Showing papers in "Seg Technical Program Expanded Abstracts in 2000"

Fast-track 'coloured' inversion

Abstract: Summary Inversion of seismic data to Acoustic Impedance is usually seen as a specialist activity, so despite the publicised benefits, inverted data are only used in a minority of cases. To help overcome this obstacle we aimed to develop a new algorithm which would not necessarily be best in class, but would be quick and easy to use and increase the use of inversion products within BPA. This new technique, ‘Coloured Inversion’, performs significantly better than traditional fast-track routes such as recursive inversion, and benchmarks well against unconstrained sparse-spike inversion. Empirical study has revealed that unconstrained sparsespike inversion can be approximately modeled as a convolutional process, with an operator whose amplitude spectrum maps the mean seismic spectrum to the mean earth AI spectrum, and has a phase of -90 o . In addition, following reflectivity work by Walden & Hosken (1985) we have found that the gross spectral form of AI logs from wells in any given field is reasonably constant. This implies that a single convolutional operator can be used to perform inversion. The approach assumes a zero-phase wavelet but does not need an explicit estimate of the wavelet amplitude spectrum. The zero-phase assumption can be compensated for by phase rotation of the inverted data in comparison with the AI logs.

Three-Dimensional Texture Attributes For Seismic Data Analysis

Abstract: In this paper we describe several new 3D texture attributes for automated interpretation of seismic data. The attributes, dip, azimuth, chaotic texture, and continuity, are robust with respect to properties such as tilted layers and amplitude variations. They are applicable to mapping of for example gas chimneys, structural features such as fault systems and salt domes and stratigraphic features such as carbonate reefs, sand channels, and fan systems.

Locating Magnetic Contacts: a Comparison of the Horizontal Gradient, Analytic Signal, And Local Wavenumber Methods

Abstract: Three methods for locating isolated magnetic contacts from magnetic anomaly data are examined. All methods are similar in that they involve passing a small window over a derivative profile or grid, searching for local maxima within the window, estimating the strike (for grids) and horizontal position of the contact from the local maxima, then estimating the depth of the contact and other parameters by fitting the derivative data within the window to a theoretical curve. The methods differ in their complexity, accuracy, and sensitivity to noise and anomaly interference. The horizontal gradient method requires first-order horizontal derivatives and a reduction-to-the-pole or pseudogravity transformation. It is the method least susceptible to noise, but results are accurate only where the magnetization is induced and the sources are of very specific types. The analytic signal method requires first-order horizontal and vertical derivatives of the magnetic field or of the first vertical integral of the magnetic field. Horizontal location of isolated sources is generally accurate, but vertical position is accurate only for specific source types. Both the horizontal gradient method and the analytic signal method can be used to estimate minimum and maximum limits on source depths. The local wavenumber method requires firstand second-order horizontal and vertical derivatives, and is the most susceptible to noise and interference effects. In the absence of these problems, it provides accurate horizontal and vertical locations of isolated sources along with structural indices for the sources. The analytic signal and local wavenumber methods can be extended to estimate the geologic dip and magnetic susceptibility contrast across isolated contacts under the assumption of induced magnetization.

De-aliased, High-Resolution Radon Transforms

Abstract: Multiple elimination methods based on the move-out discrimination between primaries and multiples rely heavily on the focusing of seismic events in the parabolic Radon domain. This focusing, however, is affected both by the finite spatial aperture and sampling of the data. As a consequence of the resulting smearing, multiple energy may be mapped into the primary model and conversely primary energy may be mapped into the multiple model. This leads to poor multiple removal and to the nonpreservation of the primary amplitudes. To overcome these pitfalls one has to make use of De-aliased, HighResolution Radon transforms. High-resolution Radon transforms have already been proposed by some authors. Here we present a novel approach that simultaneously tackles the aliasing and resolution issues in a non-iterative way.

Extended elastic impedance for fluid and lithology prediction

Abstract: Constant angle projections of seismic sections can be designed to provide maximum discrimination between fluids or lithologies. The optimum projection for a noise‐free, isotropic environment can be obtained using an extension to the elastic impedance concept, which itself is an extension of acoustic impedance (AI) to nonzero angles of incidence. To achieve this, we modify the definition of elastic impedance (EI) beyond the range of physically meaningful angles by substituting tanχ for sin2 θ in the two‐term reflectivity equation. The primary variable now becomes χ rather than θ. We allow it to vary between −90° and +90°, which gives an extension of EI for any combination of intercept and gradient. We refer to this form of elastic impedance as extended elastic impedance (EEI).In this paper we demonstrate that EEI can be tuned using different χ values to be approximately proportional to a number of elastic parameters, and we give EEI expressions for shear impedance (SI), bulk modulus, shear modulus, Lame's ...

