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- Volume 56, Issue 5, 2008
Geophysical Prospecting - Volume 56, Issue 5, 2008
Volume 56, Issue 5, 2008
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Stereotomography assisted by migration of attributes
Authors Sylvain Nguyen, Reda Baina, Mathias Alerini, Gilles Lambaré, Vincent Devaux and Mark NobleABSTRACTDepth velocity model building remains a difficult step within the seismic depth imaging sequence. Stereotomography provides an efficient solution to this problem but was limited until now to a picking of seismic data in the prestack time un‐migrated domain. We propose here a method for stereotomographic data picking in the depth migrated domain. Picking in the depth migrated domain exhibits the advantage of a better signal‐to‐noise ratio and of a more regular distribution of picked events in the model, leading to a better constrained tomographic inverse problem. Moreover, any improvement on the velocity model will improve the migrated results, again leading to improved picking. Our strategy for obtaining a stereotomographic dataset from a prestack depth migration is based on migration of attributes (and not on a kinematic demigration approach!). For any locally coherent event in the migrated image, migration of attributes allows one to compute ray parameter attributes corresponding to the specular reflection angle and dip. For application to stereotomography, the necessary attributes are the source/receiver locations, the traveltime and the data slopes. For the data slope, when the migration velocity model is erroneous, some additional corrections have to be applied to the result of migration of the attributes. Applying these corrections, our picking method is theoretically valid whatever the quality of the migration velocity model. We first present the theoretical aspects of the method and then validate it on 2D synthetic and real seismic reflection data sets.
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Diffraction imaging in depth
Authors T.J. Moser and C.B. HowardABSTRACTHigh resolution imaging is of great value to an interpreter, for instance to enable identification of small scale faults, and to locate formation pinch‐out positions. Standard approaches to obtain high‐resolution information, such as coherency analysis and structure‐oriented filters, derive attributes from stacked, migrated images. Since they are image‐driven, these techniques are sensitive to artifacts due to an inadequate migration velocity; in fact the attribute derivation is not based on the physics of wave propagation. Diffracted waves on the other hand have been recognized as physically reliable carriers of high‐ or even super‐resolution structural information. However, high‐resolution information, encoded in diffractions, is generally lost during the conventional processing sequence, indeed migration kernels in current migration algorithms are biased against diffractions. We propose here methods for a diffraction‐based, data‐oriented approach to image resolution. We also demonstrate the different behaviour of diffractions compared to specular reflections and how this can be leveraged to assess characteristics of subsurface features. In this way a rough surface such as a fault plane or unconformity may be distinguishable on a diffraction image and not on a traditional reflection image.
We outline some characteristic properties of diffractions and diffraction imaging, and present two novel approaches to diffraction imaging in the depth domain. The first technique is based on reflection focusing in the depth domain and subsequent filtering of reflections from prestack data. The second technique modifies the migration kernel and consists of a reverse application of stationary‐phase migration to suppress contributions from specular reflections to the diffraction image. Both techniques are proposed as a complement to conventional full‐wave pre‐stack depth migration, and both assume the existence of an accurate migration velocity.
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The effect of inertial coupling on seismic reflection amplitudes
Authors Ashish Arora and S.K. TomarABSTRACTA problem of reflection and transmission of elastic waves at a plane interface between a uniform elastic solid half‐space and a porous elastic half‐space containing two immiscible fluids is investigated. The theory developed by Lo, Sposito and Majer for porous media containing two immiscible fluids is employed to find out the reflection and transmission coefficients. The incident wave is assumed to propagate through the uniform elastic half‐space and two cases are considered. In the first case, a beam of plane longitudinal wave is assumed to be incident and in the second case, a beam of transverse wave is assumed to be incident at the interface. By taking granite as impervious elastic medium and columbia fine sandy loam containing air‐water mixture as porous medium, reflection and transmission coefficients are obtained. By neglecting the inertial coupling coefficients, these coefficients are reduced to those obtained by Tomar and Arora using the theory of Tuncay and Corapcioglu. It is found that the inertial coupling parameters significantly affect the phase speeds and the amplitude ratios of the transmitted waves.
