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- Volume 57, Issue 3, 2009
Geophysical Prospecting - Volume 57, Issue 3, 2009
Volume 57, Issue 3, 2009
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Velocity analysis using AB semblance
By S. FomelABSTRACTI derive and analyse an explicit formula for a generalized semblance attribute, which is suitable for the velocity analysis of prestack seismic gathers with distinct amplitude trends. While the conventional semblance can be interpreted as a squared correlation with a constant, the AB semblance is defined as a correlation with a trend. This measure is particularly attractive for analysing class II AVO anomalies and converted waves. Analytical derivations and numerical experiments show that the resolution of the AB semblance is approximately twice as low as that of the conventional semblance. However, this does not prevent it from being an effective attribute. I use synthetic and field data examples to demonstrate the improvements in velocity analysis from AB semblance.
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Stacking velocities in the presence of overburden velocity anomalies
By Emil BliasABSTRACTLateral velocity changes (velocity anomalies) in the overburden may cause significant oscillations in normal moveout velocities. Explicit analytical moveout formulas are presented and provide a direct explanation of these lateral fluctuations and other phenomena for a subsurface with gentle deep structures and shallow overburden anomalies. The analytical conditions for this have been derived for a depth‐velocity model with gentle structures with dips not exceeding 12°. The influence of lateral interval velocity changes and curvilinear overburden velocity boundaries can be estimated and analysed using these formulas. An analytical approach to normal moveout velocity analysis in a laterally inhomogeneous medium provides an understanding of the connection between lateral interval velocity changes and normal moveout velocities. In the presence of uncorrected shallow velocity anomalies, the difference between root‐mean‐square and stacking velocity can be arbitrarily large to the extent of reversing the normal moveout function around normal incidence traveltimes. The main reason for anomalous stacking velocity behaviour is non‐linear lateral variations in the shallow overburden interval velocities or the velocity boundaries.
A special technique has been developed to determine and remove shallow velocity anomaly effects. This technique includes automatic continuous velocity picking, an inversion method for the determination of shallow velocity anomalies, improving the depth‐velocity model by an optimization approach to traveltime inversion (layered reflection tomography) and shallow velocity anomaly replacement. Model and field data examples are used to illustrate this technique.
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Ground‐roll attenuation using a 2D time‐derivative filter
Authors Paulo E. M. Melo, Milton J. Porsani and Michelângelo G. SilvaABSTRACTWe present a new filtering method for the attenuation of ground‐roll. The method is based on the application of a bi‐dimensional filter for obtaining the time‐derivative of the seismograms. Before convolving the filter with the input data matrix, the normal moveout correction is applied to the seismograms with the purpose of flattening the reflections. The method can locally attenuate the amplitude of data of low frequency (in the ground‐roll and stretch normal moveout region) and enhance flat events (reflections). The filtered seismograms can reveal horizontal or sub‐horizontal reflections while vertical or sub‐vertical events, associated with ground‐roll, are attenuated. A regular set of samples around each neighbourhood data sample of the seismogram is used to estimate the time‐derivative. A numerical approximation of the derivative is computed by taking the difference between the interpolated values calculated in both the positive and the negative neighbourhood of the desired position. The coefficients of the 2D time‐derivative filter are obtained by taking the difference between two filters that interpolate at positive and negative times. Numerical results that use real seismic data show that the proposed method is effective and can reveal reflections masked by the ground‐roll. Another benefit of the method is that the stretch mute, normally applied after the normal moveout correction, is unnecessary. The new filtering approach provides results of outstanding quality when compared to results obtained from the conventional FK filtering method.
