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- Volume 53, Issue 6, 2005
Geophysical Prospecting - Volume 53, Issue 6, 2005
Volume 53, Issue 6, 2005
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A constrained 2D gravity model of the Sebastián Vizcaíno Basin, Baja California Sur, Mexico
ABSTRACTThe subsurface geometry of the Sebastián Vizcaíno Basin is obtained from the 2D inversion of gravity data, constrained by a density‐versus‐depth relationship derived from an oil exploration deep hole. The basin accumulated a thick pile of marine sediments that evolved in the fore‐arc region of the compressive margin prevalent along western North America during Mesozoic and Tertiary times. Our interpretation indicates that the sedimentary infill in the Sebastián Vizcaíno Basin reaches a maximum thickness of about 4 km at the centre of a relatively symmetric basin. At the location of the Suaro‐1 hole, the depth to the basement derived from this work agrees with the drilled interface between calcareous and volcaniclastic members of the Alisitos Formation. A sensitivity analysis strongly suggests that the assumed density function leads to a nearly unique solution of the inverse problem.
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Improving Kirchhoff migration with repeated local plane‐wave imaging? A SAR‐inspired signal‐processing approach in prestack depth imaging
Authors Isabelle Lecomte, Svein‐Erik Hamran and Leiv‐J. GeliusABSTRACTA local plane‐wave approach of generalized diffraction tomography in heterogeneous backgrounds, equivalent to Kirchhoff summation techniques when applied in seismic reflection, is re‐programmed to act as repeated synthetic aperture radar (SAR) imaging for seismic prestack depth migration. Spotlight‐mode SAR imaging quickly provides good images of the electromagnetic reflectivity of the ground via fast Fourier transform (FFT)‐based signal processing. By calculating only the Green's functions connecting the aircraft to the centre of the illuminated patch, scattering structures around that centre are also recovered. SAR technology requires us to examine seismic imaging from the local point of view, where the quantity and quality of the available information at each image point are what are important, regardless of the survey geometry. When adapted to seismics, a local image of arbitrary size and sampling is obtained by FFT of seismic energy maps in the scattering wavenumber domain around each node of a pre‐calculated grid of Green's functions. These local images can be used to generate a classic prestack depth‐migrated section by collecting only their centres. However, the local images also provide valuable information around the centre, as in SAR. They can therefore help to pre‐analyse prestack depth migration efficiently, and to perform velocity analysis at a very low cost. The FFT‐based signal‐processing approach allows local, efficient and automatic control of anti‐aliasing, noise and resolution, including optimized Jacobian weights. Repeated local imaging could also be used to speed up migration, with interpolation between local images associated with a coarse grid of Green's functions, as an alternative to interpolation of Green's functions. The local images may, however, show distortions due to the local plane‐wave approximation, and the velocity variations across their frame. Such effects, which are not necessarily a problem in SAR, should be controlled and corrected to further enhance seismic imaging. Applications to realistic models and to real data show that, despite the distortion effects, the local images can yield similar information to prestack depth migration, including common‐image‐point gathers for velocity analyses and AVO/AVA effects, at a much lower cost when a small target is considered.
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Minimum weighted norm wavefield reconstruction for AVA imaging
Authors Mauricio D. Sacchi and Bin LiuABSTRACTSeismic wavefield reconstruction is posed as an inversion problem where, from inadequate and incomplete data, we attempt to recover the data we would have acquired with a denser distribution of sources and receivers. A minimum weighted norm interpolation method is proposed to interpolate prestack volumes before wave‐equation amplitude versus angle imaging.
Synthetic and real data were used to investigate the effectiveness of our wavefield reconstruction scheme when preconditioning seismic data for wave‐equation amplitude versus angle imaging.
