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- Volume 49, Issue 5, 2001
Geophysical Prospecting - Volume 49, Issue 5, 2001
Volume 49, Issue 5, 2001
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Some SH‐wave seismic reflections from depths of less than three metres
Authors Gian Piero Deidda and Gaetano RanieriShallow SH‐wave reflections are far from routine, although their study can provide insights into important properties of near‐surface materials that cannot be inferred from P‐wave data alone. Difficulties in separating SH‐wave reflections from Love waves are generally considered the major obstacle to progress in shallow SH‐wave seismic reflection. This may be the case in surveys undertaken at great depths, but it is not necessarily true for reflection data gathered at shallow and ultra‐shallow depths. This paper shows that when SH‐wave data possess wavelengths greater than the thickness of the superficial layer, Love waves are not greatly dispersed. In this case, misinterpretation between parts of reflection hyperbolae and waveguide arrivals is sufficiently limited. In a one‐layer model earth, which well approximates typical situations of the near‐surface underground, the most energetic modes (the lowermost modes) of the dispersed surface waves have a dominant frequency band that falls below the wavelet spectrum of the shallow reflections; therefore, they can be filtered out in the frequency domain. Higher modes, although their spectral content overlaps that of the reflections, exhibit small amplitudes on seismograms and leave strong reflections unaffected.We present field examples from three different sites where we were able to obtain ultra‐shallow reflections (< 3 m) in unconsolidated sediments. The high level of resolution (vertical resolution up to 15 cm) suggests that SH‐wave reflection imaging has the potential to complement other high‐resolution techniques, such as P‐wave reflection and ground‐penetrating radar (GPR) imaging, allowing a better and more complete characterization of the near‐surface environments.
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Effective‐medium theories for fluid‐saturated materials with aligned cracks
Authors J.A. Hudson, T. Pointer and E. LiuThere is general agreement between different theories giving expressions for the overall properties of materials with dry, aligned cracks if the number density of cracks is small. There is also very fair agreement for fluid‐filled isolated cracks. However, there are considerable differences between two separate theories for fluid‐filled cracks with equant porosity. Comparison with recently published experimental data on synthetic sandstones gives a good fit with theory for dry samples. However, although the crack number density in the laboratory sample is such that first‐order theory is unlikely to apply, expressions correct to second order (in the number density) provide a worse fit. It also appears that the ratio of wavelength to crack size is not sufficiently great for any detailed comparison with effective‐medium theories, which are valid only when this ratio is large. The data show dispersion effects for dry cracks and scattering, neither of which will occur at sufficiently long wavelengths. Data from the water‐saturated samples indicate that the effect of equant porosity is significant, although the two theories differ strongly as to just how significant. Once again, and in spite of the reservations mentioned above, a reasonable fit between theory and observation can be shown.
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The seismic response to overpressure: a modelling study based on laboratory, well and seismic data
Authors José M. Carcione and Umberta TinivellaWe investigate the seismic detectability of an overpressured reservoir in the North Sea by computing synthetic seismograms for different pore‐pressure conditions. The modelling procedure requires the construction of a geological model from seismic, well and laboratory data. Seismic inversion and AVO techniques are used to obtain the P‐wave velocity with higher reliability than conventional velocity analysis. From laboratory experiments, we obtain the wave velocities of the reservoir units versus confining and pore pressures. Laboratory experiments yield an estimate of the relationship between wave velocities and effective pressure under in situ conditions. These measurements provide the basis for calibrating the pressure model. Overpressures are caused by different mechanisms. We do not consider processes such as gas generation and diagenesis, which imply changes in phase composition, but focus on the effects of pure pore‐pressure variations. The results indicate that changes in pore pressure can be detected with seismic methods under circumstances such as those of moderately deep North Sea reservoirs.
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Horizontal‐loop electromagnetic signature of a buried dike, Al Quweira area, southwest Jordan
More LessDuring 1999, horizontal‐loop electromagnetic (HLEM) measurements were made over a buried dike in the Al Quweira area, southwest Jordan, using the APEX MAX MIN III instrument, as part of a mineral exploration project. The objectives of the study were (i) to evaluate the resolution of the HLEM technique in field work in detecting and locating anomalies caused by vein‐like bodies, and (ii) to assess the capability of HLEM surveys for detecting targets in other locations throughout our geophysical survey programme. In‐phase and quadrature anomalies were recorded with 50 m and 100 m coil separations and multiple frequencies across the strike of the buried dike. Data recorded at 43 locations, spaced 10 m apart along the survey line, were interpreted quantitatively. For a 50 m separation, corresponding to shallow depths of investigation, the results do not show any recognizable response from the buried dike. The HLEM data were modelled using a three‐layer structure in order to estimate the thickness of the weathering layer along the survey line. Conversely, data obtained with a 100 m separation, corresponding to moderate depths of investigation, reveal significant anomalies from the buried dike at high frequencies. A phasor or vector diagram was used to calculate the response parameter, depth and dip of the buried dike.
