bluuebunny/crossref_metadata_2025_split huggingface dataset:
STUDY OF TRAVELTIME AND AMPLITUDE TIME‐LAPSE TOMOGRAPHY USING PHYSICAL MODEL DATA1
Authors: LEGGETT M. , GOULTY N. R. , KRAGH J. E. 
Published: April 2006

AbstractIn seismic tomography the observed traveltimes or amplitudes of direct waves are inverted to obtain an estimate of seismic velocity or absorption of the section surveyed. There has been much recent interest in using cross‐well traveltime tomography to observe the progress of fluids injected into the reservoir rocks during enhanced oil recovery (EOR) processes. If repeated surveys are carried out, then EOR processes may be monitored over a period of time.This paper describes the results of a simulated time‐lapse tomography experiment to image the flood zone in an EOR process. Two physical models were made out of epoxy resins to simulate an essentially plane‐layered sedimentary sequence containing a reservoir layer and simple geological structure. The models differed only in the reservoir layer, which was uniform in the ‘pre‐flood’ model and contained a flood zone of known geometry in the ‘post‐flood’ model. Data sets were acquired from each model using a cross‐well survey geometry. Traveltime and amplitude tomographic imaging techniques have been applied to these data in an attempt to locate the extent of the flood zone.Traveltime tomography locates the flood zone quite accurately. Amplitude tomography shows the flood zone as a region of higher absorption, but does not image its boundaries as precisely. This is primarily because of multipathing and diffraction effects, which are not accounted for by the ray‐based techniques for inverting seismic amplitudes. Nevertheless, absorption tomograms could complement velocity tomograms in real, heterogeneous reservoirs because absorption and velocity respond differently to changes in liquid/gas saturations for reservoir rocks.
THREE‐DIMENSIONAL INTERPRETATION OF MULTIPLE‐SOURCE BIPOLE‐DIPOLE RESISTIVITY DATA USING THE APPARENT RESISTIVITY TENSOR1
Authors: BIBBY H.M. , HOHMANN G.W. 
Published: April 2006

AbstractThe bipole‐dipole resistivity technique, which uses a single current source (bipole) to map variations in (apparent) resistivity has been much criticized in the past. A series of 3D models are used to show that the use of two distinct current bipoles in the same location but with different orientations, combined with analysis in the form of a previously defined tensor apparent resistivity, can greatly improve many aspects of bipole‐dipole mapping.The model study shows that, for measurement stations more than a few bipole lengths from the current source, the apparent resistivity tensor behaves, to a close approximation, as though the current bipoles are idealized dipoles, and hence is independent of the orientation of the individual current sources used. Any pair of current bipoles (in the same location but with different orientations) can therefore be used to determine the tensor resistivity properties.The invariants of the apparent resistivity tensor have considerable advantages over the normal scalar apparent resistivities. Modelling shows that although the electric field vector corresponding to a single current bipole can be highly perturbed by a local inhomogeneity for some considerable distance beyond the inhomogeneity itself, the tensor invariants are virtually unperturbed beyond the extent of the inhomogeneity. Hence false anomalies, which are a characteristic of apparent resistivity measured using only single current bipole models, are almost completely eliminated by the use of tensor invariants. Of the possible tensor invariants, the invariant given by the square root of the determinant gives the best representation of a buried 3D body. Resistivity anomalies are localized, and occur only over the causative body. Even with complex models involving several buried bodies, the tensor invariants clearly delineate the extent of each body. Outside the bounds of perturbing bodies, the tensor data can be analysed by conventional techniques, for example, to determine layered structure.
APPROXIMATIONS TO SHEAR‐WAVE VELOCITY AND MOVEOUT EQUATIONS IN ANISOTROPIC MEDIA1
Authors: LI XIANG‐YANG , CRAMPIN STUART 
Published: April 2006

AbstractBackus and Crampin derived analytical equations for estimating approximate phase‐velocity variations in symmetry planes in weakly anisotropic media, where the coefficients of the equations are linear combinations of the elastic constants. We examine the application of similar equations to group‐velocity variations in off‐symmetry planes, where the coefficients of the equations are derived numerically. We estimate the accuracy of these equations over a range of anisotropic materials with transverse isotropy with both vertical and horizontal symmetry axes, and with combinations of transverse isotropy yielding orthorhombic symmetry. These modified equations are good approximations for up to 17% shear‐wave anisotropy for propagations in symmetry planes for all waves in all symmetry systems examined, but are valid only for lower shear‐wave anisotropy (up to 11%) in off‐symmetry planes.We also obtain analytical moveout equations for the reflection of qP‐, qSH‐, and qSV‐ waves from a single interface for off‐symmetry planes in anisotropic symmetry. The moveout equation consists of two terms: a hyperbolic moveout and a residual moveout, where the residual moveout is proportional to the degree of anisotropy and the spread length of the acquisition geometry. Numerical moveout curves are computed for a range of anisotropic materials to verify the analytical moveout equations.
VARIATION OF REFLECTION AND TRANSMISSION COEFFICIENTS WITH CRACK STRIKE AND CRACK DENSITY IN ANISOTROPIC MEDIA1
Authors: LI XIANG‐YANG , CRAMPIN STUART 
Published: April 2006