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Characterization of the shallow aquifers by high‐resolution seismic data
Authors M. Giustiniani, F. Accaino, S. Picotti and U. TinivellaABSTRACTWe present the results obtained by processing high‐resolution seismic data acquired along the spring line located in the Friuli‐Venezia Giulia plain (NE of Italy), in order to characterize an important multilayered aquifer. This system is made of an unconfined layer and, at increasing depths, of several confined aquifers of variable thickness and hydraulic permeability, mainly consisting of sand and gravel material. The main targets of this study are two shallow aquifers located at about 30 m and 200 m depth respectively. The seismic method is not frequently used for this type of study but in this case, it was considered a good tool due to the depth of the targets. The detailed velocity model we obtained reveals lateral velocity variations with a maximum value of 600 m/s. The higher velocities could be associated to layers that are confined aquifers; in fact, sand and gravel are characterized by higher seismic velocity compared to clay layers. Pre‐stack depth migration using this velocity model gives a clear picture of the multilayered aquifer, highlighting lateral changes of seismic amplitude along the main reflectors. Finally, vertical variations of Poisson's ratio, computed by amplitude versus offset analysis, provide useful information about the petrophysical properties, such as the fluid content of the subsoil and lithologic changes.
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Precise arrival time detection of polarized seismic waves using the spectral matrix
More LessABSTRACTI present a statistical method for detecting the arrival times of polarized seismic waves on three‐component seismic observations in the time and frequency domains. In this method, the polarization, which is a representation of the 3D particle motion of seismic waves, is evaluated on the basis of a spectral matrix in the time and frequency domains and the statistical parameters are defined by using the eigenvalues of the spectral matrix for detecting the arrival times of linearly and elliptically polarized waves, where the idea of a statistical test of hypothesis is introduced. An evaluation of a synthetic signal revealed that the method can detect the arrival times of linearly and elliptically polarized waves within 10 sampling points at signal‐to‐noise ratios of −7 dB. Application of the method to an earthquake suggested that it can be used to detect the arrival times of both linearly and elliptically polarized waves, which are difficult to identify manually.
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Modelling production caused changes in conductivity for a siliciclastic reservoir: a differential effective medium approach
Authors Leiv‐J. Gelius and Zhong WangABSTRACTMarine controlled‐source electromagnetic sounding is a rapidly growing exploration technique that complements the seismic method in case of pore fluid discrimination between water and oil. Further development of the method is foreseen to be within reservoir monitoring involving permanent instrumentation on both seafloor and within wells. Such advanced applications demand a good understanding and description of the electric Earth model to ensure accurate modelling of possible Earth responses as well as successful inversion of measurement data. Based on earlier works carried out within petrophysics and resistivity‐logging, an extended effective‐medium scheme is derived that can be applied to model reservoir production effects. This rock‐physics model has the potential of taking into account the effect of important parameters like grain‐shape distribution, grain alignment, shaliness, salinity, saturation, temperature and stress, based on a uniform formulation. It has been calibrated by employing various core sample measurements. The effective‐medium scheme can easily be interfaced with an existing standard electromagnetic modelling program and several such examples are presented.
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Seismic scale saturation relations in turbidite reservoirs undergoing waterflood
Authors Colin MacBeth and Karl StephenABSTRACTEstimates of the effective fluid modulus from seismic cannot be directly converted to the true pore‐volume weighted mean saturation determined from fluid flow principles by using the saturation laws currently in use. One of the reasons is that seismic waves sample the reservoir geology and production induced saturation heterogeneity in a different way from the fluids. This mismatch prevents accurate quantitative evaluation of saturation changes from 4D seismic analysis. To tackle this problem, a reservoir‐related saturation law is developed for a turbidite reservoir – this geology being chosen because the architecture for a single sand package can be modelled as a stack of horizontal beds. An effective medium and perturbation theory are applied to the determination of the seismic properties of this model. This calculation provides a relationship that connects the true saturation to the effective fluid modulus from seismic via statistical measures of the vertical spread of the porosity and saturation variations in the reservoir. These statistics can be extracted from the simulation model and if known, enable the new saturation law to deliver a significant improvement in accuracy when estimating compared to other well‐known laws. The relationship that has been developed also captures the effect of inter‐bedded shales and can therefore be used to estimate true saturation in regions of the reservoir with moderate to low net‐to‐gross, provided the fraction of the shale component is known. In practice, the final choice of saturation law depends upon the reservoir information available, the assumptions that can be tolerated and the accuracy required in any particular reservoir characterization study.
