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53rd EAEG Meeting
- Conference date: 26 May 1991 - 30 May 1991
- Location: Florence, Italy
- ISBN: 978-90-73781-03-0
- Published: 28 May 1991
1 - 20 of 315 results
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Interpolation beyond aliasing by (t,x)-domain PEFs
Authors J. F. Claerbout and D. NicholsA great many processes are limited by the simple statement of the spatial Nyquist requirement-that no wavelength be shorter than twice the sampling interval on the data wavefield. This condition forces costly expenditures in 3-D reflection data acquisition. The sirnple statement of the Nyquist requirement oversimplifies reallife. At the 1989 SEG meeting, S. Spitz showed astonishing 2-D interpolations that seem to violate the Nyquist requirement and so they give us a deeper understanding of it. The real Nyquist limitation is on the spectral bandwidth, not on the maximum frequency, and Spitz' results can be explained by saying that the bandwidth need not be contiguous. We often characterize data as "good" or "noisy" when we really mean it contains "few" or "many" plane wave events. For noisy data there is no escaping the simple form of the Nyquist limitation but with good data it seems we can.
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A semblance-guided median filter
Authors E. E. Reiter and M. N. ToksözWe describe and implement a slowness selective median filter based on information from a local set of traces. The filter is constructed in two steps, the first being an estimation of a preferred slowness and the second the selection of a median or trimmed mean value to replace the original data point. A symmetric window of traces is selected about each trace to be filtered and the filter repeatedly applied to each time point. The preferred slowness is determined by scanning a range of linear moveouts within the user specified slowness passband. Semblance is computed for each trial slowness and the preferred slowness selected from the peak semblance value.
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3D Reconstruction of irregularly sampled wave fields
Authors N. A. Kinneging, A. J. W. Duijndam and L. OnkiehongA complication in 3D marine acquisition is that streamer feathering causes a crossline distribution of the actual midpoints. In conventional processing the individual traces are gathered in bin sorts and stacked after NMO correction. The stacked traces are posted at the centre of the bin, thus causing a posting error. This approach has some disadvantages. Depending on the feathering conditions some bin sorts will be overpopulated, while others will be underpopulated producing inconsistent stacking responses. The binstack smears the data over the bin area and thus puts a limit on the resolution. On the other hand this latter effect may be considered as a kind of spatial anti-alias filter.
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The impact of processing on the APF.VO responses of a marine seismic dataset
Authors A. Mazzotti and G. RavagnanIt is weIl known that the diagnostic potential of seismic data may be severely dependent on the acquisition and processing procedures. This is certainly true for any kind of interpretation approach or inversion algorithm but it becomes of the utmost importance when we deal with prestack seismic data. Namely, the Amplitude Vs. Offset (AVO) and the cornbined Amplitude, Phase and Frequency Vs. Offset (APF.VO) analyses may be very sensitive to the previous acquisition and processing. The main purpose of this paper is to show the impact that processing procedures have on the APF.VO responses of reflected signals pertaining to an offshore seismic line. We focus on two separate targets that give rise to bright reflections on the stack section: a cineritic layer and a gas-sand bed.
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Integration of trend and detail
Authors N. P. van Dijk, G. J. M. Lörtzer and A. J. BerkhoutLinearized inversion of amplitude versus offset (AVO) from multi-offset data consists of weighted stacking. It yields the elastic parameters as traces in terms of bandlimited relative contrasts. (Smith and Gidlow 1987; Lörtzer and Berkhout 1989). The weighted stacking results are displayed as seismie traces. These traces may be considered as spiky relative contrasts convolved with the seismic wavelet.
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The signal coherence as a versatile diagnostic tool to improve seismic data processing effectiveness
Authors B. Boiardi and M. CardamoneThe increasingly large amount of seismic data involved in 20 and 30 seismic data processing projects requires the personal skill of the analyst to be assisted in the choose of algorithms and parameters. At the same time the increasing complication of nowadays exploration themes wants the seismic method to provide highly reliable and uniform results.
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The wave equation stack
By R. SilvaIt is generally known that common midpoint (CMP) stacking and Kirchhoff migration are similar operations. This similarity arises from the fact that they both perform summation over hyperbolic trajectories defined by two-way travel times. Since migration can be considered as a shifting of energy with respect to the midpoint axis, stacking can be described in terms of shifting energy with respect to the offset axis.
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New perspectives in inverse VSP method by using explosives in the open hole
Authors P. C. Layotte, C. Naville and P. FroidevauxA borehole seismic experiment was carried out in the southeast part of France in the Balazuc-1 well belonging to the B.R.G.M.'s scientific project G.P.F. (Géologie Profonde de la France: Deep Geology of France). This experiment was supported by the ARTEP* association.
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The influence of acquisition system; coupling environment and sensor type on VSP image quality
Authors K. J. Dodds, T. Spencer, G. Ballantyne and R. BuntThe influence of the acquisition system type and its coupling environment on the quality of a VSP reflector signature is a source of conjecture prior to any VSP survey. We present a case history data set that involves variations of both acquisition system and environment including two different types of sensors and evaluate the differences evident in both the raw data and final processed image.
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Secondary shear waves generated from source boreholes
Authors J. A. Meredith, M. N. Toksöz and C. H. ChengIn cross weIl tomography experiments there are two types of sources commonly used. The most common are point sources, such as the airgun or dynamite, and axial sources. In general, the point sources generate tube waves which travel up and down the borehole and these tube waves contain more than 99% of the energy generated by the source (Winbow, 1989). AdditionaIly, point and radial sources also generate shear waves (Heelan, 1953, Lee and Balch, 1982). Axial sources do not generate tube waves (Winbow, 1989; Meredith, 1990) and therefore will not be discussed in this abstract.
