bluuebunny/crossref_metadata_2025_split huggingface dataset:
ACCURATE FINITE‐DIFFERENCE OPERATORS FOR MODELLING THE ELASTIC WAVE EQUATION1
Authors: JASTRAM CORD , BEHLE ALFRED 
Published: April 2006

AbstractA method is presented for computing the coefficients of finite‐difference operators for the elastic wave equation. As opposed to other algorithms, in this case the amplitude spectrum of the source function is taken into account. Test calculations show that this approach gives more accurate results than operators based on a Taylor expansion or on minimizing the maximum error of the group velocity in a spatial frequency interval.
PARABOLIC AND HYPERBOLIC PARAXIAL TWO‐POINT TRAVELTIMES IN 3D MEDIA1
Authors: SCHLEICHER J. , TYGEL M. , HUBRAL P. 
Published: April 2006

AbstractThe 4 × 4 T‐propagator matrix of a 3D central ray determines, among other important seismic quantities, second‐order (parabolic or hyperbolic) two‐point traveltime approximations of certain paraxial rays in the vicinity of the known central ray through a 3D medium consisting of inhomogeneous isotropic velocity layers. These rays result from perturbing the start and endpoints of the central ray on smoothly curved anterior and posterior surfaces. The perturbation of each ray endpoint is described only by a two‐component vector. Here, we provide parabolic and hyperbolic paraxial two‐point traveltime approximations using the T‐propagator to feature a number of useful 3D seismic models, putting particular emphasis on expressing the traveltimes for paraxial primary reflected rays in terms of hyperbolic approximations. These are of use in solving several forward and inverse seismic problems. Our results simplify those in which the perturbation of the ray endpoints upon a curved interface is described by a three‐component vector. In order to emphasize the importance of the hyperbolic expression, we show that the hyperbolic paraxial‐ray traveltime (in terms of four independent variables) is exact for the case of a primary ray reflected from a planar dipping interface below a homogeneous velocity medium.
VISCOELASTIC SEISMIC RESPONSES OF 2D RESERVOIR MODELS1
Authors: KANG I. B. , McMECHAN GEORGE A. 
Published: April 2006

AbstractThe use of relaxation mechanisms has recently made it possible to simulate viscoelastic (Q) effects accurately in time‐domain numerical computations of seismic responses. As a result, seismograms may now be synthesized for models with arbitrary spatial variations in compressional‐ and shear‐wave quality factors (Q9, and Qs, as well as in density (ρ) and compressional‐ and shear‐wave velocities (Vp, and Vs).Reflections produced by Q contrasts alone may have amplitudes as large as those produced by velocity contrasts. Q effects, including their interaction with Vp, Vs and p, contribute significantly to the seismic response of reservoirs. For band‐limited data at typical seismic frequencies, the effects of Q on reflectivity and attenuation are more visible than those on dispersion.Synthetic examples include practical applications to reservoir exploration, evaluation and monitoring. Q effects are clearly visible in both surface and offset vertical seismic profile data. Thus, AVO analyses that neglect Q may produce erroneous conclusions.
A COMPARISON OF LABORATORY AND FIELD MEASUREMENTS OF P‐WAVE ANISOTROPY1
Authors: SAMS M. S. , WORTHINGTON M. H. , KING M. S. , KHANSHIR M. SHAMS 
Published: April 2006

AbstractP‐wave velocity anisotropy determined from a cross‐hole survey at the Imperial College borehole test site compares favourably with that measured in the laboratory on core from the holes. The holes penetrate a layered sequence of sandstones, shales and carbonates of the Namurian Upper Limestone Group. The Laboratory measurements of the vertical and horizontal velocities of core samples indicate that the shales exhibit P‐wave anisotropies of over 20% but that the sandstones and limestones are only slightly anisotropic. These discrete measurements have been used in combination with wireline data to produce a log of P‐wave anisotropy. Including the anisotropic information vastly improves the match between observed and synthetic traveltimes from the cross‐hole data set. This implies that there is little frequency dependence of intrinsic P‐wave anisotropy.Inversion of the cross‐hole traveltimes highlights the need for good angular coverage in order to resolve the anisotropy parameters. The observed P‐wave anisotropy of the field data is due to the combined effect of sedimentary layering and the intrinsic anisotropy of the rocks. The intrinsic anisotropy is found to be the dominant factor.
3D KINEMATIC INVERSION FROM A SET OF LINE PROFILES1
Authors: SORIN VALERY , KEYDAR SHEMER , LANDA EVGENY 
Published: April 2006

