northern portion of the Gulf. An increase in prominence of the multiple reflections appears associated with decreasing depth to a dipping highvelocity layer.

Inversion of a second‐order polynomial time‐distance function, fitted to the observed refraction onset time‐distance values, gives the velocity‐depth function for sediments between the water bottom and a high‐velocity layer. Velocities thus determined increase non‐linearly with depth from a value near water velocity at the water bottom. Depths to the high‐velocity layer are obtained from the associated head‐wave linear time‐distance function and by ray tracing in the overlying sediments.

As the high‐velocity layer approaches the water bottom from sub‐water depths exceeding 0.6 km to a depth of 56 m, intensity of the multiple reflections increases to the extent of completely dominating individual records to a time of at least 3 s. The estimated plane‐wave normal‐incident reflection coefficient at the top of the high‐velocity layer increases with decreasing depth to this layer, approaching 0.5 at the shallowest depth. This strong reflection coefficient further substantiates the existence of multiple reflections between the high‐velocity layer and water layer. However, existence of water‐layer multiples cannot be ruled out. The estimated water‐bottom reflection coefficient is approximately 0.3, a substantial value. Multiple reflections of considerably less intensity are apparent where the high‐velocity layer is deepest, and it is likely that such are waterlayer multiple reflections. Unfortunately, water‐layer multiple reflections and multiple reflections between the water surface and high‐velocity layer cannot be separated by their coincidence with time‐distance (normal moveout) curves, the configuration of each visibly matching the curves equally well.
RESISTIVITY SOUNDING ON A LAYERED TRANSITIONAL EARTH*
Authors: MALLICK K. , JAIN S. C.
Published: April 2006

Abstract

Master curves are presented for three‐layer earth‐sections, the electrical resistivity in the intermediate layer of which has a linear variation with depth. A new approach is proposed to interpret the sounding data with the help of RMS difference in apparent resistivity values.
IMPULSE RESPONSE MODELS FOR NOISY VIBROSEIS† DATA*
Authors: LINES L.R. , CLAYTON R.W. , ULRYCH T.J.
Published: April 2006

Abstract

A new method of Vibroseis deconvolution has been recently proposed by the authors. This discussion describes the effects of noise on the application of this method. The initial deconvolution step involves estimating the spectrum of the Vibroseis wavelet by homomorphic filtering. It is shown that noise causes problems with phase estimation. Hence, the Vibroseis wavelet is assumed to be zero phase. Examples demonstrate that zero phase cepstral filtering is a robust wavelet estimation approach for noisy data. The second step of the deconvolution method forms an impulse response model by a spectral extension method. Although this step can improve the resolution of seismic arrivals, it must be applied with caution in view of the deleterious effects of noise.
WAVE EQUATION MIGRATION WITH THE ACCURATE SPACE DERIVATIVE METHOD*
Authors: GAZDAG J.
Published: April 2006

Abstract

A stacked seismic section represents a wave‐field recorded at regularly spaced points on the surface. The seismic migration process transforms this recorded data into a reflectivity display. In recent years, Jon F. Claerbout and his co‐workers developed migration techniques based on the numerical approximation of the wave equation by finite difference methods. This paper describes an alternative method, termed ASD (for Accurate Space Derivative), and its application to the wave equation migration problem. In this approach to the numerical solution of partial differential equations, partial derivatives are computed by finite Fourier transform methods. This migration method can accommodate media with vertical as well as horizontal velocity variations.
ON THE BREMMER SERIES DECOMPOSITION: EQUIVALENCE BETWEEN TWO DIFFERENT APPROACHES*
Authors: AMINZADEH F. , MENDEL J.M.
Published: April 2006

Abstract

A Bremmer Series decomposition of the solution y(t) to the lossless wave equation in layered media is

image

where the yj(t) are physically meaningful constituents (i.e., y1(t) are primaries, y2(t) are secondaries, etc.). This paper reviews Mendel's state space models for generating the constituents; reviews Bremmer's integral equation models for generating the constituents; and demonstrates how Mendel's state space models can be obtained by a careful decomposition of Bremmer's integral equation models. It shows that Mendel's equations can be viewed as approximate numerical solutions of Bremmer's integral equations. In a lossless homogeneous medium, the approximations become exact.
FINITE‐DIFFERENCE EVALUATION OF APPARENT RESISTIVITY CURVES*
Authors: MUFTI I.R.
Published: April 2006

Abstract

The problem of numerical evaluation of apparent resistivity curves is treated by finite difference modeling. The models proposed are set up in cylindrical coordinates and yield the potential field due to a point source located in a radially symmetric environment. The Schlumberger configuration, widely used for surface measurements, is emphasized. However, the treatment is equally applicable to other similar situations such as the computation of synthetic electric logs when the resistivity of the borehole fluid is different from that of the surrounding uniform or stratified medium. Moreover, the individual layers may not necessarily be isotropic.