AbstractRecent observations show that the differential amplitudes between the faster and slower split shear‐waves in reflection surveys contain information about lateral variations of crack density in cracked reservoirs. However, the variation of amplitude with crack geometry when the crack strike changes with depth has not been reported previously. In this paper, we derive expressions for reflection and transmission coefficients of plane split shear‐waves at vertical incidence at an interface separating two cracked (anisotropic) media with different crack strikes. We examine the effects on these coefficients as crack strike and crack density vary. For interfaces with large velocity‐contrasts, the reflection coefficients carry little information about crack geometry, and the effects of crack strike varying with depth are negligible. In such cases, the polarization and time‐delay of the split shear‐waves are the only features which reliably diagnose anisotropy and contain information about the variation of crack strike and density. However, for interfaces with small velocity‐contrasts, the effects of any variation of crack strike with depth cannot be neglected. In such cases, in addition to the polarization and time‐delays of split shear‐waves, both the differential amplitude of faster and slower shear‐waves and the amplitude ratio of the two off‐diagonal elements in the reflected data matrix after separation of split shear‐waves, contain information about the variation of crack strike and crack density. In contrast, effects of crack strike changing with depth on transmitted waves are more sensitive regardless of the velocity‐contrast and the degree of anisotropy.
IMAGING CAPABILITY OF CROSS‐HOLE SEISMIC REFLECTION SURVEYS1
Authors: ROWBOTHAM P. S. , GOULTY N. R. 
Published: April 2006

AbstractIn seismic migration, it is important to sample a range of dips around the local structural dip at each image point. Meaningful images are obtained only where this condition holds. For cross‐hole seismic reflection surveys, the distribution of dips sampled at each image point is controlled principally by the survey geometry, including source and receiver array lengths and their element spacings. Using a real data set as an example, we show how survey geometry can limit imaging capability close to the boreholes and even in the middle of the section between the boreholes.At the processing stage, effective removal of direct waves and accurate estimation of the velocity field are essential for optimizing image quality. For migration, we propose a generalized Berryhill (GB) scheme which is based on the Kirchhoff integral and takes into account both the near‐field and far‐field terms. This should improve the ability to image close to source and receiver arrays, provided that the element spacing in the nearby array is small enough.
THE FRACTAL DIMENSION OF GRAVITY DATA SETS AND ITS IMPLICATION FOR GRIDDING1
Authors: KEATING PIERRE 
Published: April 2006

AbstractGravity survey station locations are, in general, inhomogeneously distributed. This inevitably results in interpolation errors in the computation of a regular grid from the gravity data. The fractal dimension of the station distribution can be used to determine if an interpolated map is aliased at a specific wave‐length and, moreover, it is often possible to determine an optimum gridding interval. Synthetic distributions of gravity station locations have been used for theoretical studies and it is found that for randomly distributed data there is a range of sizes for which the spatial data distribution has a fractal dimension of 2; that is, Euclidean. The minimum length scale at which the distribution ceases to be Euclidean is the optimum interpolation interval obeying Shannon's sampling theorem. For dimensions less than 2, the optimum interpolation interval is the shortest length at which the scaling regime is constant. In this case the gravity field cannot be interpolated without introducing some aliasing. As the fractal dimension characterizes the data distribution globally over the whole study area, the actual gridding interval, in some cases, will be smaller in order to represent short‐wavelength features properly in the more densely sampled sub‐areas, but this may generate spurious anomalies elsewhere. The proposed technique is applied to the station distribution of the Canadian national gravity data base and a series of sub‐areas. A fractal dimension of 1.87 is maintained over a range of sizes from 15 km to over 1600 km. Although aliasing occurs, since the gravity field certainly contains much shorter wavelength anomalies, aliasing errors may be minimized by selecting the proper interpolation interval from the fractal analysis.
ALGEBRAIC PROCESSING TECHNIQUES FOR ESTIMATING SHEAR‐WAVE SPLITTING IN NEAR‐OFFSET VSP DATA: THEORY1
Authors: ZENG XINWU , MACBETH COLIN 
Published: April 2006

