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- Volume 50, Issue 2, 2002
Geophysical Prospecting - Volume 50, Issue 2, 2002
Volume 50, Issue 2, 2002
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Successful application of ground‐penetrating radar in the exploration of gem tourmaline pegmatites of southern California
Authors Jeffrey E. Patterson and Frederick A. CookABSTRACTApplication of ground‐penetrating radar has been successful in delineating gem‐bearing zones in the Himalaya pegmatite mine of the Mesa Grande district of southern California. The high frequency of the electromagnetic signal allows features as small as a few centimetres to be resolved within 1–2 m of the surface of a mine wall. Careful initial set‐up consisted of: (i) selection of antennae with sufficiently high central frequencies; (ii) recording with a short time of scan to reduce end‐of‐scan noise levels; (iii) choosing appropriate colour schemes to highlight extreme amplitude variations. Operation during data collection consisted of pre‐painting marking points on the mine face and air launching the signal to reduce false anomalies caused by rocking of the antenna on the rough surfaces. Data processing using the Hilbert transform provided images of the cavity geometry that were then used by the blasting captain for accurate placement of explosives. The instantaneous frequency plot was found to be effective in distinguishing air‐filled from clay‐filled pockets, and the instantaneous phase plot was helpful in selecting potential targets where the amplitude was less than the maximum range. When carefully used in conjunction with good knowledge of the geological conditions, the method promises to provide an important tool for mapping internal structures of pegmatites, thus assisting future mining activities.
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An approach to combined rock physics and seismic modelling of fluid substitution effects
Authors Tor Arne Johansen, Åsmund Drottning, Isabelle Lecomte and Håvar GjøystdalABSTRACTThe aim of seismic reservoir monitoring is to map the spatial and temporal distributions and contact interfaces of various hydrocarbon fluids and water within a reservoir rock. During the production of hydrocarbons, the fluids produced are generally displaced by an injection fluid. We discuss possible seismic effects which may occur when the pore volume contains two or more fluids. In particular, we investigate the effect of immiscible pore fluids, i.e. when the pore fluids occupy different parts of the pore volume.
The modelling of seismic velocities is performed using a differential effective‐medium theory in which the various pore fluids are allowed to occupy the pore space in different ways. The P‐wave velocity is seen to depend strongly on the bulk modulus of the pore fluids in the most compliant (low aspect ratio) pores. Various scenarios of the microscopic fluid distribution across a gas–oil contact (GOC) zone have been designed, and the corresponding seismic properties modelled. Such GOC transition zones generally give diffuse reflection regions instead of the typical distinct GOC interface. Hence, such transition zones generally should be modelled by finite‐difference or finite‐element techniques.
We have combined rock physics modelling and seismic modelling to simulate the seismic responses of some gas–oil zones, applying various fluid‐distribution models. The seismic responses may vary both in the reflection time, amplitude and phase characteristics. Our results indicate that when performing a reservoir monitoring experiment, erroneous conclusions about a GOC movement may be drawn if the microscopic fluid‐distribution effects are neglected.
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Increasing the vertical resolution of conventional sub‐bottom profilers by parametric equalization
Authors P. Cobo, C. Ranz and M. CerveraABSTRACTVertical resolution, i.e. the ability to resolve two close reflectors, is a crucial aspect of pulses used in geo‐acoustic exploration of sea sub‐bottoms. This paper deals with the problem of exploring the shallowest unconsolidated layers of the seafloor with conventional piezo‐electric sonar pulses. Such transducers do not have a sufficiently broad transmission response to enable them to radiate short high‐resolution pulses. Therefore, some kind of equalization process must be applied to broaden the transmission response. Here, inverse filtering is used to calculate the transducer driving waveform so that the subsequent acoustic pulse has a zero‐phase cosine‐magnitude nature. Within a specified bandwidth, this pulse has minimum length, i.e. maximum resolution.
The method has been applied to compress the acoustic pulses radiated by two piezo‐electric transducers. In conventional performance, these transducers radiate narrowband pulses which contain several cycles at the natural resonance frequency. Under equalized driving, both transducers emit broadband pulses, with resolving power greatly increased, at the cost of some amplitude loss. That is, the pulses radiated by both transducers have been shortened from 1 ms (low‐frequency transducer) and 0.274 ms (high‐frequency transducer) in conventional performance to 0.13 ms and 0.038 ms in equalized mode, with amplitude losses of 33% and 56%, respectively. The great improvement in the resolution of this technique is demonstrated by comparing the synthetic echograms that should be obtained when exploring a wedge model using zero‐phase cosine‐magnitude pulses with conventional ping pulses.
