Showing papers in "Seg Technical Program Expanded Abstracts in 1998"

Imaging dispersion curves of surface waves on multi-channel record

Abstract: Summary Real and synthetic data verifies the wavefield transformation method described here converts surface waves on a shot gather directly into images of multi-mode dispersion curves. Pre-existing multi-channel processing methods require preparation of a shot gather with exceptionally large number of traces that cover wide range of source-to-receiver offsets for a reliable separation of different modes. This method constructs high-resolution images of dispersion curves with relatively small number of traces. The method is best suited for near-surface engineering project where surface coverage of a shot gather is often limited to near-source locations and higher-mode surface waves can be often generated with significant amount of energy.

Wave Equation Prediction And Removal of Interbed Multiples

Abstract: The succes of multiple suppression for seismic data often hinges on the accuracy with which the multiple behaviour can be predicted. This is especially true for interbed multiples where small details can be essential for distinguishing the multiples from surrounding primaries. In this paper we describe a new wave equation method of predicting interbed multiples from primary events. This scheme can be used to estimate multiples directly from measured data and has been incorporated into a new suppression technique.

Automated 3D Tomographic Velocity Analysis of Residual Moveout In Prestack Depth Migrated Common Image Point Gathers.

Abstract: Residual moveout analysis of prestack depth migrated common image point gathers is used for velocity model building in areas of complex geology. Where the velocity varies smoothly, the model is commonly built from stacking velocity analysis of the residual curvature on the gathers after conversion back to time. Where the velocity varies more rapidly, the model must be built instead by projecting the residual depth errors back over individually traced raypaths in a full tomographic inversion in depth. In this paper we show results of the latter method applied to a Gulf Coast data set. The 3D field example was run in a highly automated fashion. The depth errors (residual moveout) were first picked in batch mode throughout a sparsely sampled volume of common image point gathers and then minimized through a global inversion of all picks at all depths simultaneously. The inverse problem was constrained with preconditioning to solve for the smoothest part of the velocity field first. This automated approach improves turnaround by minimizing human intervention.

Faster shot-record depth migrations using phase encoding

Abstract: Phase encoding of shot records provides a means of imaging a number of shots within a single migration. This results in a reduction in the required computation for a complete image, a reduction by the number of shots used in each individual migration, trading this increase in speed for additional noise in the resulting image. Some methods for phase encoding have been shown to limit this noise to a tolerable range when combining several shots, enabling speed ups of a factor of a few. In this paper, the authors present a use of phase encoding which allows faster imaging by an order of magnitude or more, with the additional benefit that the individual migrations can be stopped whenever the answer is good enough. This approach may ultimately render 3-D frequency-domain prestack depth migration cost effective.

An Acoustic Wave Equation For Anisotropic Media

Abstract: A wave equation, derived using the acoustic medium assumption for P-waves in transversely isotropic (TI) media with a vertical symmetry axis (VTI media), yields a good kinematic approximation to the familiar elastic wave equation for VTI media. The wavefield solutions obtained using this VTI acoustic wave equation are free of shear waves, which significantly reduces the computation time compared to the elastic wavefield solutions for exploding‐reflector type applications. From this VTI acoustic wave equation, the eikonal and transport equations that describe the ray theoretical aspects of wave propagation in a TI medium are derived. These equations, based on the acoustic assumption (shear wave velocity = 0), are much simpler than their elastic counterparts, yet they yield an accurate description of traveltimes and geometrical amplitudes. Numerical examples prove the usefulness of this acoustic equation in simulating the kinematic aspects of wave propagation in complex TI models.

Ground Roll As a Tool to Image Near-surface Anomaly

Abstract: On an uncorrelated field record obtained using a monotonic sweep, ground roll is displayed in increasing or decreasing order of frequency with each frequency well separated from all others Phase velocity and attenuation characteristics of each frequency contain the average elastic property of nearsurface materials down to approximately half the wavelength An uncorrelated field record, therefore, by itself can be associated with a two-dimensional display of the change in near-surface elastic property Through the redundancy in data acquisition and a simple data processing step, the uncorrelated field records can be transformed into a stacked section that can be correlated directly to the image of the change in elastic property of near-surface materials This method can be effectively used to detect near-surface anomalies of various kinds

Common Angle Image Gather - a Strategy for Imaging Complex Media

Abstract: Common image panels are commonly used for migration based velocity analysis and for migration based AVO analysis. The advantages of common offset image gather with respect to common shot image gathers are now well established [4] but also their poor quality in case of complex models [3]. The reasons of this failure may appear when the problem is analyzed in the frame of the asymptotic linearized inverse theory [7, 6]. We propose here an analysis of the relevance of common image gather analysis in case of velocity models with triplicated ray fields. We show that individual common shot or common offset may provide inconsistent images of the subsurface, and we propose a strategy for common anale imaging avoiding the former artefacts. Comparisons of common shot, common offset and common angle image gathers are presented for 2D complex Marmousi dataset.