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Estimation of gas‐hydrate concentration and free‐gas saturation at the Norwegian‐Svalbard continental margin
Authors José M. Carcione, Davide Gei, Giuliana Rossi and Gianni MadrussaniABSTRACTWe estimate the concentration of gas hydrate and free gas at an area located to the north of the Knipovich Ridge (western Svalbard margin). The method is based on P‐wave velocities computed by reflection tomography applied to multicomponent ocean‐bottom seismometer data. The tomographic velocity field is fitted to theoretical velocities obtained from a poro‐elastic model based on a Biot‐type approach (the interaction between the rock frame, gas hydrate and fluid is modelled from first physical principles). We obtain average hydrate concentrations of 7% and maximum free‐gas saturations of 0.4% and 9%, depending on the saturation model.
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Least‐squares wave‐path migration
Authors Yike Liu, Hongchuan Sun and Xu ChangABSTRACTWe present a new least‐squares migration method called least‐squares wave‐path migration. The proposed method combines an iterative conjugate‐gradient solver with a stationary‐phase wave‐path migration operator. Numerical tests demonstrate that (i) least‐squares wave‐path migration is computationally more efficient than and almost as accurate as Kirchhoff least‐squares migration, and (ii) many of the artefacts seen in wave‐path migration images are suppressed after several conjugate‐gradient iterations. Previous results have shown that 3D wave‐path migration is up to 100 times faster than a standard 3D Kirchhoff migration, but sometimes at the cost of reduced quality. With the proposed least‐squares wave‐path migration method, the image quality in wave‐path migration can be improved at an acceptable increase in computational cost.
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Mapping the geometry of an aquifer system with a high‐resolution reflection seismic profile
Authors Roberto Francese, Mauro Giudici, Douglas R. Schmitt and Annalisa ZajaABSTRACTA high‐resolution P‐wave seismic reflection survey was conducted in the area of Lambro Park within the city of Milan (northern Italy). Several high permeability channel sequences were identified and the stratigraphy of the infill was accurately mapped. The seismic signature imaged a previously undefined unit and also established the lateral correlation of some depositional units within the three major aquifer groups along the survey line.
The complexity of the acoustic framework and the reduced depth of the source location limited effective elastic‐wave generation. The geological setting also caused propagation of strong coherent noise patterns. Further interference, observed in the recorded data, was due to the traffic noise from the nearby highway. The attenuation of the undesired events required the design of specific filters and their multistep implementation.
The results of forward modelling based on borehole information and of noise tests were crucial factors in the design of the processing parameters and in the stratigraphic interpretation of the final stacked section.
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Simulation of two‐phase fluid flow through compactible reservoirs
Authors Vladimir Grechka and Leigh SoutterABSTRACTFluid‐flow simulators used in the oil industry model the movement of fluids through a porous reservoir rock. These simulators either ignore coupling between the flow and concurring deformation of the solid rock frame or take it into account approximately, in the so‐called loose or staggered‐in‐time mode. In contrast to existing simulators, the one we describe here fully couples two‐phase (oil and water) flow to subsurface deformation and simultaneously accounts for all relevant physical phenomena. As such, our flow simulator inherently links time‐dependent fluid pressures, saturations, permeabilities and flow velocities to stresses in the whole subsurface. These stresses relate to strains through the non‐linear theory of elasticity, allowing us to model time‐lapse changes in seismic velocities and anisotropy. The velocity variations manifest themselves in time shifts and reflection amplitudes that are conventionally measured from 4D seismic data. Changes in anisotropy produce time‐dependent shear‐wave splitting that can be used for monitoring the horizontal stresses.
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Finite‐difference modelling of wave propagation in acoustic tilted TI media
Authors Linbin Zhang, James W. Rector III and G. Michael HoverstenABSTRACTBased on an acoustic assumption (that the shear‐wave velocity is zero) and a dispersion relationship, we derive an acoustic wave equation for P‐waves in tilted transversely isotropic (TTI) media (transversely isotropic media with a tilted symmetry axis). This equation has fewer parameters than an elastic wave equation in TTI media and yields an accurate description of P‐wave traveltimes and spreading‐related attenuation. Our TTI acoustic wave equation is a fourth‐order equation in time and space. We demonstrate that the acoustic approximation allows the presence of shear waves in the solution. The substantial differences in traveltime and amplitude between data created using vertical transversely isotropic (VTI) and TTI assumptions is illustrated in examples.
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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)