The medium under investigation is discretized by using a very coarse system of horizontal and vertical grid lines whose distance from the source increases logarithmically; consequently, the physical dimensions of the medium can be made “infinite” without affecting the numerical size of the model. Finer features such as a thin but anomalously resistive or conductive bed which would ordinarily be missed in coarse discretization are accurately taken into account, since the calculations are done in terms of the Dar Zarrouk parameters derived from the exact resistivity distribution of the model. This enables one to compute the potential field by inverting a small sparse matrix. When the medium comprises only a few layers, the efficiency of the finite‐difference model is comparable to that of the known analytical methods; for more complicated structures, however, the finite‐difference model becomes more efficient. The accuracy of finite‐difference results is demonstrated by comparing them with the corresponding analytically obtained data.
INFLUENCE OF A CONDUCTING SHIELD IN THE ONE‐LOOP VERSION OF THE TRANSIENT PULSE INDUCTION METHOD*
Authors: NAGENDRA R. , RAO I.B. RAMAPRASADA , BHIMASANKARAM V.L.S.
Published: April 2006

Abstract

The transient response of a conductive shell‐shell model in the one‐loop version was obtained analytically. The results indicate that four zones, namely early, late early, intermediate, and late zone can be identified in the total transient characteristic of the model. In case the measurements are carried out in the late early zone, a conductive target appears as a resistive one. It is suggested that the optimum time of measurement should be so selected as to fall in the intermediate zone.
TRACING BY GRAVITY OF A NARROW BURIED GRABEN STRUCTURE, DETECTED BY SEISMIC REFRACTION, FOR GROUND‐WATER INVESTIGATIONS IN NORTH CHILE*
Authors: VAN OVERMEEREN R.A.
Published: April 2006

Abstract

In the Andes mountains of North Chile seismic refraction measurements revealed the existence of a buried graben structure. The special geological environment permitted the deduction, by detailed analyses of diffraction patterns and delayed arrival times, of a low velocity layer underneath a high velocity ignimbritic sheet.

The gravity method was chosen as an economic secondary aid to trace the course of the buried structure. This unusual combination of detailed gravity measurements following a general seismic survey was not only successful in detecting and tracing a buried structure, but also provided complementary data about the deeper subsurface conditions.
AUTOREGRESSIVE PATTERN RECOGNITION APPLIED TO THE DELIMITATION OF OIL AND GAS RESERVOIRS*
Authors: BOIS P.
Published: April 2006

Abstract

It is often difficult to precisely determine the boundaries of oil and gas reservoirs in the horizontal and vertical directions. Autoregressive pattern recognition is used to reveal lateral facies variations and to specify reservoir boundaries. The method is based on the calculation of autoregressive coefficients for short trace sectors between the top and bottom boundaries of the reservoir. These coefficients are determined by applying the maximum entropy principle. Once these coefficients are known, an estimate can be made of the power spectra of these trace sectors. Then these coefficients will act as characters for a pattern recognition algorithm.

A decision criterion is used to distinguish the trace sector corresponding to layers impregnated with oil or gas and those impregnated with water.

Two examples are given to show how autoregressive pattern recognition allows to accurately delimit gas or oil reservoirs.
DEBUBBLING: A GENERALIZED LINEAR INVERSE APPROACH*
Authors: LEVY S. , CLOWES R.M.
Published: April 2006

Abstract

On seismograms recorded at sea bubble pulse oscillations can present a serious problem to an interpreter. We propose a new approach, based on generalized linear inverse theory, to the solution of the debubbling problem. Under the usual assumption that a seismogram can be modelled as the convolution of the earth's impulse response and a source wavelet we show that estimation of either the wavelet or the impulse response can be formulated as a generalized linear inverse problem. This parametric approach involves solution of a system of equations by minimizing the error vector (ΔX = Xobs– Xcal) in a least squares sense. One of the most significant results is that the method enables us to control the accuracy of the solution so that it is consistent with the observational errors and/or known noise levels.