Elastic modeling initiative, part III: 3-D computational modeling

Abstract: A 3-D finite-difference elastic wave propagation code that incorporates a number of advanced computational and physics-based enhancements has been developed. These enhancements include full 3-D elastic, viscoelastic, and topographic modeling (anisotropic capabilities arc being added), low-level optimization, propagating and variable density grids, hybridization, and parallelization. This code takes advantage of high performance computing and massively parallel processing to make 3-D full-physics simulations of seismic problems feasible. This computational tool will be used to generate an elastic subset of the SEG/EAEG acoustic data set. The acoustic and elastic data will be compared to examine pitfalls with traditional processing, and to test the effectiveness of using elastic data as an aid to seismic imaging.

The Simplest Discrete Radon Transform

Abstract: Pitfalls in the evaluation of the discrete Radon transform are reviewed and methods are identified that overcome the problems. A commonly used discrete form of the Radon integral formula results in two types of artifacts being generated: operator aliasing artifacts and truncation artifacts. Operator aliasing artifacts can be avoided by taking into account the bandlimited nature of the integrand during the conversion of the Radon integral into a discrete summation. Truncation artifacts are due to aperture limitation of the input data (missing offsets) and can be overcome by imposing a minimum entropy constraint in model space that forces the solution to be as simple as possible. Eliminating these two types of artifacts produces a clean, simple, and more highly resolved image in the Radon domain for any application such as multiple elimination, predictive deconvolution or migration.

Airborne Electromagnetic Systems - 50 Years of Development

Abstract: The initial successful test flights of the Stanmac-McPhar fixed-wing airborne EM (AEM) system in Canada during the summer of 1948 can nominally be called the birth of this branch of exploration geophysics. The discovery of the Heath Steele deposit in New Brunswick, Canada in 1954, as a result of an AEM survey, proved to be the catalyst for the development of additional AEM systems and the eventual application of AEM surveys worldwide.

Noise attenuation with wavelet transforms

Abstract: The wavelet domain is convenient for various seismic data processing tasks, such as noise attenuation, because data are represented in time and frequency simultaneously. A method that combines a wavelet transform and localized semblanceweighted hyperbolic velocity filtering is presented for different types of noise attenuation. One benefit of applying noise attenuation in the wavelet domain is that some spectral components of signal are preserved untouched, and only the part of the signal whose frequency band overlaps with noise is filtered. A test example with symmetric-sampled 3D cross spread data shows that after wavelet transform ground roll noise attenuation, the data quality is largely improved, particularly for low CMP fold areas and deep portions of the data which are contaminated by noise.

Applying statistical rock physics and seismic inversions to map lithofacies and pore fluid probabilities in a North Sea reservoir.

Abstract: Summary Reliably predicting lithologic and saturation heterogeneities is one of the key problems in reservoir characterization. In this study we show how statistical rock physics techniques combined with seismic information can be used to classify reservoir lithologies and pore fluids. One of the innovations was to use a new seismic impedance attribute (related to VP/VS ratio) that incorporates far-offset data, but at the same time can be practically obtained using normal incidence inversion algorithms. The methods were applied to a North Sea turbidite system. We incorporated well log measurements with calibration from core data to estimate the near and far-offset reflectivity and impedance attributes. Multivariate probability distributions were estimated from the data to identify the attribute clusters and their separability for different facies and fluid saturations. A training data was set up using Monte Carlo simulations based on the well log derived probability distributions. Fluid substitution b y Gassmann’s equation was used to extend the training data, thus accounting for pore fluid conditions not encountered in the well. Seismic inversion of near offset and far offset stacks gave us two 3-D cubes of impedance attributes in the interwell region. The near offset stack approximates a zero offset section giving an estimate of the normal incidence acoustic impedance (rV). The far offset stack gives an estimate of a VP/VS related elastic impedance attribute that is equivalent to the acoustic impedance for non-normal incidence. These impedance attributes obtained from seismic inversion were then used with the training probability distribution functions to predict the probability of occurrence of the different lithofacies in the interwell region. Simple statistical classification techniques, as well as geostatistical indicator simulations were applied on the 3-D seismic data cube. A Markov-Bayes technique was used to update the probabilities obtained from the seismic data by taking into account the spatial correlation as estimated from the facies indicator variograms. The final results are spatial 3-D maps of not only the most likely facies and pore fluids, but also their occurrence probabilities. A key ingredient in this study was the exploitation of physically-based seismic-toreservoir properties transforms optimally combined with statistical techniques.