Advantages of calculating shear-wave velocity from surface waves with higher modes

Abstract: Summary The Rayleigh-wave phase velocity of a layered earth model is a function of frequency and four groups of earth parameters: compressional (P)-wave velocity, shear (S)-wave velocity, density, and thickness of layers. For the fundamental mode of Rayleigh waves, analysis of the Jacobian matrix for high frequencies (5-40 Hz) provides a measure of dispersion curve sensitivity to earth model parameters. S-wave velocities are the dominant influence of the four earth model parameters. With the lack of sensitivity of the Rayleigh wave to P-wave velocities and densities, estimations of these parameters can be made for a layered earth model such that dispersive data vary predominantly with S-wave velocities (Xia et al., 1999a). This thesis is valid for higher modes of Rayleigh waves as well. Experimental analysis indicates that energy of higher modes tends to become more dominant as the source distance becomes larger (Park et al., 1999a). In some cases, higher mode data are necessary since shorter wavelength components of fundamental mode Rayleigh waves are obscured by these higher frequency data where higher modes of Rayleigh waves dominate. As well, our modeling results demonstrate at least two quite exciting higher mode properties. First, for fundamental and higher mode Rayleigh wave data with the same wavelength, higher modes can “see” deeper (longer than the wavelength) than fundamental modes (normally shorter than the wavelength). Second, higher mode data can increase the resolution of the inverted S-wave velocities. A much better S-wave velocity picture can be produced from inversion of surface wave data if higher-mode data are included. Real world examples show how resolution can be improved.

Beamlet migration based on local perturbation theory

Abstract: Beamlet migration based on local perturbation theory is proposed. The method is formulated with a local bac kground velocity and local perturbations for each window of the wave eld decomposition using GaborDaubechies frame and local cosine basis. The propagators and phase-correction operators are obtained analytically for the G-D tight-frame, and numerically for the local cosine basis. The numerical test using the SEG-EAEG salt model poststac kdata demonstrates the great potential of this approach.

Seismic low‐frequency effects from fluid‐saturated reservoir

Abstract: The normal incidence of Pelastic wave reflected from a thin porous dry and water saturated layer is considered. The effect of stronger reflections and travel time delays from water saturated layer is observed at low frequencies. We compared the results of laboratory modeling with “frictional-viscous” theoretical model and found that low (< 5) values of attenuation parameter Q and its approximate proportionality to frequency can explain the effect. The values of Q were determined in a separate experiment using recordings of a transmitted field for a thick porous layer, where water saturated layer had attenuation about two times higher then in a dry layer. These findings can be used for detecting and monitoring liquid saturated areas in thin porous layers.

Joint 3-D Inversion of Gravity, Magnetic and Tensor Gravity Fields For Imaging Salt Formations in the Deepwater Gulf of Mexico

Abstract: A very robust inversion process was developed to produce geologic models based on vector and tensor potential methods data. We have successfully jointly modeled gravity and magnetic fields over areas as large as several thousand square miles down to prospect areas in the deepwater Gulf of Mexico. In some cases tensor gravity was also included in the joint inversions. In order to accomplish this we have had to overcome many of the pitfalls associated with this type of modeling. The resulting software runs very efficiently on desktop computers such as a Sun Ultra-60.

Seismic Monitoring of CO2 Injected Into a Marine Acquifer

Abstract: Since October 1996 Statoil has injected CO2 into a saline aquifer for disposal. Monitoring the behaviour of the CO2 in the sand formation and the sealing capacity of the overlying shale cap rock are key elements in understanding the dynamics of the injection process. A repeated 3D seismic dataset was therefore acquired in 1999, after injection of about 2 million metric tons of CO2. The time-lapse data show a large increase in reflectivity and a large push-down of reflections caused by the injected CO2. Gas at different levels within the sand are probably trapped by thin shale layers. Only a small part has reached the top of Utsira Fm., and no signs of CO2 are observed above the top seal.

Common angle imaging conditions for pre-stack depth migration

Abstract: Summary New classes of wave equation based wavefield extrapolators now allow us to produce accurate depth images of complex structural targets. The most common imaging condition extracts the zero time wavefield at zero offset, yielding a single image at each depth. Migration-inversion theory provides a framework for constructing imaging conditions that exploit redundancy in the extrapolated wavefields to provide more information on subsurface properties. The most important of these are background velocity and amplitude as a function of incidence angle. The common angle imaging condition (CAI) extracts the zero time wavefield at constant offset ray parameter. Images at a number of angles (or offset ray parameter values) can be extracted to provide the equivalent of migrated taup gathers for each surface CMP location. Since offset ray parameter is proportional to angle of incidence at the reflector, CAI gathers provide amplitude versus angle information that automatically incorporates reflector dip. Residual velocity analysis is performed on CAI gathers by scanning for residual elliptical moveout using familiar tau-p formulations. The combination of fast, accurate wavefield extrapolators (for example, the common offset pseudo-screen operators reported by Jin and Wu, 1999) with the common angle imaging condition now provides a complete set of tools for structural imaging, AVO, and velocity analysis with wave equation based techiques.