AbstractIn the case of 3D multilayered structures the 2D interval velocity analysis may be inaccurate. This fact is illustrated by synthetic examples.The method proposed solves the 3D inverse problem within the scope of the ray approach. The solution, i.e. the interval velocities and the reflection interface position, is obtained using data from conventional 2D line profiles arbitrarily located and from normal incidence time maps. Although the input information is essentially limited, the method presented reveals only minor biased velocity estimates.In order to implement the proposed 3D inversion method, we developed a processing procedure. The procedure performs the evaluation of reflection time and ray parameters along line profiles, 3D interval velocity estimation, and time‐to‐depth map migration. Tools to stabilize the 3D inversion are investigated.The application of the 3D inversion technique to synthetic and real data is compared with results of the 2D inversion.
MODEL‐BASED STACK: A METHOD FOR CONSTRUCTING AN ACCURATE ZERO‐OFFSET SECTION FOR COMPLEX OVERBURDENS1
Authors: LANDA E. , THORE P. , RESHEF M. 
Published: April 2006

AbstractThe interpretation of stacked time sections can produce a correct geological image of the earth in cases when the stack represents a true zero‐offset section. This assumption is not valid in the presence of conflicting dips or strong lateral velocity variations. We present a method for constructing a relatively accurate zero‐offset section. We refer to this method as model‐based stack (MBS), and it is based on the idea of stacking traces within CMP gathers along actual traveltime curves, and not along hyperbolic trajectories as it is done in a conventional stacking process. These theoretical curves are calculated for each CMP gather by tracing rays through a velocity‐depth model. The last can be obtained using one of the methods for macromodel estimation. In this study we use the coherence inversion method for the estimation of the macromodel since it has the advantage of not requiring prestack traveltime picking. The MBS represents an accurate zero‐offset section in cases where the estimated macromodel is correct. Using the velocity–depth macromodel, the structural inversion can be completed by post‐stack depth migration of the MBS.
TEMPERATURE EFFECTS ON AIRGUN SIGNATURES1
Authors: LANGHAMMER JAN , LANDRØ MARTIN 
Published: April 2006

AbstractExperiments in an 850 litre water tank were performed in order to study temperature effects on airgun signatures, and to achieve a better understanding of the physical processes that influence an airgun signature. The source was a bolt airgun with a chamber volume of 1.6 cu.in. The pressure used was 100 bar and the gun depth was 0.5 m. The water temperature in the tank was varied between 5°C and 45°C. Near‐field signatures were recorded at different water temperatures. Typical signature characteristics such as the primary‐to‐bubble ratio and the bubble time period increased with increasing water temperature. For comparison and in order to check whether this is valid for larger guns, computer modelling of airguns with chamber volumes of 1.6 and 40 cu.in. was performed. In the modelling the same behaviour of the signatures with increasing water temperature can be observed. The increase in the primary‐to‐bubble ratio and the bubble time period with increasing water temperature can be explained by an increased mass transfer across the bubble wall.
ON THE COMPUTATION OF THE MAGNETIC FIELD DUE TO A D.C. POINT ELECTRODE AT THE VERTICAL BOUNDARY BETWEEN TWO QUARTER‐SPACES1
Authors: HVOŽDARA M. 
Published: April 2006

AbstractNew formulae are presented for the calculation of the horizontal magnetic field due to a point electrode situated at the vertical boundary between two quarter‐spaces of different electrical conductivities. Previously the solution was obtained using a double Fourier transform of the expression for the vertical component of the magnetic field. The new formulae are given in the form of single integrals.
BOREHOLE EFFECTS ON DOWNHOLE SEISMIC MEASUREMENTS1
Authors: PENG CHENGBIN , CHENG C. H. , TOKSÖZ M. N. 
Published: April 2006

