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- Volume 58, Issue 3, 2010
Geophysical Prospecting - Volume 58, Issue 3, 2010
Volume 58, Issue 3, 2010
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Elastic interferometry for ocean bottom cable data: Theory and examples‡
Authors J.E. Gaiser and I. VasconcelosABSTRACTElastic imaging from ocean bottom cable (OBC) data can be challenging because it requires the prior estimation of both compressional‐wave (P‐wave) and shear‐wave (S‐wave) velocity fields. Seismic interferometry is an attractive technique for processing OBC data because it performs model‐independent redatuming; retrieving ‘pseudo‐sources’ at positions of the receivers. The purpose of this study is to investigate multicomponent applications of interferometry for processing OBC data. This translates into using interferometry to retrieve pseudo‐source data on the sea‐bed not only for multiple suppression but for obtaining P‐, converted P to S‐wave (PS‐wave) and possibly pure mode S‐waves. We discuss scattering‐based, elastic interferometry with synthetic and field OBC datasets.
Conventional and scattering‐based interferometry integrands computed from a synthetic are compared to show that the latter yields little anti‐causal response. A four‐component (4C) pseudo‐source response retrieves pure‐mode S‐reflections as well at P‐ and PS‐reflections. Pseudo‐source responses observed in OBC data are related to P‐wave conversions at the seabed rather than to true horizontal or vertical point forces. From a Gulf of Mexico OBC data set, diagonal components from a nine‐component pseudo‐source response demonstrate that the P‐wave to S‐wave velocity ratio (VP/VS) at the sea‐bed is an important factor in the conversion of P to S for obtaining the pure‐mode S‐wave reflections.
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Seismic interferometry, intrinsic losses and Q‐estimation*
Authors Deyan Draganov, Ranajit Ghose, Elmer Ruigrok, Jan Thorbecke and Kees WapenaarABSTRACTSeismic interferometry is the process of generating new seismic traces from the cross‐correlation, convolution or deconvolution of existing traces. One of the starting assumptions for deriving the representations for seismic interferometry by cross‐correlation is that there is no intrinsic loss in the medium where the recordings are performed. In practice, this condition is not always met. Here, we investigate the effect of intrinsic losses in the medium on the results retrieved from seismic interferometry by cross‐correlation. First, we show results from a laboratory experiment in a homogeneous sand chamber with strong losses. Then, using numerical modelling results, we show that in the case of a lossy medium ghost reflections will appear in the cross‐correlation result when internal multiple scattering occurs. We also show that if a loss compensation is applied to the traces to be correlated, these ghosts in the retrieved result can be weakened, can disappear, or can reverse their polarity. This compensation process can be used to estimate the quality factor in the medium.
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Synthesis of a seismic virtual reflector*
Authors Flavio Poletto and Biancamaria FarinaABSTRACTWe describe a method to process the seismic data generated by a plurality of sources and registered by an appropriate distribution of receivers, which provides new seismic signals as if in the position of the receivers (or sources) there was an ideal reflector, even if this reflector is not present there. The data provided by this method represent the signals of a virtual reflector. The proposed algorithm performs the convolution and the subsequent sum of the real traces without needing subsurface model information. The approach can be used in combination with seismic interferometry to separate wavefields and process the reflection events. The application is described with synthetic examples, including stationary phase analysis and with real data in which the virtual reflector signal can be appreciated.
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Identification of lateral discontinuities via multi‐offset phase analysis of surface wave data
Authors Giulio Vignoli and Giorgio CassianiABSTRACTSurface wave methods are based on the inversion of observed Rayleigh wave phase‐velocity dispersion curves. The goal is to estimate mainly the shear‐wave velocity profile of the investigated site. The model used for the interpretation is 1D, hence results obtained wherever lateral variations are present cannot be considered reliable.
In this paper, we study four synthetic models, all with a lateral heterogeneity. When we process the entire corresponding seismograms with traditional f‐k approach, the resulting 1D profiles are representative of the subsurface properties averaged over the whole length of the receivers lines. These results show that classical analysis disregards evidences of sharp lateral velocity changes even when they show up in the raw seismograms.
In our research, we implement and test over the same synthetic models, a novel robust automated method to check the appropriateness of 1D model assumption and locate the discontinuities. This new approach is a development of the recent multi‐offset phase analysis with the following further advantages: it does not need previous noise evaluation and more than one shot.