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Acquisition and processing of VSP data obtained in horitzontal wells drilled from floating platforms
Authors S. A. Petersen and R. HeggernesWithin the last couple of years the use of horizontal wells for oil production purposes has increased. Clearly this new acquisition environment has an impact on the logging tools and deployment methods employed and on the post-survey dataprocessing. This is also the case for the Vertical Seismie Profiling (VSP) method. In the following the problems met in collecting and processing VSP data from floating platforms are discussed for an example from the central part of the North Sea.
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Multicomponent VSP radon transform for transversely isotropic media
By J. E. GaiserShear-wave particle motion directions measured in vertical seismie profiles (VSP) are very sensitive to anisotropic properties of the medium; however, in the case of compressional waves this is not true (Crampin 1981). Analyzing polarization diagrams (holograms) to measure these directions can be inaccurate because individual 3-component geophones are subject to interference from ambient noise and ot her coherent signals. An alternative is to investigate polarization characteristics along wavefronts using multicomponent Radon transforms.
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VSP Noise suppression through spectral equalisation
By P. B. DillonSpectral equalisation or spectral balancing is the suppression of seismic noise through power-spectral processing only, the phase spectra remaining unchanged. Tufekcic et al (1981) demonstrated a multichannel scheme in which the power spectra were spatially smoothed, over a large number of traces. Coppens and Mari (1984) showed that selectively weighting the power spectrum over the useful frequency band also gave good results. Both these schemes target the distortion introduced by surface-seismic ground roll. Ground roll is usually characterised by high-amplitude narrow-band spectral interference. That is, the noise is non-Gaussian. Since the character of VSP noise is very of ten non- Gaussian, it would seem that spectral equalisation should solve some of the noise problems experienced in the borehole environment.
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VSP and acoustic logging. Case history on the Cere La Ronde structure
Authors J. L. Mari and F. VerdierThe comparison of seismic data recorded at very different frequencies (a few hundred Hz for VSP and a few ten KHz for acoustic logging) and of dipmeter information can help for the interpretation of seismic anomalies. The VSP can have a lateral investigation of several ten or even hundred meters and a 1 to 10 meters vertical resolution. The acoustic logging used in reflected or diffracted modes has a vertical resolution of a few ten centimeters an 1 to 20 meters lateral investigation.
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3D kinematic inversion from a set of line profiles
More Less2-D interval velocity analysis is inaccurate in the case of tree-dimensional structures. This is demonstrated on the isotropic two-Iayered model where two reflecting interfaces have conflicted dip azimuths (Figure 1). We consider the profile Y=5km. The stacked section (Figure 2) is obtained from CMP gathers calculated by 3-D ray tracing. Interval velocity analysis along this profile is carried out by the 2-D coherency inversion (Landa et al.,1989). This analysis provides a high coherency (normalized semblance is about 0.8) for both reflection horizons on the CMP gathers.
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Velocity analysis with tomographic inversion on shot record migration
More LessPrestack time migration can still be a suitable alternative to image complex structures with moderate lateral velocity variations and it is less sensitive to the knowledge of the migration-velocity model than prestack depth migration.
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Advanced 3D macro-model estimation: The Sistre Project
By J. L. GuizouGetting a reliable depth image of the subsurface requires the identification of the main geological bodies beneath the surface of the earth, the estimation of the wave propagation velocities through these bodies, and an accurate positioning in depth of the interfaces which separate the geological layers. The exploration industry needs in the field of macro-model estimation have recently evolved from the conventional 2D framework towards the more attractive 3D framework. As a consequence, our geophysical industry must he prepared to offer reliable tools which make it possible to estimate structurally complex 3D geological bodies from seismic data. Several authors have reported methods to carry out three-dirnensional macro-model estimation using ray-tracing techniques and traveltime inversion techniques (Fawcett (1983), Chiu and Stewart (1987), and Guiziou and Haas (1988) ). However, these methods are limited to layer-cake-type subsurface models.
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A review of surface consistent imaging and DMO for irregular topography
Authors A. N. Ellis, R. A. Bale and P. W. KitchensideIn the processing of land data, static time corrections are used for improving velocity analyses, the enhancement of stacked sections and correcting for long-wavelength structural distortion due to irregular topography. Staties can be split into three categories, namely short-, medium- and long-wavelength, where medium wavelengths are of the order of one spread length. However static corrections are only valid when ray paths are verticaI near the surface. Therefore when the ray paths deviate from the verticaI, owing to steeply dipping reflectors or high velocities in the near surface (such as in permafrost environments), they must be replaced by more accurate wave-equation techniques.
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Impedance inversion using constant-offset reflection seismic data
Authors D. Wang, A. B. Weglein and T. J. UlrychAperture migration (aperture-compensated migration inversion) [1,2] inverts the zero-offset reflectivity from prestack reflection seismic data correctly. This method accommodates the incident angle of the wave, the reflector's dip and the actual receiver apert ure and is superior to conventional migration methods.
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Computation of true-amplitude reflections from offset data
Authors J. Schleicher, M. Tygel and P. HurbalCompressional primary seismic non-zero offset reflections as, e.g., common-midpoint or common-offset reflections are frequently used in seismic parameter estimation and imaging. Seismic migration procedures oper- . ating on such data are currently employed with increasing success. However, the dynamic aspects of wavefield migration methods still pose basic unsolved problems. One of the the questions is how the observed amplitudes can provide a measure of angle-dependent reflection coefficients.
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