AbstractAn exact formulation for borehole coupling, which is valid for all frequencies and all azimuthally symmetric and non‐symmetric components, is presented. The borehole effects on downhole seismic measurements are studied in detail as functions of frequency, angle of incidence and polarization of an incident wave as well as geophone orientation. We found that correction for the borehole effect on downhole measurements should be made for frequencies above 500 Hz in a hard formation. In a soft formation, if the angle of incidence is well away from the resonance angle for SV incidence, no borehole correction is needed for frequencies below 300 Hz, while for frequencies above 300 Hz, the borehole can cause severe problems in downhole measurements. The borehole can also significantly alter the particle motion direction which implies that horizontal component rotation from data itself is unreliable for experiments with frequencies above 1 kHz in the hard formation and around 500 Hz in the soft formation.
Using the pseudospectral technique on curved grids for 2D acoustic forward modelling1
Authors: Nielsen Per , If Flemming , Berg Per , Skovgaard Ove 
Published: April 2006

AbstractThe Fourier pseudospectral method has been widely accepted for seismic forward modelling because of its high accuracy compared to other numerical techniques. Conventionally, the modelling is performed on Cartesian grids. This means that curved interfaces are represented in a ‘staircase fashion‘causing spurious diffractions. It is the aim of this work to eliminate these non‐physical diffractions by using curved grids that generally follow the interfaces.A further advantage of using curved grids is that the local grid density can be adjusted according to the velocity of the individual layers, i.e. the overall grid density is not restricted by the lowest velocity in the subsurface. This means that considerable savings in computer storage can be obtained and thus larger computational models can be handled.One of the major problems in using the curved grid approach has been the generation of a suitable grid that fits all the interfaces. However, as a new approach, we adopt techniques originally developed for computational fluid dynamics (CFD) applications. This allows us to put the curved grid technique into a general framework, enabling the grid to follow all interfaces. In principle, a separate grid is generated for each geological layer, patching the grid lines across the interfaces to obtain a globally continuous grid (the so‐called multiblock strategy).The curved grid is taken to constitute a generalised curvilinear coordinate system, where each grid line corresponds to a constant value of one of the curvilinear coordinates. That means that the forward modelling equations have to be written in curvilinear coordinates, resulting in additional terms in the equations. However, the subsurface geometry is much simpler in the curvilinear space.The advantages of the curved grid technique are demonstrated for the 2D acoustic wave equation. This includes a verification of the method against an analytic reference solution for wedge diffraction and a comparison with the pseudospectral method on Cartesian grids. The results demonstrate that high accuracies are obtained with few grid points and without extra computational costs as compared with Cartesian methods.
Elastic modelling on a grid with vertically varying spacing1
Authors: Jastram Cord , Tessmer Ekkehart 
Published: April 2006

AbstractWe present a discrete modelling scheme which solves the elastic wave equation on a grid with vertically varying grid spacings. Spatial derivatives are computed by finite‐difference operators on a staggered grid. The time integration is performed by the rapid expansion method. The use of variable grid spacings adds flexibility and improves the efficiency since different spatial sampling intervals can be used in regions with different material properties. In the case of large velocity contrasts, the use of a non‐uniform grid avoids spatial oversampling in regions with high velocities. The modelling scheme allows accurate modelling up to a spatial sampling rate of approximately 2.5 gridpoints per shortest wavelength. However, due to the staggering of the material parameters, a smoothing of the material parameters has to be applied at internal interfaces aligned with the numerical grid to avoid amplitude errors and timing inaccuracies. The best results are obtained by smoothing based on slowness averaging. To reduce errors in the implementation of the free‐surface boundary condition introduced by the staggering of the stress components, we reduce the grid spacing in the vertical direction in the vicinity of the free surface to approximately 10 gridpoints per shortest wavelength. Using this technique we obtain accurate results for surface waves in transversely isotropic media.
Cased borehole effects on downhole seismic measurements1
Authors: Peng Chengbin , Cheng Chuen Hon , Toksöz M. Nafi 
Published: April 2006