A Monte Carlo Method to Quantify Uncertainty in the Inversion of Zero-Offset VSP Data

Abstract: Summary We describe in this paper a method that uses zero-offset Vertical Seismic Profile (VSP) data to infer the characteristics of a layered medium below the receivers. We quantify the non-uniqueness, or uncertainty, of the solution by adopting a Bayesian approach that defines a posterior distribution of layered media for the given VSP data and prior knowledge (for example, on likely values of acoustic impedance). This formulation also ensures that the sampled layered medium solutions are simple, i.e., do not contain layers that are not required by the data. We describe a variation of the commonly used Monte Carlo Metropolis-Hastings algorithm that efficiently obtains a sample from the posterior distribution of layered media. The Monte Carlo method is illustrated with an example where VSP data were acquired for pore pressure prediction. The method quantifies the uncertainty inherent to predicting the depth of overpressured zones from VSP data, and demonstrates how this uncertainty is reduced by additional travel time-depth ties acquired during drilling.

Acquisition of marine point receiver seismic data with a towed streamer.

Abstract: Summary A new concept for the acquisition of point receiver towed streamer data is introduced. Recording of point receiver data, rather than the analogue outputs of hardwired arrays, enables both dynamic group forming and enhanced attenuation of noise. Dynamic group forming allows better signal preservation and enhanced frequency recovery, particularly at far offsets. Data adaptive noise attenuation methods allows improved attenuation of the low frequency noise induced by water flow around the streamer, enabling the enhancement of the low frequency signal-to-noise ratio through avoidance of low cut filters. Point receiver marine seismic data will improve the dynamic range of the seismic signal and enhance the viability of time-lapse monitoring of reservoirs with towed streamer data.

Estimation and interpretation of P and S impedance volumes from simultaneous inversion of P-wave offset seismic data

Abstract: Summary Seismic amplitude variation with offset holds information on density and two elastic parameters: compressional and shear velocities (or impedances). We simultaneously invert multiple offset stacks to transform P-wave offset seismic reflection data to these parameters. Prior to the inversion, wavelets are estimated separately for each offset stack. This enables the inversion to compensate for offset-dependent phase, bandwidth and tuning and nmo stretch effects. The impedance volumes can be interpreted separately or combined to estimate other geophysical parameters which might optimally discriminate between facies. In this regard, we have found the Lame’ parameters, Lambda and Mu particularly useful. From well log analysis we expect that reservoir sands have lower Lambda (incompressibility) and higher

Velocity, Density and Modulus of Hydrocarbon Fluids --Data Measurement

Abstract: Density and ultrasonic velocity of numerous hydrocarbon fluids (oil, oil based mud filtrate, hydrocarbon gases and miscible CO2-oil) were measured at in situ conditions of pressure up to 50 Mpa and temperatures up to100 °C. Dynamic moduli are derived from velocities and densities. Newly measured data refine correlations of velocity and density to API gravity, Gas Oil ratio (GOR), Gas gravity and in situ pressure and temperature. Gas in solution is largely responsible for reducing the bulk modulus of the live oil. Phase changes, such as exsolving gas during production, can dramatically lower velocities and modulus, but is dependent on pressure conditions. Distinguish gas from liquid phase may not be possible at a high pressure. Fluids are often supercritical. With increasing pressure, a gas-like fluid can begin to behave like a liquid

Globally optimized fourier finite-difference migration method

Abstract: Summary To accurately image complex structures with strong lateral velocity variations and steep dips, we develop a globally optimized Fourier finite-difference method that uses a rational approximation of the square-root operator in the one-way wave equation. The method uses a split-step Fourier operator coupled with a one-term optimized finite-difference operator. The two coefficients in the rational approximation are obtained by an optimization scheme that maximizes the maximum dip angle of the method for a given model. Our optimized method uses the same coefficients throughout a model in contrast

Velocity, Density And Modulus of Hydrocarbon Fluids -- Empirical Modeling

Abstract: Improved velocity and density models were developed based on an extensive suite of new data. Nearly 70 gas-free (dead) and gas-charged (live) oil samples provided by industry sponsors were measured at pressures up to 55.2 Mpa (8000 Psi) and temperatures up to 100 C. This new data suggest that the velocity model developed by Batzle and Wang, (1992) overestimates the gas-oil ratio (GOR) effect on velocity of hydrocarbon liquids. Two techniques are employed to fit the data: a model based on engineering concepts of ideal liquids, and on purely empirical forms. These results have been generalized and incorporated in a program (FLAG) to calculate and plot fluid properties as functions of composition, pressure, and temperature.