AbstractA vector convolutional model for multicomponent data acquired in an anisotropic earth is used as a basis for developing algebraic solutions to interpret near‐offset VSP data. This interpretation of the cumulative or interval medium response (Green's tensor) for shear waves, determines a polarization azimuth for the leading shear wave and the time‐delay between the fast and slow split waves. The algebraic solutions effectively implement least‐squares eigenanalysis or singular value decomposition. Although the methodology for shear‐wave analysis is strictly relevant to a transmission response, it can be adapted to surface data for a uniform anisotropic overburden. The techniques perform well when calibrated and tested using synthetic seismograms from various anisotropic models. Noise tests demonstrate the sensitivity of the interval measurements to local interferences, particularly if the shear waves are generated by one source. Although the algorithms are faster than numerical search routines, this is not seen as their major advantage. The solutions may have potential in near real‐time interpretation of shear‐wave data in well logging, where they may be coded on a microchip to provide a direct stream of separated shear waves, or polarization and birefringence information. There may also be some benefit for large prestack multicomponent surface data sets, where the solutions provide a direct transformation to the split‐shear‐wave components, reducing the storage space for further processing.
On reducing ambiguity in the interpretation of transient electromagnetic sounding data1
Authors: Goldman M. , Du Plooy A. , Eckard M. 
Published: April 2006

AbstractAn attempt to resolve non‐uniqueness in the interpretation of transient electromagnetic (TEM) sounding data using measured data alone is made. It is shown in the various examples studied that sufficiently early time measurements can be the determining factor in reducing the ambiguity caused by model equivalence. The early delay times thus play a dual role in transient soundings: they are responsible for resolving shallow structures and they may eliminate the ambiguity in the interpretation of geoelectric parameters of deeper targets. This is illustrated by the results of a follow‐up TEM survey at the Dead Sea coast of Israel where the use of supplementary early time measurements allowed non‐uniqueness in the determination of the depth to fresh/saline groundwater interface to be resolved.
Seismic velocities in fractured rocks: an experimental verification of Hudson's theory1
Authors: Peacock S. , McCann C. , Sothcott J. , Astin T.R. 
Published: April 2006

AbstractFlow of fluids in many hydrocarbon reservoirs and aquifers is enhanced by the presence of cracks and fractures. These cracks could be detected by their effects on propagation of compressional and shear waves through the reservoir: several theories, including Hudson's, claim to predict the seismic effects of cracks. Although Hudson's theory has already been used to calculate crack densities from seismic surveys, the predictions of the theory have not yet been tested experimentally on rocks containing a known crack distribution. This paper describes an experimental verification of the theory. The rock used, Carrara marble, was chosen for its uniformity and low porosity, so that the effect of cracks would not be obscured by other influences. Cracks were induced by loading of laboratory specimens. Velocities of compressional and shear waves were measured by ultrasound at 0.85 MHz in dry and water‐saturated specimens at high and low effective pressures. The cracks were then counted in polished sections of the specimens. In ‘dry’ specimens with both dry and saturated cracks, Hudson's theory overpredicted observed crack densities by a constant amount that is attributed to the observed value being systematically underestimated. The theory made poor predictions for fully saturated specimens. Shear‐wave splitting, caused by anisotropy due to both crystal and crack alignment, was observed. Cracks were seen to follow grain boundaries rather than the direction of maximum compression due to loading. The results demonstrate that Hudson's theory may be used in some cases to determine crack and fracture densities from compressional‐ and shear‐wave velocity data.
Migration to multiple offset and velocity analysis1
Authors: Ferber R.‐G. 
Published: April 2006

AbstractThe stacking velocity best characterizes the normal moveout curves in a common‐mid‐point gather, while the migration velocity characterizes the diffraction curves in a zero‐offset section as well as in a common‐midpoint gather. For horizontally layered media, the two velocity types coincide due to the conformance of the normal and the image ray. In the case of dipping subsurface structures, stacking velocities depend on the dip of the reflector and relate to normal rays, but with a dip‐dependent lateral smear of the reflection point. After dip‐moveout correction, the stacking velocities are reduced while the reflection‐point smear vanishes, focusing the rays on the common reflection points. For homogeneous media the dip‐moveout correction is independent of the actual velocity and can be applied as a dip‐moveout correction to multiple offset before velocity analysis.Migration to multiple offset is a prestack, time‐migration technique, which presents data sets which mimic high‐fold, bin‐centre adjusted, common‐midpoint gathers. This method is independent of velocity and can migrate any 2D or 3D data set with arbitrary acquisition geometry. The gathers generated can be analysed for normal‐moveout velocities using traditional methods such as the interpretation of multivelocity‐function stacks. These stacks, however, are equivalent to multi‐velocity‐function time migrations and the derived velocities are migration velocities.
The fast Hankel transform1
Authors: Mohsen Adel A. , Hashish E.A. 
Published: April 2006