AbstractBoth approximate and exact formulations for the interaction of an incident elastic wave with a cased borehole are presented. In the approximate method, simple and explicit formulae are derived for the pressure in fluid at low frequencies. In the exact method, elastic potentials in each annulus are represented as a superposition of fundamental solutions to the Helmholtz equations. Continuity of displacements and stresses across layer boundaries are used to determine unknown coefficients. A global matrix algorithm is employed to compute simultaneously these coefficients in individual layers. Calculations show that, in cased boreholes, the borehole effects on downhole seismic measurements are more significant than in open boreholes. A strong resonance occurs in the fluid for SV‐wave incidence from a soft formation. This resonance is prominent even at very high frequencies because the tube‐wave velocity is raised well above the formation shear velocity by the steel pipe. At a particular angle of incidence of a plane P‐wave, the pressure in the fluid is near zero at low frequencies (the cased borehole screening phenomenon). For hard formations and frequencies above 1 kHz, the cased borehole influence on a downhole geophone measurement is significant, especially at grazing incidence. For soft formations, both the pressure in the fluid and the solid displacement on the borehole wall show strong dependence on frequency and angle of incidence, even at low frequencies.
Gravity interpretation of sedimentary basins with hyperbolic density contrast1
Authors: Rao C. Visweswara , Pramanik A.G. , Kumar G.V.R.K. , Raju M.L. 
Published: April 2006

AbstractThe variation in the density of sediments with depth in a sedimentary basin can be represented by a hyperbolic function. Gravity anomaly expressions for a 2D vertical prism and an asymmetric trapezium with a hyperbolic density distribution are derived in a closed form. These are used in inverting the gravity anomaly of a sedimentary basin with variable density. Firstly, the basin is viewed as a series of prisms juxtaposed with each other. The initial thickness of each prism is obtained from the gravity anomaly at its centre, based on the gravity anomaly of an infinite slab with a hyperbolic density contrast. These thicknesses are improved, based on the differences between the observed and the calculated anomalies. For an improved rate of convergence of the solution, these thicknesses may alternatively be refined using the well‐known ridge regression technique. Secondly, the basin is approximated by an asymmetric trapezium and its anomalies are inverted for the parameters of the trapezium using the ridge regression. Since this approximation serves to oversimplify the floor of the basin, it must be used only when the sediment‐basement interface has minor undulations. The results of a hypothetical case and two field cases (the San Jacinto Graben, California and the Godavari Graben, southern India) are presented. In both field cases, the interpreted depths are comparable with the real ones, proving the validity of the assumption of a hyperbolic density distribution of the sediments in the two basins considered.
A sample of controlled experiments in diff radion tomography1
Authors: Gelius L.‐J. 
Published: April 2006

AbstractVarious applications of a new geophysical reconstruction method, generalized acoustical diffraction tomography (GADT), which is based on transmission data as input are considered. Conventional diffraction tomography methods normally require linearization with respect to a uniform reference medium and regular sampling along a straight line. Thus, these methods will not work well when the background is strongly non‐uniform and/or the acquisition geometry is arbitrary. However, GADT can, in principle, handle both irregularly spaced data, curved acquisition lines, and non‐uniform background models.A number of controlled model tank and field experiments, where the model and the test object(s) are known a priori, have been carried out. After acquiring the tomographic data in each experiment, these are used to compute a reconstruction of the model, which can then be compared with the actual, known model. The method's ability to yield high‐quality images of the different targets is demonstrated.
2D modelling of resistivity and magnetotelluric data from the Belvedere Spinello salt mine, Italy1
Authors: Iliceto V. , Santarato G. , Zerilli A. 
Published: April 2006

AbstractThe Belvedere Spinello salt mine is located in the Catanzaro Province of Calabria in Southern Italy. An extensive mining program has caused the development of Underground cavities filled with brine and the migration of this brine has been of great environmental concern to the mine owners. This paper presents the results of a multidimensional interpretation of a two‐phase resistivity and magnetotelluric (MT) survey that was performed in an attempt to determine the complex conductivity structure of the mine area and to gain information on brine development and migration pathways. Key resistivity soundings were interpreted using a 2.5D algorithm based on the Polozhii decomposition method. The MT data were interpreted using a 2D finite‐element code. A conductivity model was developed, integrating available geological and drill‐hole information. The interpretation of the MT data, collected five years after the acquisition of the resistivity data, shows a conductive feature of depth that is not resolved in the resistivity interpretation. This feature has been interpreted as a thick brine zone that has developed as a result of mining during the interval between the resistivity and the MT measurements.
Tracking the amplitude versus offset (AVO) by using orthogonal polynomials1
Authors: Johansen T.A. , Bruland L. , Lutro J. 
Published: April 2006