Only once the discontinuities are clearly identified, we confidently perform classical f‐k dispersion curve extraction and inversion separately on both sides of the discontinuity. Thus the final results, obtained by putting side by side the 1D profiles, are correct 2D reconstructions of the discontinuous S‐wave distributions obtained without any additional ad‐hoc hypotheses.
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Nonlinear structure‐enhancing filtering using plane‐wave prediction*
Authors Yang Liu, Sergey Fomel and Guochang LiuABSTRACTAttenuation of random noise and enhancement of structural continuity can significantly improve the quality of seismic interpretation. We present a new technique, which aims at reducing random noise while protecting structural information. The technique is based on combining structure prediction with either similarity‐mean filtering or lower‐upper‐middle filtering. We use structure prediction to form a structural prediction of seismic traces from neighbouring traces. We apply a non‐linear similarity‐mean filter or an lower‐upper‐middle filter to select best samples from different predictions. In comparison with other common filters, such as mean or median, the additional parameters of the non‐linear filters allow us to better control the balance between eliminating random noise and protecting structural information. Numerical tests using synthetic and field data show the effectiveness of the proposed structure‐enhancing filters.
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Kinematic characteristics and the influence of reference velocities of phase‐shift‐plus‐interpolation and extended‐split‐step‐Fourier migration methods
Authors Jing‐Bo Chen and Shu‐Yuan DuABSTRACTThe phase‐shift‐plus‐interpolation and extended‐split‐step‐Fourier methods are wavefield‐continuation algorithms for seismic migration imaging. These two methods can be applied to regions with complex geological structures. Based on their unified separable formulas, we show that these two methods have the same kinematic characteristics by using the theory of pseudodifferential operators. Numerical tests on a Marmousi model demonstrate this conclusion. Another important aspect of these two methods is the selection of reference velocities and we explore the influence of the selection of reference velocities by comparing the geometric progression method and the statistical method. We show that the geometric progression method is simple but does not take into account the velocity distribution while the statistical approach is relatively complex but reflects the velocity distribution.
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Time‐frequency seismic data de‐noising
Authors Thomas Elboth, Ida Vik Presterud and Dag HermansenABSTRACTMarine seismic data are always affected by noise. An effective method to handle a broad range of noise problems is a time‐frequency de‐noising algorithm. In this paper we explain details regarding the implementation of such a method. Special emphasis is given to the choice of threshold values, where several different strategies are investigated. In addition we present a number of processing results where time‐frequency de‐noising has been successfully applied to attenuate noise resulting from swell, cavitation, strumming and seismic interference. Our seismic interference noise removal approach applies time‐frequency de‐noising on slowness gathers (τ−p domain). This processing trick represents a novel approach, which efficiently handles certain types of seismic interference noise that otherwise are difficult to attenuate. We show that time‐frequency de‐noising is an effective, amplitude preserving and robust tool that gives superior results compared to many other conventional de‐noising algorithms (for example frequency filtering, τ−p or fx‐prediction). As a background, some of the physical mechanisms responsible for the different types of noise are also explained. Such physical understanding is important because it can provide guidelines for future survey planning and for the actual processing.
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Inversion of controlled‐source electromagnetic data using a model‐based approach
Authors Maokun Li, Aria Abubakar, Tarek M. Habashy and Yan ZhangABSTRACTA two‐and‐half dimensional model‐based inversion algorithm for the reconstruction of geometry and conductivity of unknown regions using marine controlled‐source electromagnetic (CSEM) data is presented. In the model‐based inversion, the inversion domain is described by the so‐called regional conductivity model and both geometry and material parameters associated with this model are reconstructed in the inversion process. This method has the advantage of using a priori information such as the background conductivity distribution, structural information extracted from seismic and/or gravity measurements, and/or inversion results a priori derived from a pixel‐based inversion method. By incorporating this a priori information, the number of unknown parameters to be retrieved becomes significantly reduced. The inversion method is the regularized Gauss‐Newton minimization scheme. The robustness of the inversion is enhanced by adopting nonlinear constraints and applying a quadratic line search algorithm to the optimization process. We also introduce the adjoint formulation to calculate the Jacobian matrix with respect to the geometrical parameters. The model‐based inversion method is validated by using several numerical examples including the inversion of the Troll field data. These results show that the model‐based inversion method can quantitatively reconstruct the shapes and conductivities of reservoirs.