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Effective permittivity of porous media: a critical analysis of the complex refractive index model
Authors Alessandro Brovelli and Giorgio CassianiABSTRACTThe availability of reliable constitutive models linking the bulk electric properties of porous media to their inner structure is a key requirement for useful quantitative applications of noninvasive methods. This study focuses on the use of dielectric measurements to monitor fluid saturation changes in porous materials. A number of empirical, semi‐empirical and theoretical relationships currently exists that link the bulk dielectric constant with volumetric water content. One such relationship, named complex refractive index model or Lichteneker‐Rother model has been extensively applied in recent years. Here we first analyse the characteristics of this Lichteneker‐Rother model by means of theoretical considerations. This theoretical analysis indicates that the Lichteneker‐Rother exponent is dependent upon the geometrical properties of the porous structure, as well as the permittivity contrast between the different phases. Pore‐scale modelling and experimental data further support this result. The parameter estimation robustness in presence of synthetic data error is also assessed. This demonstrates that Lichteneker‐Rother parameters cannot, in general, be independently identified on the basis of bulk dielectric constant versus moisture content data.
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First‐order error propagation formulae for the Gassmann relations
More LessABSTRACTThe Gassmann relations of poroelasticity provide a connection between the dry and the saturated elastic moduli of porous rock and are useful in a variety of petroleum geoscience applications. Because some uncertainty is usually associated with the input parameters, the propagation of error in the inputs into the final moduli estimates is immediately of interest. Two common approaches to error propagation include: a first‐order Taylor series expansion and Monte‐Carlo methods. The Taylor series approach requires derivatives, which are obtained either analytically or numerically and is usually limited to a first‐order analysis. The formulae for analytical derivatives were often prohibitively complicated before modern symbolic computation packages became prevalent but they are now more accessible. We apply this method and present formulae for uncertainty in the predicted bulk and shear moduli for two forms of the Gassmann relations. Numerical results obtained with these uncertainty formulae are compared with Monte‐Carlo calculations as a form of validation and to illustrate the relative characteristics of the two approaches. Particular emphasis is given to the problem of correlated variables, which are often ignored in naïve approaches to error analysis. Going out to the error level that the two methods were compared, the means agree and the variance of the Monte Carlo method for bulk modulus grows with input error.
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Two‐dimensional joint inversion of radiomagnetotelluric and direct current resistivity data
Authors M. Emin Candansayar and Bülent TezkanABSTRACTAn algorithm for the two‐dimensional (2D) joint inversion of radiomagnetotelluric and direct current resistivity data was developed. This algorithm can be used for the 2D inversion of apparent resistivity data sets collected by multi‐electrode direct current resistivity systems for various classical electrode arrays (Wenner, Schlumberger, dipole‐diplole, pole‐dipole) and radiomagnetotelluric measurements jointly. We use a finite difference technique to solve the Helmoltz and Poisson equations for radiomagnetotelluric and direct current resistivity methods respectively. A regularized inversion with a smoothness constrained stabilizer was employed to invert both data sets. The radiomagnetotelluric method is not particularly sensitive when attempting to resolve near‐surface resistivity blocks because it uses a limited range of frequencies. On the other hand, the direct current resistivity method can resolve these near‐surface blocks with relatively greater accuracy. Initially, individual and joint inversions of synthetic radiomagnetotelluric and direct current resistivity data were compared and we demonstrated that the joint inversion result based on this synthetic data simulates the real model more accurately than the inversion results of each individual method. The developed 2D joint inversion algorithm was also applied on a field data set observed across an active fault located close to the city of Kerpen in Germany. The location and depth of this fault were successfully determined by the 2D joint inversion of the radiomagnetotelluric and direct current resistivity data. This inversion result from the field data further validated the synthetic data inversion results.
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Gravimetric study of a potential mineral deposit in the Itapororoca region, Brazil
More LessABSTRACTA semi‐detailed gravity survey was carried out over an area of 650 km2 localized in the Eo‐Neoproterozoic coastal zone of Paraiba State where 548 new gravity stations were added to the existing database. Gravity measurements were made with a LaCoste and Romberg model G meter with a precision of 0.04 mGal. The altitude was determined by barometric levelling with a fixed base achieving a 1.2 m measure of uncertainty, corresponding to an overall accuracy of 0.24 mGal for the Bouguer anomaly. The residual Bouguer map for a 7th degree regional polynomial showed a circumscribed negative anomaly coincident with a localized aero‐magnetic anomaly and with hydro‐thermally altered outcrops, near the city of Itapororoca. The 3D gravity modelling, constrained by geologic mapping was interpreted as a low density, fractured and/or altered material with a most probable volume of approximately 23 km3, extending to about 8,500 m depth. This result is in accordance with a volcanic body associated with hydrothermal processes accompanied by surface mineralization evidence, which may be of interest to the mining industry.
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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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