bluuebunny/crossref_metadata_2025_split huggingface dataset:
A NEW APPROACH TO VIBROSEIS DECONVOLUTION*
Authors: LINES L. R. , CLAYTON R. W. 
Published: April 2006

AbstractA 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

AbstractFor 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

AbstractAn 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

AbstractIn 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

AbstractA 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

AbstractThe 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

AbstractInferior 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.
IMPULSE RESPONSE MODELS FOR NOISY VIBROSEIS† DATA*
Authors: LINES L.R. , CLAYTON R.W. , ULRYCH T.J. 
Published: April 2006

AbstractA 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

AbstractA 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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractIn 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

AbstractIt 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

AbstractOn 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

AbstractLateral 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

AbstractOn 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

AbstractThe 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

AbstractThirty‐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

AbstractThe 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

AbstractA 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
image

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

AbstractGeoelectric 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.