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Conforming Falcon‡ gravity and the global gravity anomaly
More LessABSTRACTGravity derived only from airborne gravity gradient measurements with a normal error distribution will have an error that increases with wavelength. It is straightforward in principle to use sparsely sampled regional gravimeter data to provide the long wavelength information, thereby conforming the derived gravity to the regional gravity. Regional surface or airborne gravimeter data are not always available and can be difficult and expensive to collect in many of the areas where an airborne gravity gradiometer survey is flown. However the recent release by the Danish National Space Centre of the DNSC08 global gravity anomaly data has provided regional gravity data for the entire earth of adequate quality for this purpose. Studies over three areas, including comparisons with ground, marine and airborne gravimetry, demonstrate the validity of this approach. Future improvements in global gravity anomaly data are expected, particularly as the product from the recently launched Gravity field and steady‐state Ocean Circulation Explorer (GOCE) satellite becomes available and these will lead directly to an improvement in the very wide bandwidth gravity available after conforming gravity derived from gravity gradiometry with the global gravity.
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Correlation of vertical electrical sounding and electrical borehole log data for groundwater exploration
Authors G.K. Hodlur and Ratnakar DhakateABSTRACTWe have correlated the longitudinal unit conductance CL obtained from interpreted vertical electrical sounding data with the formation resistivity Rt and the formation resistivity factor F, obtained by carrying out electrical borehole logging. Interpreted geophysical data of eleven soundings and two electrical borehole log records are used for the analysis. The geophysical data used were acquired in a sedimentary basin. The study area is called Lower Maner Basin located in the province of Andhra Pradesh, India. Vertical electrical soundings were carried out using a Schlumberger configuration with half current electrode separation varying from 600–1000 m. For logging the two boreholes, a Widco logger‐model 3200 PLS was used. True formation resistivity Rt was calculated from a resistivity log. Formation resistivity factor F was also calculated at various depths using Rt values. An appreciable inverse relation exists between the correlated parameters. The borehole resistivity Rt and the formation resistivity factor F decrease with the increase in the longitudinal unit conductance CL. We have shown the use of such a relation in computing borehole resistivity Rt and formation resistivity factor F at sites that posses only vertical electrical sounding data, with a fair degree of accuracy. Validation of the correlation is satisfactory. Scope for updating the correlation is discussed. Significance and applications of the relation for exploration of groundwater, namely to update the vertical electrical sounding data interpretation by translating the vertical electrical sounding data into electrical borehole log parameters, to facilitate correlations studies and to estimate the porosity (φ), permeability (K) and water saturation Sw of water bearing zones are discussed.
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Controlled source electromagnetic three‐dimensional grid‐modelling based on a complex resistivity structure of the seafloor: effects of acquisition parameters and geometry of multi‐layered resistors
Authors Anwar H. Bhuyian, Bård P. Thrane, Martin Landrø and Ståle E. JohansenABSTRACTA comprehensive controlled source electromagnetic (CSEM) modelling study, based on complex resistivity structures in a deep marine geological setting, is conducted. The study demonstrates the effects of acquisition parameters and multi‐layered resistors on CSEM responses. Three‐dimensional (3D) finite difference time domain (FDTD) grid‐modelling is used for CSEM sensitivity analysis. Interpolation of normalized CSEM responses provides attributes representing relative sensitivity of the modelled structures. Modelling results show that fine grid, 1 × 1 km receiver spacing, provides good correlations between CSEM responses and the modelled structures, irrespective of source orientation. The resolution of CSEM attributes decreases for receiver spacing >2 × 2 km, when using only in‐line data. Broadside data in the grid geometry increase data density by 100 – approximately 200% by filling in in‐line responses and improve the resolution of CSEM attributes. Optimized source orientation (i.e., oblique to the strike of an elongated resistor) improves the structural definition of the CSEM anomalies for coarse‐grid geometries (receiver spacing ≥3 × 3 km). The study also shows that a multi‐resistor anomaly is not simply the summation but a cumulative response with mutual interference between constituent resistors. The combined response of constituent resistors is approximately 50% higher than the cumulative response of the multi‐resistor for 0.5 Hz at 4000 m offset. A gradual inverse variation of offset and frequency allows differentiation of CSEM anomalies for multi‐layered resistors. Similar frequency‐offset variations for laterally persistent high‐resistivity facies show visual continuity with varying geometric expressions. 3D grid‐modelling is an effective and adequate tool for CSEM survey design and sensitivity analysis.
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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 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)