Prestack depth migration using a hybrid pseudo‐screen propagator

Abstract: A hybrid pseudo-screen propagator is applied to prestack depth migration for recursive wave eld backpropagation. The propagator is decomposed into a phase-screen term and a large-angle compensation term, the latter is computed by a FD scheme. Downw ardextrapolated waveelds are calculated rst by a phase shift of the primary w ave eld propagating in the bac kground medium. The scattering contributions generated by the velocity heterogeneities are then superposed. Numerical examples show the ability of the method to handle the steep dip events in the case of strong lateral velocity variation and to pro vide a good image when applied to the Marmousi dataset.

Calibration And Inversion of Non-Zero Offset Seismic

Abstract: Summary Many basins throughout the world show significant seismic AVO response within at least some depth window. The effect of this on stacked seismic data cannot be ignored when tying to well logs and in many cases can provide valuable additional information. However the extraction of the AVO information from large volumes of 3D data presents many practical and theoretical difficulties. Because of the enormous storage requirements of prestack 3D a common compromise is to assess AVO characteristics using a small number of substacks or angle stacks. I present here a method of constructing substacks to correspond to specific incidence angles and an expression for a petrophysical curve which is analogous to acoustic impedance but for non-zero incidence angles. I refer to this as Elastic Impedance. The combination of a constant angle substack and an Elastic Impedance curve for the same angle provides a framework for the calibration and inversion of non-zero offset seismic data using routinely available software applications.

Fracture Detection Using P-P And P-S Waves In Multicomponent Sea-floor Data

Abstract: The azimuthal variations in P -P amplitude, velocity, and interval moveout show elliptical variations in azimuthally anisotropic media. This can be used to determine the fracture strike of the medium and has been veri ed from real data. However, the P wave e ects only occur at su ciently large o sets with multi-azimuths, and are often complicated by other factors. This limits the application of P -wave analysis to some extent. Analysis of P -S waves may thus prove to be bene cial. For near vertical propagating P -S waves, the polarization and time delay of the shear-wave provide a direct measurement of the fracture orientation and intensity. The fracture strike (polarization azimuth) is diagnosed by a polarity change and amplitude dimming in the azimuthal gathers of the transverse-geophone component. Based on this, a robust method is presented for recovering the fracture strike using a 3D cross geometry where the source boat sails across the receiver cable.

Seismic Reflectivity of a Thin Porous Fluid-saturated Layer Versus Frequency

Abstract: We pay attention to seismic reflection from thin reservoir layers using both results of field experiment and physical modeling. Each case is compared with numerical modeling applying both elastic and poroelastic Biot model. The comparison shows that even at low frequencies there are some differences in reflection response of elastic and poroelastic layers although they strongly depend on medium parameters.

Nonhyperbolic Reflection Moveout For Azimuthally Anisotropic Media

Abstract: Reflection moveout in azimuthally anisotropic media is not only azimuthally dependent but it is also nonhyperbolic. As a result, the conventional hyperbolic normal moveout (NMO) equation parameterized by the exact NMO (stacking) velocity loses accuracy with increasing offset (i.e., spreadlength). This is true even for a single-homogeneous azimuthally anisotropic layer. The most common azimuthally anisotropic models used to describe fractured media are the horizontal transverse isotropy (HTI) and the orthorhombic (ORT) symmetry. Here, we introduce an analytic representation for the quartic coefficient of the Taylor's series expansion of the two-way traveltime for pure mode reflection (I.e., no conversion) in arbitrary anisotropic media with arbitrary strength of anisotropy. In addition, we present an analytic description of the long-spread (large-offset) nonhyperbolic reflection moveout (NHMO). In multilayered azimuthally anisotropic media, the NMO (stacking) velocity and the quartic moveout coefficient can be calculated with good accuracy using the known averaging equations for VTI media. The interval NMO velocities and the interval quartic coefficients, however, are azimuthally dependent. This allows us to extend the nonhyperbolic moveout (NHMO) equation, originally designed for VTI media, to more general horizontally stratified azimuthally anisotropic media. As a result, our formalism allows rather simple transition from VTI to azimuthally anisotropic media. Numerical examples from reflection moveout in orthorhombic media, the focus of this paper, show that this NHMO equation accurately describes the azimuthally-dependent P-wave reflection traveltimes, even on spreadlengths twice as large as the reflector depth. This work provides analytic insight into the behavior of nonhyperbolic moveout, and it has important applications in modeling and inversion of reflection moveout in az-

Have a look at the resolution of prestack depth migration for any model, survey and wavefields.