AbstractAn efficient algorithm is presented to compute the Hankel transform. The algorithm yields simultaneously all the required weights for a given order of the Bessel function using the fast Fourier transform. An additional shift is introduced to the filter abscissa besides Koefoed's shift to give a better filter performance.
Layer‐stripping reverse‐time migration1
Authors: Shih Ruey‐Chyuan , Levander Alan R. 
Published: April 2006

AbstractWe present a layer‐stripping method of migration for irregularly layered media in which first‐order velocity discontinuities separate regions of constant or smoothly varying velocity. We use the reverse‐time method to migrate seismic data layer by layer, from the surface downwards. As part of the migration of a given layer, the bottom boundary of the layer is defined based on power in the migrated signal, and a seismic section is collected along it. This new section serves as the boundary condition for migration in the next layer. This procedure is repeated for each layer, with the final image formed from the individual layer images. Layer‐stripping migration consists of three steps: (1) layer definition, (2) wavefield extrapolation and imaging, and (3) boundary determination. The migration scheme when used with reverse‐time extrapolation is similar to datuming with an imaging condition. The reverse‐time method uses an explicit fourth‐order time, tenth‐order space, finite‐difference approximation to the scalar wave equation.The advantages of layer‐stripping reverse‐time migration are: (1) it preserves the benefits of the reverse‐time method by handling strong velocity contrasts between layers and steeply dipping structures; (2) it reduces computer memory and saves computation time in high‐velocity layers, and (3) it allows interpretational control of the image.Post‐stack layer‐stripping reverse‐time migration is illustrated with a synthetic CMP data example. Prestack migration is illustrated with a synthetic data set and with a marine seismic reflection profile across the Santa Maria Basin and the Hosgri Fault in central California.
Two‐level magnetovariational measurements for the determination of underground resistivity distributions1
Authors: Patella D. , Siniscalchi A. 
Published: April 2006

AbstractWe investigate, from a theoretical point of view, the possibility of performing marine two‐level magnetovariational measurements. An apparent resistivity function is denned and calculated after solving the differential equation governing the behaviour of the natural magnetic field variations inside a one‐dimensional earth. In order to generalize the problem, a frequency‐dependent resistivity is assumed to characterize the layers and the distortions caused by the polarization effects are carefully analysed. The computation of three‐layer amplitude and phase diagrams for the apparent resistivity function shows that, in the case of an intermediate polarizable layer, sandwiched between a non‐dispersive overburden and substratum, the H‐type sequence results are the most affected by the dispersion phenomenon as it occurs in magnetotellurics. Finally we consider the problem of the sensitivity of the method, since, in practice, it requires top and bottom sensors separated by a vertical finite distance. It is found that in the higher‐frequency range, due to the strong attenuation of the relative components of the field, the depth of the bottom sensor must be small enough to guarantee detectable signals, well above the full‐scale resolution of the acquisition system. Conversely, in the lower‐frequency range such a depth must be large enough to allow the difference between the top and bottom signals to be above the same recording sensitivity threshold.
AVO signatures of actual and synthetic reflections from different petrophysical targets1
Authors: Mazzotti A. , Melis A.M. , Ravagnan G. , Bernasconi G. 
Published: April 2006

AbstractWe use a marine seismic dataset to examine the reflections from two gas sands, a lignitic sand and a cineritic bed, by means of their amplitude versus offset (AVO) responses. This offset‐dependent signature is related to specific petrophysical and thus elastic situations or to peculiar interference patterns and may help to distinguish the nature of the amplitude anomalies on the stack sections.The prestack analysis is carried out on seismic data which have undergone an accurate true‐amplitude processing.It is found that the lignitic‐sand reflections exhibit a decreasing AVO while the two‐gas sands show markedly increasing AVO trends. Also the reflections from the cineritic layer show increasing amplitudes with offset that may be due either to the petrophysical nature of the cinerites or to thin‐layer interference or to both.In order to verify the reliability of the actual AVO responses we develop a detailed model from well data and compute a synthetic CMP seismogram. In order to account for mode conversions and thin‐layer effects, the synthetic seismograms are computed using the reflectivity method. The wavelets used in the synthetics are retrieved from actual seismic and borehole data by means of wavelet processing. When finely layered structures are present, the estimation of a reliable wavelet is extremely important to get the correct synthetic AVO response. In particular, the AVO responses of the cineritic layer differ substantially if we make use in the computation of the synthetics of a Ricker wavelet or of a wavelet estimated through wavelet processing.The good match between the observed and modelled data confirms the reliability of the processing sequence and of the final AVO signatures.