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ADI plus interpolation: accurate finite‐difference solution to 3D paraxial wave equation
By Yanghua WangAn accurate finite‐difference solution is developed for the paraxial wave equation in 3D seismic migration. The conventional alternating‐direction‐implicit (ADI) scheme used in migration causes errors, because the variables in the migration problem are complex‐valued, not real‐valued, and the imaginary part of the higher‐order spatial derivatives cannot be ignored. The accuracy of the 3D paraxial extrapolator is preserved by (i) retaining these higher‐order terms so that it does not produce the apparent azimuthal anisotropy in conventional migration, and (ii) filtering the non‐physical evanescent waves during the downward extrapolation. The implementation of the accurate solution consists of two steps: firstly, the application of ADI to solve two tridiagonal systems sequentially, and secondly, an interpolation between the extrapolated wavefields of successive extrapolation levels. The method is computationally efficient as it uses the ADI scheme and, in addition, couples the correction for azimuthal anisotropy and the suppression of evanescent waves into a single operation, the interpolation step.
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Fast repeat‐modelling of time‐lapse seismograms
Authors Andreas Kirchner and Serge A. ShapiroIn order to plan surveys for seismic time‐lapse studies, seismograms are usually simulated for a set of earth models corresponding to different stages of the oil production process. This can be expensive because, for reasons of accuracy, finite‐difference (FD) modelling is very often applied.We present the Born repeat‐modelling technique (BRM), which is a combination of FD modelling and perturbation theory. Production effects are considered as small perturbations of the earth model, and the Born approximation is used to compute the change in the wavefield due to production‐induced modifications to the reservoir properties. This is very efficient in the situation described above.We give a detailed description of the proposed modelling scheme and discuss its applicability. The theoretical foundations of BRM are presented for onshore seismic experiments, and an extension to the case of marine (pressure) seismograms is derived. In addition, the performance of BRM is demonstrated with several numerical examples.
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On the effectiveness of 2D electrical inversion results: an agricultural case study
Authors C. Panissod, D. Michot, Y. Benderitter and A. TabbaghElectrical resistivity tomography was used in Beauce (France) to assess the water extraction by corn plants (evapotranspiration). The acquired pseudosections show conductive anomalies under the plants. A 2D inversion of measurements led us to identify clear resistive features associated with the water losses under the corn‐plant rows. New models have been calculated with two different 3D algorithms (finite‐difference and moment‐method) to take into account 3D structure of the ground and to confirm that periodic resistive features may generate shifted apparent‐resistivity anomalies.
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Limiting apparent‐resistivity values for dipping‐bed earth models
Authors Paul Hunt, Nigel Powell and Kathryn A. WatsonConsideration is given to some special features of normalized apparent‐resistivity (NAR) curves, resulting from the deployment of Wenner electrode arrays on the surface above dipping earth structures. Limiting values of the potential are derived when a direct‐current source is located at points on the surface above a simple two‐region dipping‐bed earth model and their influence on corresponding NAR curve characteristics is investigated. Particular attention is given to the exploitation of such features to provide a new and direct approach to model parameter estimation, either as an alternative to traditional curve‐matching techniques or as a source of supporting information when other earth model characterization methods have been employed. Throughout, the emphasis is on the single‐dipping‐bed model, but application to more complex structures is discussed, including examples of two dipping beds, dipping dikes and more general tilted unconformities.
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Improved amplitude preservation for prestack depth migration by inverse scattering theory
Authors Changsoo Shin, Seonghyung Jang and Dong‐Joo MinA prestack reverse time‐migration image is not properly scaled with increasing depth. The main reason for the image being unscaled is the geometric spreading of the wavefield arising during the back‐propagation of the measured data and the generation of the forward‐modelled wavefields. This unscaled image can be enhanced by multiplying the inverse of the approximate Hessian appearing in the Gauss–Newton optimization technique. However, since the approximate Hessian is usually too expensive to compute for the general geological model, it can be used only for the simple background velocity model.We show that the pseudo‐Hessian matrix can be used as a substitute for the approximate Hessian to enhance the faint images appearing at a later time in the 2D prestack reverse time‐migration sections. We can construct the pseudo‐Hessian matrix using the forward‐modelled wavefields (which are used as virtual sources in the reverse time migration), by exploiting the uncorrelated structure of the forward‐modelled wavefields and the impulse response function for the estimated diagonal of the approximate Hessian. Although it is also impossible to calculate directly the inverse of the pseudo‐Hessian, when using the reciprocal of the pseudo‐Hessian we can easily obtain the inverse of the pseudo‐Hessian. As examples supporting our assertion, we present the results obtained by applying our method to 2D synthetic and real data collected on the Korean continental shelf.
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Numerical simulation of 2D wave propagation on unstructured grids using explicit differential operators
Authors Martin Käser and Heiner IgelWe present a numerical method of simulating seismic wave propagation on unstructured 2D grids. The algorithm is based on the velocity–stress formulation of the elastic wave equation and therefore uses a staggered grid approach. Unlike finite‐element or spectral‐element methods, which can also handle flexible unstructured grids, we use explicit differential operators for the calculation of spatial derivatives in each time step. As shown in previous work, three types of these operators are used, and their particular performance is analysed and compared with standard explicit finite‐difference operators on regular quadratic and hexagonal grids. Our investigations are especially focused on the influence of grid irregularity, sampling rate (i.e. gridpoints per wavelength) and numerical anisotropy on the accuracy of numerical seismograms. The results obtained from the various methods are therefore compared with analytical solutions. The algorithm is then applied to a number of models that are difficult to handle using (quasi‐)regular grid methods. Such alternative techniques may be useful in modelling the full wavefield of bodies with complex geometries (e.g. cylindrical bore‐hole samples, 2D earth models) and, because of their local character, they are well suited for parallelization.
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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)