Common-Reflection-Surface Stack And Conflicting Dips

Abstract: Summary The recently introduced common-reflection-surf ace (CRS) stack simulates a zero-offset (ZO) section from multi-coverage seismic reflection data for 2-D media in a data-driven way, i e, without explicit knowledge of the macro-velocity model The “best” stacking operators are determined by an optimization of the coherency along different test stacking operators in the multi-coverage data Previous implementations determine only one optimum stacking operator for each ZO sample to be simulated Consequently, conflicting dips are not taken into account but only the most prominent event contributes to a particular stack sample In this work, I show how this limitation can be overcome The pragmatic search strategy of the original CRS stack implementation consists of three one-parametric search steps to determine the stacking operators In the first step, an automatic CMP stack, conflicting dips can hardly be considered because the respective stacking velocities might be quite similar However, I observe that conflicting dips can still be detected and separated in the subsequent search steps that are applied to the result of the automatic CMP stack I propose an extension of the pragmatic approach to account for conflicting dips For ZO samples where conflicting dips are detected, an additional one-parametric search is required This provides a set of three kinematic wavefield attributes for each of the conflicting events and allows to simulate their interference in the simulated ZO section

Tau domain migration velocity analysis using angle CRP gathers and geologic constraints

Abstract: SUMMARY We present an alternative approach for ray based migration velocity analysis. Instead of using Kirchoff migration and offset domain Common Reflection Point (CRP) gathers we use wave equation ray parameter CRP gathers. By performing tomography in vertical travel-time (tau) space, we avoid estimating mapping velocity, instead concentrating on focusing velocity. By introducing anisotropic preconditioning oriented along bedding planes, we quickly guide the inversion towards a geologically reasonable model. We illustrate the benefits of our tomography method by comparing it to more traditional methods on a synthetic anticline model.

(t,x) domain, pattern‐based ground roll removal

Abstract: We demonstrate the use of a new, t x domain, pattern-based signal/noise separation technique to separate ground roll from primary reflection events. Ground roll is notoriously difficult to model with generality, but the technique requires a kinematically correct model of the noise. We obtain an imperfect model of the ground roll directly from the data itself, by application of a suitable lowpass filter. On a 2-D receiver line gather taken from a 3-D shot gather, in which the ground roll is spatially aliased and has nonlinear moveout, the separation results improve markedly over direct subtraction of the noise model from the data.

One-way Acoustic Reciprocity And Its Applications In Multiple Elimination And Time-lapse Seismics

Abstract: In this paper we review our reciprocity theorems for one-way wave fields, modified for lossy media, and we discuss applications in multiple elimination and time-lapse seismics. Introduction One-way wave equations have played a prominent role in seismic processing since the pioneering work of Claerbout [4], Berkhout [2] and others. The reason for this is that a seismic experiment can be explained in terms of ‘downgoing’ waves traveling from the source at the Earth’s surface to a target in the subsurface and ‘upgoing’ waves traveling from the target to the receivers at the surface. One-way wave equations naturally honor this distinction between downgoing and upgoing waves. This paper starts with a review of reciprocity theorems for oneway wave fields. These reciprocity theorems formulate general relations between the one-way wave fields in two different ‘states’. One of these states is an actual seismic experiment, while the other state can either be a computational state (e.g. a wave field propagator), a desired state (e.g. multiple-free data) or another seismic measurement (characterizing time-lapse differences in the target). Fokkema and van den Berg [8] derived seismic processing techniques from Rayleigh’s reciprocity theorem for total acoustic wave fields. In the current paper the oneway reciprocity theorems form the starting point. These theorems provide a theoretical frame-work for current seismic processing techniques based on the one-way wave equations. Some applications will be indicated. One-way reciprocity theorems in media with losses The one-way wave equation and its symmetry properties. We review the acoustic one-way wave equation for downgoing and upgoing waves in an inhomogeneous medium with losses. We introduce a one-way wave vector and a one-way source vector , according to and (1)

An Approximation For the Xu-White Velocity Model

Abstract: In 1995, S. Xu and R. E. White described a method for estimating compressional and shear-wave velocities of shaley sandstones from porosity and shale content. Their model was able to predict the effect of increasing clay content on compressional-wave velocity observed in laboratory measurements. A key step in the Xu-White method estimates dry rock bulk and shear moduli for the sand/shale mixture. This step is performed numerically by applying the differential effective medium method to the Kuster-Toksoz equations for ellipsoidal pores. This step is computationally intensive. Using reasonable assumptions about dry rock elastic properties, this step can be replaced with a set of approximations for dry rock bulk and shear moduli. Numerical experiments show an extremely close match between velocities obtained with these approximations and velocities computed with the differential effective medium method. These approximations simplify the application of the Xu-White method, and make the method computationally more efficient. They also provide insight into the Xu-White method. For example, these approximations show how the Xu-White model is related to the critical porosity model.