The complete debubbling procedure can be described in four steps: (1) apply minimum entropy deconvolution to the observed data to obtain a deconvolved spike trace, a first approximation to the earth's response function; (2) use this trace and the observed data as input for the generalized linear inverse procedure to compute an estimated basic bubble pulse wavelet; (3) use the results of steps 1 and 2 to construct the compound source signature consisting of the primary pulse plus appropriate bubble oscillations; and (4) use the compound source signature and the observed data as input for the generalized linear inverse method to determine the estimated earth impulse response—a debubbled, deconvolved seismogram. We illustrate the applicability of the new approach with a set of synthetic seismic traces and with a set of field seismograms.

A disadvantage of the procedure is that it is computationally expensive. Thus it may be more appropriate to apply the technique in cases where standard analysis techniques do not give acceptable results. In such cases the inherent advantages of the method may be exploited to provide better quality seismograms.
DETERMINATION OF LATERAL INHOMOGENEITIES IN REFLECTION SEISMICS BY INVERSION OF TRAVELTIME RESIDUALS*
Authors: NEUMANN G.
Published: April 2006

Abstract

Lateral inhomogeneities generate fluctuations in the traveltime of seismic waves. By evaluation of these traveltime fluctuations from different source and receiver positions, lateral inhomogeneities can be located using a pseudo inverse matrix method (Aki, Christoffersson and Husebye 1977). The formulation of the problem is possible for transmitted waves as well as for reflected and refracted waves. In reflection seismics this method is of importance, if no reflections from the inhomogeneities themselves, but only reflections from lower boundaries can be observed.

The basic assumptions for the mathematical formulation are (1) the average velocities and depths of the reflecting horizons are known already from standard processing methods, and (2) the traveltime residuals are due to lateral velocity changes between different reflectors or between reflectors and the surface. The area of the earth to be considered is divided into layers and the layers into rectangular blocks. The parallel displacement of a ray after passing a disturbed block is neglected, only the traveltime residual is taken into account.

In this paper the method and its application to data obtained with two‐dimensional models are described.
THE RATIO OF THE PP‐ TO THE SS‐REFLECTION COEFFICIENT AS A POSSIBLE FUTURE METHOD TO ESTIMATE OIL AND GAS RESERVOIRS*
Authors: MEISSNER R. , HEGAZY M.A.
Published: April 2006

Abstract

On the basis of known relationships between various elastic moduli and porosity in a porous medium and of various parameters from well‐logs, the reflection coefficient for normal incidence of P‐waves is compared to that of S‐waves. If pores contain gas, the reflection coefficient in S is much smaller than that of P but shows large differences for different gas concentration in one of the two layers while the reflection coefficient in P is insensitive to the gas concentration. It is found that the ratio of the two reflection coefficients provides a powerful tool for discriminating between layers of different degrees of gas concentration and for the extension of the sensitivity of the bright‐spot technique to greater depths.
E‐POLARIZATION BOUNDARY CONDITIONS ABOVE LATERALLY NON‐UNIFORM CONDUCTORS FOR A SIMPLE INDUCTION MECHANISM*
Authors: SPITZ S.
Published: April 2006

Abstract

The induction problem for an inhomogeneous two‐dimensional conductor presenting a vertical contact between two media of diverse conductivities is considered. The general solution of the Helmholtz equation in the atmosphere is constructed for a simple induction mechanism. It is found that in the E‐polarization case the anomalous field behavior in the boundary region and beyond differs from the generally assumed one. The conclusion of this paper is that the boundary conditions widely utilized in numerical computations have to be modified.
STATISTICAL ANALYSIS OF THE GRAVITY METHOD OF CALCULATING THE DEPTH TO THE PRE‐CENOZOIC SURFACE IN YUCCA FLAT, NEVADA TEST SITE*
Authors: BRETHAUER G.E. , HEALEY D.L. , MILLER C.H.
Published: April 2006

Abstract

Thirty‐eight gravity method estimates of the depth to the pre‐Cenozoic surface under Yucca Flat are used to determine the statistical accuracy associated with the gravity method. Results indicate that the gravity method tends to overestimate the depth to the pre‐Cenozoic surface, on the average, by 3.5 m with a standard deviation of 75.2 m. No significant change of this statistical estimate was observed with (1) depth, (2) location, or (3) date when the gravity‐method estimate was made.
NEW ASPECTS IN THE INVERSION OF GRAVITY PROFILES BY POLYNOMIAL METHOD*
Authors: BALDI P.
Published: April 2006