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P‐wave stacking‐velocity tomography for VTI media
Authors Vladimir Grechka, Andres Pech and Ilya TsvankinABSTRACTA major complication caused by anisotropy in velocity analysis and imaging is the uncertainty in estimating the vertical velocity and depth scale of the model from surface data. For laterally homogeneous VTI (transversely isotropic with a vertical symmetry axis) media above the target reflector, P‐wave moveout has to be combined with other information (e.g. borehole data or converted waves) to build velocity models for depth imaging. The presence of lateral heterogeneity in the overburden creates the dependence of P‐wave reflection data on all three relevant parameters (the vertical velocity VP0 and the Thomsen coefficients ε and δ) and, therefore, may help to determine the depth scale of the velocity field.
Here, we propose a tomographic algorithm designed to invert NMO ellipses (obtained from azimuthally varying stacking velocities) and zero‐offset traveltimes of P‐waves for the parameters of homogeneous VTI layers separated by either plane dipping or curved interfaces. For plane non‐intersecting layer boundaries, the interval parameters cannot be recovered from P‐wave moveout in a unique way. Nonetheless, if the reflectors have sufficiently different azimuths, a priori knowledge of any single interval parameter makes it possible to reconstruct the whole model in depth. For example, the parameter estimation becomes unique if the subsurface layer is known to be isotropic. In the case of 2D inversion on the dip line of co‐orientated reflectors, it is necessary to specify one parameter (e.g. the vertical velocity) per layer.
Despite the higher complexity of models with curved interfaces, the increased angle coverage of reflected rays helps to resolve the trade‐offs between the medium parameters. Singular value decomposition (SVD) shows that in the presence of sufficient interface curvature all parameters needed for anisotropic depth processing can be obtained solely from conventional‐spread P‐wave moveout. By performing tests on noise‐contaminated data we demonstrate that the tomographic inversion procedure reconstructs both the interfaces and the VTI parameters with high accuracy. Both SVD analysis and moveout inversion are implemented using an efficient modelling technique based on the theory of NMO‐velocity surfaces generalized for wave propagation through curved interfaces.
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Modelling the seismic attributes of overpressured siliciclastic rocks, with a genetic annealing (GAN) algorithm
Authors Patricia Domnesteanu, Robert Domnesteanu and Clive McCannABSTRACT A genetic annealing (GAN) algorithm is used to derive an empirical model which predicts compressional‐wave velocity values for overpressured siliciclastic rocks. The algorithm involves non‐linear random searching and mutation techniques and its annealing component imposes a very strict control over the rate of convergence of the search. This technique provides an alternative to the standard calculations involving the effective stress coefficient (n). The pore pressure is introduced into the model as an explicit variable and as part of an overpressure coefficient, (Pp/Pc) − the ratio of pore to confining pressure. Empirical model‐derived data and known laboratory data are compared and their differences are shown to be within statistically acceptable error limits. The empirical equation fits all under‐ and overpressured data simultaneously, irrespective of pore fluid pressure level, with the same parameters. It is used to predict seismic velocities very accurately for extreme levels of overpressure, starting from normally pressured experimental data. The model highlights the effect of pore pressure on the compressional‐wave velocity of fully saturated samples with different clay contents. It can be used when the experimental data available are sparse and particularly when a prediction of material behaviour is necessary at specific pore fluid pressure and depth conditions.
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Effective velocities in fractured media: a numerical study using the rotated staggered finite‐difference grid
Authors Erik H. Saenger and Serge A. ShapiroABSTRACTThe modelling of elastic waves in fractured media with an explicit finite‐difference scheme causes instability problems on a staggered grid when the medium possesses high‐contrast discontinuities (strong heterogeneities). For the present study we apply the rotated staggered grid. Using this modified grid it is possible to simulate the propagation of elastic waves in a 2D or 3D medium containing cracks, pores or free surfaces without hard‐coded boundary conditions. Therefore it allows an efficient and precise numerical study of effective velocities in fractured structures. We model the propagation of plane waves through a set of different, randomly cracked media. In these numerical experiments we vary the wavelength of the plane waves, the crack porosity and the crack density. The synthetic results are compared with several static theories that predict the effective P‐ and S‐wave velocities in fractured materials in the long wavelength limit. For randomly distributed and randomly orientated, rectilinear, non‐intersecting, thin, dry cracks, the numerical simulations of velocities of P‐, SV‐ and SH‐waves are in excellent agreement with the results of the modified (or differential) self‐consistent theory. On the other hand for intersecting cracks, the critical crack‐density (porosity) concept must be taken into account. To describe the wave velocities in media with intersecting cracks, we propose introducing the critical crack‐density concept into the modified self‐consistent theory. Numerical simulations show that this new formulation predicts effective elastic properties accurately for such a case.