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Comparison between the Fourier finite‐difference method and the generalized‐screen method
Authors Jin‐Hai Zhang, Wei‐Min Wang and Zhen‐Xing YaoABSTRACTThe Fourier finite‐difference propagator and the generalized‐screen propagator are two general high‐order forms of one‐way dual‐domain methods. We compare these two propagators mainly on phase accuracy, computational efficiency and 3D extension. A comparison of phase accuracy shows that the high‐order generalized‐screen propagator is preferable to the Fourier finite‐difference propagator for heterogeneous media with a weak velocity contrast and wide dip angle. With increasing velocity contrast, the accuracy improvement gained by the high‐order generalized‐screen propagator declines rapidly. The Fourier finite‐difference propagator is more robust and flexible to lateral velocity variations than the generalized‐screen propagator. The 2D Fourier finite‐difference propagator is superior to the 2D generalized‐screen propagator when the velocity contrast is stronger than 23%. Despite the two‐way splitting error, the 3D Fourier finite‐difference propagator is more accurate than the second‐order generalized‐screen propagator when the velocity contrast is stronger than 20% and is more accurate than the fourth‐order generalized‐screen propagator when the velocity contrast is stronger than 40%. Numerical experiments on the SEG/EAGE salt model demonstrate that the Fourier finite‐difference propagator behaves better than the generalized‐screen propagator when imaging steep salt boundary and faults beneath the salt body. Under the same hardware and software conditions, the computational cost of the Fourier finite‐difference propagator in our implementation is greater than that of the second‐order generalized‐screen propagator but smaller than that of the third‐order generalized‐screen propagator. Compared with the Fourier finite‐difference propagator, the generalized‐screen propagator requires fewer grid points per wavelength and has more potential to improve running speed in the presence of a much faster Fourier transform. These analyses are applicable for both forward modelling and depth migration.
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3D finite‐difference synthetic acoustic logging in cylindrical coordinates
Authors D. Pissarenko, G.V. Reshetova and V.A. TcheverdaABSTRACTOn the basis of the application of a finite‐difference approximation of an initial boundary value problem for elastic wave equations (velocity/stress formulation), a numerical method and its algorithmic implementation have been developed in order to perform a computer simulation of sonic logging. A very general statement is dealt with – the surrounding medium is allowed to be 3D heterogeneous and the source can be located at any point inside or outside the well. To provide the most precise description of the sharpest interface of the problem – the interface of the well, we formulate the problem in cylindrical coordinates with a z axis directed along the well. In order to avoid excessive azimuth inflation of grid cells with radius increase, we perform periodical refinement of the grid step in the azimuth direction. This guarantees the quasi‐uniform size of the grid cells throughout the area of computation.
In order to truncate area of computation, a classical version of a perfectly matched layer is developed and implemented for cylindrical coordinates. Its main advantage in comparison with other approaches is an extremely low level of artificial reflections and it is not necessary to perform splitting of variables in the azimuth direction.
Implementation of parallel computations is performed via domain decomposition: the area of computation is sliced into a number of adjoining discs and each disc is assigned to its own processor unit. Data exchange between processor units is performed with the help of a message‐passing interface library. Results of numerical experiments for an elastic medium with a circular crack are presented and discussed.
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Imaging of offset VSP data acquired in complex areas with modified reverse‐time migration
Authors D. Neklyudov and I. BorodinABSTRACTA modified reverse‐time migration algorithm for offset vertical seismic profiling data is proposed. This algorithm performs depth imaging of target areas in the borehole vicinity without taking into account the overburden. Originally recorded seismograms are used; reliable results can be obtained using only the velocity profile obtained along the well. The downgoing wavefield emitted from a surface source is approximated in the target area using the transmitted P‐wave, recorded by the receivers deployed in the well. This is achieved through a reverse‐time extrapolation of the direct transmitted P‐wave into the target area after its separation in offset vertical seismic profiling seismograms generated using a finite‐difference scheme for the solution of the scalar wave equation.
The proposed approach produces ‘kinematically’ reliable images from reflected PP‐ and PS‐waves and, furthermore, can be applied as a salt proximity tool for salt body flank imaging based on the transmitted PS‐waves. Our experiments on synthetic data demonstrate that the modified reverse‐time migration provides reliable depth images based on offset vertical seismic profiling data even if only the velocity profile obtained along the borehole is used.