Abstract

The gravity anomaly caused by a deviation of a density interface that is of constant profile but finite extension (a so‐called 2 1/2‐dimensional structure) can be expressed in polynomial form that can be inverted. Numerical experiments with synthetic structures show the viability of this inversion method.
A COMPARISON OF IP TRANSIENT SHAPES OVER DISSEMINATED AND MASSIVE SULFIDE SHEETS IN THE LOWER PILLOW LAVAS OF CYPRUS*
Authors: MALIOTIS G. , KHAN M. AFTAB
Published: April 2006

Abstract

A number of time‐domain IP traverses were carried out across two parallel mineralized sheets in the Lower Pillow Lavas, near Mitsero, Cyprus with Huntec Mark III equipment using the pole‐dipole array. In one sheet the mineralization was disseminated (2%S), and in the other it was massive (30%S). The transients were recorded at separation n= 2 at a number of points to give the complete shape of the curves. The normalized time integrals were anomalous over the two sheets, but were not significantly different; the highest values being observed over the disseminated sheet. Both sheets were also associated with high electromagnetic components of the decay curve. The chargeability and resistivity values obtained over the disseminated body were considerably higher. The metal factor was also of value in discriminating between massive ore, disseminated mineralization, and barren rock. The values of P2 and P3 for the two bodies were also compared (P2 and P3 are defined by

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where M1 to M4 are the amplitudes of the decay curve at 55, 130, 280 and 580 ms respectively). For the massive ore, P was inversely related to M, but for the disseminated ore P was independent of M. Four simple parameters from the decay curves show that indices of curve shape offer the best prospect of grade discrimination.
GEOELECTRIC INVESTIGATIONS OF A FAULT SYSTEM IN QUATERNARY DEPOSITS*
Authors: VANDENBERGHE J.
Published: April 2006

Abstract

Geoelectric soundings were used to investigate the younger fault activity (Middle and Early Pleistocene) of the southwestern fault margin area of the Central Graben (and extension of the Rhine Graben) in the southern Netherlands and northern Belgium. Some effects of this fault activity can still be observed in the present geomorphology and hydrography. Investigations have been concentrated on the uppermost 20–30 m, consisting mainly of Middle and Early Pleistocene fluviatile deposits. Marshy clays and loams are most conductive. They alternate with large sand beds and gravelly gully infillings, characterized by high specific resistivities. Problems in the interpretation of the geoelectric data, caused by the variability of the deposits, are partially solved by in‐situ resistivity measurements (“mini‐electric” measurements). In spite of the lithologic inhomogeneity, a few marker horizons allow the geologic structure to be determined. Some new tectonic boundaries have been traced. The movements along the faults during the Quaternary are very small (less than 10 m). Therefore, the network of soundings has to be very dense and a very intensive analysis of the soundings is necessary. The results also have a hydrogeologic significance. This project illustrates the possibility of locating small tectonic structures in relatively inhomogeneous deposits by a detailed and carefully designed geoelectric survey.
USE OF THE RESISTIVITY METHOD IN GEOLOGICAL MAPPING—CASE HISTORIES FROM RANIGANJ COALFIELD, INDIA*
Authors: VERMA R. K. , BANDYOPADHYAY T. K.
Published: April 2006

Abstract

The effectiveness of the electrical resistivity method has been studied using various configurations with different spacings over the Southern Boundary Fault in the northwestern part, and across a dolerite dyke named “Salma dyke” in the central part, of the Raniganj Coalfield, India. It has been observed that the delineation of the fault and the dyke was made possible under differential tropical weathering conditions. Geoelectric sections across the fault and the dyke have been prepared on the basis of Schlumberger sounding results. In profiling, Wenner, two‐electrode, half‐Schlumberger and part of Al‐Chalabi's configurations were applied over the Southern Boundary Fault. Across the dyke, alpha‐, beta‐, and gamma‐Wenner, Schlumberger, half‐Schlumberger and two‐electrode configurations were employed. Azi‐muthal two‐electrode sounding was also studied over the fault.

The interpretation of the results of apparent resistivity profiles across the Southern Boundary Fault suggests that the Wenner and two‐electrode configurations possess certain diagnostic features which help in mapping a single lithological contact, provided sufficient resistivity contrast exists. Although Schlumberger configuration seems to be quite suitable, other configurations may also be usefully employed over the dyke.