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Rough seas and time‐lapse seismic
Authors Robert Laws and Ed KraghABSTRACTTime‐lapse seismic surveying has become an accepted tool for reservoir monitoring applications, thus placing a high premium on data repeatability. One factor affecting data repeatability is the influence of the rough sea‐surface on the ghost reflection and the resulting seismic wavelets of the sources and receivers. During data analysis, the sea‐surface is normally assumed to be stationary and, indeed, to be flat. The non‐flatness of the sea‐surface introduces amplitude and phase perturbations to the source and receiver responses and these can affect the time‐lapse image.
We simulated the influence of rough sea‐surfaces on seismic data acquisition. For a typical seismic line with a 48‐fold stack, a 2‐m significant‐wave‐height sea introduces RMS errors of about 5–10% into the stacked data. This level of error is probably not important for structural imaging but could be significant for time‐lapse surveying when the expected difference anomaly is small. The errors are distributed differently for sources and receivers because of the different ways they are towed. Furthermore, the source wavelet is determined by the sea shape at the moment the shot is fired, whereas the receiver wavelet is time‐varying because the sea moves significantly during the seismic record.
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Finding the shape of a local heterogeneity by means of a structural inversion with constraints
By Pavel DitmarABSTRACTVarious aspects of structural inversion are considered. The aim of the inversion is limited to finding the shape of an isolated 2D homogeneous body, although the technique may be generalized to the case of interfaces with steep fragments, faults or overhangs. The unknown parameters are shifts of border points. The shift directions can be normal to the initial heterogeneity configuration or to another contour. Medium properties (seismic velocities, densities, etc.) within the heterogeneity are assumed to be known. The optimal shape is determined iteratively, a quadratic objective function being minimized at each iteration with the conjugate‐gradient method. Special attention is paid to preventing self‐intersections, for which purpose each unknown parameter is forced to lie within a certain predetermined interval. In order to achieve this, the classical conjugate‐gradient method has been modified accordingly. Three numerical examples are considered. These illustrate how the developed technique can be applied to different practical problems. The first example is devoted to monitoring an oil/steam interface by gravity gradiometry measurements. In the second example, a cross‐hole seismic experiment is simulated. It is shown that a structural inversion can restore the configuration of a local body much more accurately than traditional seismic tomography. In the third example, the shape of a salt dome is reconstructed by joint inversion of refracted traveltimes and gravity measurements. This example demonstrates how different kinds of data, used simultaneously in a structural inversion, can complement each other.
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Improved NMO correction with a specific application to shallow‐seismic data
More LessABSTRACTThe application of traditional NMO‐correction techniques in the processing of seismic data may result in severe distortion. This distortion is observed as a decrease in frequency content (NMO stretch) or even a total loss of data for steep velocity gradients. It is shown that the specific nature of the observed distortion is introduced by the particular implementation of the traditional NMO‐correction technique. It is also shown that other techniques that have been proposed in the literature suffer from similar artefacts. An alternative approach is suggested, based on the correction of tapered blocks of seismic data, followed by a coherence filter to compensate for the specific artefacts thus introduced. Correction of seismic amplitudes (e.g. geometrical spreading and attenuation) is implemented as an integral part of the NMO‐correction method thus allowing for both dynamic and kinematic reconstruction of interfering events. From the application of this method to synthetic and field data, it is concluded that the proposed technique may be particularly useful in the processing of shallow seismic (shear‐wave) data. Although not illustrated in this paper, it is emphasized that application of the proposed method is by no means restricted to shear waves; application to P‐wave data will prove equally useful, although the stretch effects involved may not be as severe as in the examples given in this paper.
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Series approximation of the Kirchhoff method for wavefield computation on a rough subsurface
Authors Selma Kadioğlu and Turan KayiranABSTRACT Computation of the wavefield due to reflection from an irregular surface is carried out for subsurfaces with large radii of curvature. The Kirchhoff approximation is proved to be sufficiently accurate provided that the acoustic wavelength is sufficiently small with respect to the asperities of the rough surface. For cases where the irregular surface does not fulfil this condition, a series solution is proposed. The first term of this series appears to be the result obtained by conventional Kirchhoff approximation. The series, initially developed in the space–wavenumber domain by Meecham, is transformed into the space–time domain, and the general expression for the series is obtained by calculation of the normal derivative of the field function. The series solution, restricted to the first two terms, is illustrated by application to three synthetic examples. Applications show that the series approximation obtained by the Kirchhoff method contributes significantly to the modelling of narrow, steep and deep structures and consequently it appears that the second term in the series cannot be ignored in the computation of the wavefields arising from a rough surface.
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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)