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Low‐frequency microtremor anomalies at an oil and gas field in Voitsdorf, Austria
Authors Marc‐André Lambert, Stefan M. Schmalholz, Erik H. Saenger and Brian SteinerABSTRACTResults of a passive microtremor survey at an oil and gas field in Voitsdorf, Austria, are presented. The survey consists in six parallel profiles approximately 9 km long over two hydrocarbon reservoirs. For each profile the seismic wavefield was recorded synchronously at 11 in‐line stations. The measurements were conducted with broadband seismometers and lasted, for each profile, at least 12 hours overnight. Data interpretation is based on a comprehensive data set and on the analysis of four different spectral attributes. These attributes quantify the characteristic features of the wavefield's Fourier spectra in the low‐frequency range (<10Hz). One attribute quantifies the spectral energy in the vertical wavefield component, another attribute quantifies the maxima in vertical‐to‐horizontal spectral ratios and two attributes describe the frequency shifts of peaks within the spectra of vertical and horizontal wavefield components. Due to temporal variations of the signals we combine the long‐term measurements (several hours of continuous records) of multiple profiles. This procedure considerably enhances the consistency of each spectral attribute and makes them suitable to quantify lateral variations of the wavefield. The results show that using a combination of several attributes significantly increases the reliability of detecting anomalies in the microtremor wavefield that are presumably caused by hydrocarbon reservoirs. A numerical study of two‐dimensional seismic wave propagation is applied to investigate the peak frequency shift attributes. The results of the study indicate that the attributes may contain information on the depth of hydrocarbon reservoirs, assuming that the reservoir acts as a (secondary) source of low‐frequency seismic waves.
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Estimation of petrophysical parameters by linearized inversion of angle domain pre‐stack data
Authors F. Chiappa and A. MazzottiABSTRACTWe describe a linear Bayesian inversion method to estimate the relevant petrophysical properties of the media forming a reflecting interface from the observations of amplitude variation with incidence angle. Three main steps characterize the proposed approach:
– information from borehole logs are statistically analysed to estimate the empirical models that describe the functional relationship between petrophysical (e.g. porosity, saturation, pressure or depth) and seismic variable(P and S velocities and density);
– the pure‐mode (PP) reflection coefficient is parameterized in terms of the relevant petrophysical variables and is linearized in order to implement the linear inversion;
– the sought petrophysical parameters are estimated from the seismic reflected amplitudes by applying the linearized inversion where a priori information, data and model errors and solutions are described by probability density functions.
We test the method on synthetic and real data relative to reflections from a shale/gas‐sand interface where the amplitude versus angle response, besides the lithological contrast, is mainly controlled by the saturation and porosity of the sand layer. The outcomes of the linearized inversion are almost identical to those obtained by a previously developed non‐linear inversion method demonstrating the applicability of the linear inversion. It turns out that the gas‐sand saturation in the range 0%–95% is a poorly resolved parameter while the porosity is the best resolved parameter. The issues of robustness and resolution of the inversion are discussed either through singular value decomposition analysis or the observation of the a posteriori probability density functions.
The linear inversion algorithm, compared with the previously developed non‐linear method, reduces significantly the computation time allowing for more extensive applications.
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Deriving effects of pressure depletion on elastic framework moduli from sonic logs
Authors Anne‐Kari Furre, Mona Andersen, Anita Smalø Moen and Randi Kristin TønnessenABSTRACTWe have developed a method to determine the effect of pore pressure depletion on elastic framework moduli, by utilizing sonic logs from wells drilled at different locations through a reservoir at varying depletion stages. This is done by first inverting the sonic logs for elastic framework bulk and shear moduli, thus carefully removing pressure dependent fluid effects. By crossplotting these elastic framework moduli against an increase in net stress (which is directly related to depletion), we derive the stress sensitivity of the elastic framework moduli. We found that the observed stress sensitivity was consistent with time‐lapse seismic results and that the sensitivity derived from the sonic logs was much smaller than predicted by hydrostatic measurements on core samples. This method is applicable to depletion scenarios in mature fields and can be used both for modelling and inverting time‐lapse seismic data for effects of pore pressure depletion on seismic data.