Abstract: Summary Integral equation approaches to acoustic imaging within the Born or Kirchhoff approximation give compact algorithms for depth migration, using pre-calculated Green’s functions. They moreover give direct access to a fundamental parameter: the scattering wavenumber. This parameter controls the resolution of the depth migrated section. Using efficient methods to obtain asymptotic Green’s functions, resolution functions can now be calculated numerically in both 2D and 3D. Any location in any background model can be considered. All kind of surveys and subselections of shot/ receiver couples are possible. As multi-field imaging is available, new set of scattering wavenumbers may be considered. Resolution functions are important at a survey planning stage to define the optimal acquisition. They are also essential for the interpretation of depth migrated sections, i.e., to discriminate between true reflections and artificial events due to bandlimited signals and limited-aperture. Such resolution effects should be corrected prior to any measurements of reflectivity.

Inversion of Airborne Electromagnetic Data

Abstract: A BSTR A C T A irborne electrom agnetic geophysics is based on analysis o f the interaction o f an electrom agnetic field w ith the geoelectric properties o f the earth. Inversion, or inverse m odelling, o f airborne electrom agnetic (AEM ) data refers to a particular m athem atical m ethodology for solving the A E M inverse problem , that is. ded u cin g the e a rth ’s geoelectric p ro p erties fro m o b served electro m ag n etic interactions. This is a difficult problem for several reasons. First, like m ost geophysical inverse problem s, the A EM inverse problem w ith a finite num ber o f noisy data is illposed, and consequently, the geoelectric properties o f the earth cannot be uniquely determ ined. To generate a unique solution a priori inform ation m ust be added to the inverse problem : a procedure referred to as regularisation. Second, since the geoelectric properties of the earth and the observed A EM data are not linearly related the inverse problem is nonlinear and requires solution by an iterative m ethod. Third, the forw ard problem o f calculating the response from a given geoelectric earth m odel, w hich is an essential part o f the inverse problem , is itself a difficult and tim e consum ing problem for 2.5D or 3D models. Fourth, A E M geophysics is characterised by enorm ous q u an titie s o f data. T hese difficulties and how they can be addressed are the focus o f this paper. Particular em phasis is placed on the non­ uniqueness o f the A E M inverse problem and how it can be resolved through regularisation using a priori inform ation. The applicability o f ID inversion in m ulti-dim ensional environm ents and the advantages o f m ulti-dim ensional inversion are dem onstrated, as is the potential value o f joint inversion o f A EM data and other geophysical data. IN T R O D U C T IO N

AVO attributes and noise: pitfalls of crossplotting

Abstract: Summary AVO crossplotting has been widely used in the past few years as a way of deriving improved hydrocarbon indicators from seismic data. By crossplotting the standard AVO attributes of intercept and gradient, it is possible to obtain an optimum combination of the two (the fluid factor), which is insensitive to the AVO effect of brine-saturated shales and sands. Any remaining AVO anomaly can then be attributed to hydrocarbons or other lithologic factors. In addition, the physical location of an AVO anomaly on the crossplot gives an indication as to the geological setting of the potential reservoir. However, like the stack, intercept and gradient are sensitive to noise. While the intercept standard-deviation is slightly higher than the stack, the gradient standard-deviation is much larger and dramatically increases with travel-time. This partly explains the scale difference between the two attributes that is always observed with real data. Furthermore, in the presence of noise, intercept and gradient become statistically correlated. This correlation biases fluid factor calculations so that this attribute is reduced to a mere far-offset stack. Since stack and gradient do not correlate statistically, their crossplot can be used to validate or dismiss a trend observed in an intercept versus gradient crossplot. If the trend still exists, albeit slightly rotated, in the stack versus gradient crossplot, it is a lithologic effect; if not, it is a statistical artifact. Applications to real data have shown that, in general, what could be interpreted as a background shale trend is in fact noise-related. Consequently, the calculated fluid factor corresponds to a far-offset stack, which is actually a legitimate hydrocarbon indicator, but not as good as the theoretical fluid factor. Improved hydrocarbon indicators can nonetheless be obtained using the statistical independence of stack and gradient.

A Cross-equalization Processing Flow For Off-the-shelf 4D Seismic Data

Abstract: SUMMARY Seismic reservoir monitoring is a new technology that involves interpreting differences between different generations of 3-D seismic data in terms of changes in reservoir properties over time. A vast number of 3-D seismic surveys are repeated, in whole or in part, for reasons other than reservoir monitoring. In this paper, we cross-equalize two off-the-shelf migrated datacubes over a Gulf of Mexico field with favorable reservoir properties, to see whether their time-lapse nature can be exploited for reservoir monitoring purposes. Key elements in the processing flow include spatial realignment to a common grid, wavelet equalization by matched-filtering, warping to compensate for different stacking/migration velocities, and amplitude balancing. A balance is struck between global and local operators to ensure a clean cross-equalization result without inadvertently removing changes due to fluid production.