AbstractInversion for S‐wave velocities from the amplitude variation with offset of P‐wave data is far from being a standard routine in the seismic processing sequence. However, the need for tracking the amplitude versus offset (AVO) occurs in several situations, for example in order to estimate the zero‐offset amplitude, to reveal areas with particular AVO characteristics, or to compress the AVO so that it is more easily obtainable at a later stage of the seismic processing. Furthermore, weak reflections can occasionally, due to the effect of the angle‐dependent reflectivity, have a polarity‐shift with offset, resulting in a very poor, or even vanishing, stack response. In such cases, the reflection event has to be represented by some other property than its mean amplitude or stack value.We outline how the AVO of seismic data may be extracted and classified by the use of orthogonal polynomials. The main advantage of this method compared to a general polynomial fit is that the AVO may be classified by a unique Spectrum of polynomial coefficients. This is in analogy to Fourier coefficients where the orthogonal basis is harmonic functions. The set of orthogonal polynomials is constructed entirely from the set of offset coordinates, and these polyno‐mials are defined only on the offset window considered. Compared to a Fourier transform, this is a major advantage since there is no effect of a limited spatial bandwidth.The AVO of normal‐moveout corrected data may be represented by a data gather where the orthogonal polynomial coefficients are given as time traces with each trace revealing a certain AVO characteristic. For instance, the stack is proportional to the zeroth‐order coefficient, the mean gradient is given by the firstorder coefficient, while the second‐order coefficient indicates whether the AVO increases and then decreases, or vice versa.
Phase inversion deconvolution for long and short period multiples attenuationl1
Authors: Lichrnan Eugene , Northwood E. John 
Published: April 2006

AbctractA new approach to deconvolution has been developed to improve the attenuation of multiple energy. This approach to deconvolution is unique in that it not only eliminates the usual assumptions of a minimum phase lag wavelet and a random distribution of impulses, but also overcomes the noise limitation of the homomorphic deconvolution and its inherent instability to phase computation.We attempt to analyse the continuous alteration of the acoustic waveform during the propagation through a linear system. Based on the results of this analysis, the surface‐related measurements are described as a convolution of the impulse response of the system with the non‐stationary forward wavelet which includes all multiple terms generated within the system.The amplitude spectrum of the forward wavelet is recovered from the amplitude spectrum of the recorded signal, using the difference between the rate of decay of the source wavelet and the duration of the measurement.The phase spectrum of the forward wavelet is estimated using the Hilbert transform and the fact that the mixed phase lag wavelet can be presented as a convolution of the minimum and maximum phase lag wavelets.The multiples are discriminated from primaries by comparison of the phase spectrum of the seismic signal and the inverse of the forward wavelet. Therefore, the technique is called phase inversion deconvolution (PID). This approach requires no velocity information in order to recognize and attenuate multiple energy. Therefore, primary energy is recovered in the near‐offset region where the velocity differential between primary and multiple energies is very small.
Quantitative evaluation of crosshole seismic reflection images using physical model data1
Authors: Rowbotharn Peter S. , Goulty Neil R. 
Published: April 2006

AbstractPhysical models give us a known geometry with which to compare our processed reflection images and therefore our imaging techniques. We show how this comparison may be quantified in order to evaluate processed images properly. A crosshole data set was acquired through a model interrogated at ultrasonic frequencies using Durham University's physical modelling system. Various reflectivity images were obtained using processing sequences which include deconvolution, wavefield separation and migration. An error‐energy scheme was used to assess the quality of these images, by comparing them against a best‐fit depth model. A synthetic data set was also used to evaluate the imaging capability of the crosshole geometry and the effectiveness of the different processing schemes.