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The effect of mineralization on the ratio of normal to tangential compliance of fractures
Authors Colin M. Sayers, Arash Dahi Taleghani and José AdachiABSTRACTThe effect of a fracture on the propagation of seismic waves can be represented in terms of the normal compliance BN and tangential compliance BT of the fracture. If BN/BT= 1 for all fractures, the effective elastic stiffness tensor of an isotropic background containing an arbitrary orientation distribution of fractures is orthotropic (i.e., has three orthogonal planes of mirror symmetry) in the long‐wave limit. However, deviations from orthotropy may occur if BN/BT differs significantly from unity and this can cause the principal axes of the P‐wave NMO ellipse and of the variation in the PP‐reflection amplitude as a function of azimuth, to deviate from the fast and slow polarization direction of a vertically propagating S‐wave. Simple models of a fracture in terms of a planar distribution of cracks suggest that BN/BT≈ 1 for dry fractures. However, naturally occurring fractures often exhibit mineralization in the form of bridges between opposing faces of the fracture. The presence of such bridges leads to significant departures of BN/BT from unity.
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Interpretation case study of the Sahl El Qaa area, southern Sinai Peninsula, Egypt
Authors Taha Rabeh, Jorge Miguel Miranda, Joao Carvalho and Andrei BocinABSTRACTThe study area is located in the Sinai Peninsula, which is considered one of the most promising regions for oil resources. Three different tectonic forces affect the area in the triple junction structures associated with the opening of the Gulf of Suez and the strike slip movement along the Gulf of Aqaba. The main goal of this work is to model the structure of the basement rocks in the study area using magnetic methods. To achieve this, a high‐resolution land magnetic survey was acquired and the results were combined with existing seismic reflection data. The magnetic interpretation was carried out using the analytical signal, horizontal gradient, Euler and Werner deconvolution and 3D magnetic modelling methods. We concluded that most of the deduced structures are trending in N–S, N35°–N45° west and E–W directions. The Aqaba trend (N15°–N25° east) is barely noticeable. The depth to the basement rocks ranges from 1 km to more than 2 km below sea level and these results are in agreement with the available well log data. In addition, interpretation of seismic reflection sections was carried out and compared with overlapping magnetic profiles interpreted using Euler deconvolution. They show that the sedimentary section was affected by the basement tectonics, with faults extending from the basement upwards through the sedimentary cover. These faults constitute good potential structural traps for oil accumulation.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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Volume 64 (2015 - 2016)
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Volume 63 (2015)
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Volume 62 (2014)
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Volume 61 (2013)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 58 (2010)
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Volume 57 (2009)
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Volume 56 (2008)
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Volume 55 (2007)
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Volume 54 (2006)
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Volume 53 (2005)
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Volume 52 (2004)
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Volume 51 (2003)
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Volume 50 (2002)
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Volume 49 (2001)
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Volume 48 (2000)
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Volume 47 (1999)
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Volume 46 (1998)
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Volume 45 (1997)
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Volume 44 (1996)
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Volume 43 (1995)
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Volume 42 (1994)
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Volume 41 (1993)
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Volume 40 (1992)
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Volume 39 (1991)
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Volume 38 (1990)
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Volume 37 (1989)
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Volume 36 (1988)
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Volume 35 (1987)
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Volume 34 (1986)
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Volume 33 (1985)
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Volume 32 (1984)
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Volume 31 (1983)
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Volume 30 (1982)
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Volume 29 (1981)
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Volume 28 (1980)
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Volume 27 (1979)
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Volume 26 (1978)
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Volume 25 (1977)
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Volume 24 (1976)
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Volume 23 (1975)
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Volume 22 (1974)
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Volume 21 (1973)
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Volume 20 (1972)
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Volume 19 (1971)
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Volume 18 (1970)
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Volume 17 (1969)
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Volume 16 (1968)
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Volume 15 (1967)
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Volume 14 (1966)
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Volume 13 (1965)
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Volume 12 (1964)
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Volume 11 (1963)
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Volume 10 (1962)
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Volume 9 (1961)
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Volume 8 (1960)
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Volume 7 (1959)
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Volume 6 (1958)
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Volume 5 (1957)
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Volume 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)