41 Magazines & Journals found

Magazines & Journals

  • 2.5-D frequency-domain seismic wave modeling in heterogeneous, anisotropic media using a Gaussian quadrature grid technique

    2.5-D frequency-domain seismic wave modeling in heterogeneous, anisotropic media using a Gaussian quadrature grid technique

    We present an extension of the spectral element method (SEM), called the Gaussian quadrature grid technique, for 2.5-D frequency-domain seismic wave modeling in heterogeneous, anisotropic media having arbitrary free-surface topography. The technique has two new features. First, it employs a point-gridded model sampled by irregular Gaussian quadrature abscissa rather than a hexahedral-element mesh so as to simplify the procedures of matching the ...

  • 2.5D modelling of elastic waves in transversely isotropic media using the spectral element method

    2.5D modelling of elastic waves in transversely isotropic media using the spectral element method

    The spectral-element method provides an accurate alternative to the finite-element method for modelling elastic waves in anisotropic media. With the aim of reducing the high computational overheads of 3D modelling, we have implemented 2.5D spectral-element modelling of elastic waves, initially for vertically transversely isotropic media, and then extended to a tilted transversely isotropic medium with a dipping symmetry-axis. We have ...

  • 2.5-D/3-D resistivity modelling in anisotropic media using Gaussian quadrature grids

    2.5-D/3-D resistivity modelling in anisotropic media using Gaussian quadrature grids

    We present a new numerical scheme for 2.5-D/3-D direct current resistivity modelling in heterogeneous, anisotropic media. This method, named the ‘Gaussian quadrature grid’ (GQG) method, cooperatively combines the solution of the Variational Principle of the partial differential equation, Gaussian quadrature abscissae and local cardinal functions so that it has the main advantages of the spectral element method. The formulation shows that the GQG ...

  • 3-D frequency-domain seismic wave modelling in heterogeneous, anisotropic media using a Gaussian quadrature grid approach

    3-D frequency-domain seismic wave modelling in heterogeneous, anisotropic media using a Gaussian quadrature grid approach

    We present an extension of the 3-D spectral element method (SEM), called the Gaussian quadrature grid (GQG) approach, to simulate in the frequency-domain seismic waves in 3-D heterogeneous anisotropic media involving a complex free-surface topography and/or sub-surface geometry. It differs from the conventional SEM in two ways. The first is the replacement of the hexahedral element mesh with 3-D Gaussian quadrature abscissae to directly sample ...

  • A crosswell seismic experiment for nickel sulphide exploration

    A crosswell seismic experiment for nickel sulphide exploration

    A crosshole seismic tomography experiment was conducted in the Kambalda nickel district to image the space between two dipping exploration boreholes separated by 80 m. A downhole electromagnetic (EM) survey had disclosed a conductor (possible mineralisation) between the holes. Excellent data quality was obtained from explosive sources at 2-m increments and recorded on a hydrophone array at 2-m spacing. Over 3200 travel times were converted into ...

  • A damping method for the computation of the 2.5-D Green's function for arbitrary acoustic media

    A damping method for the computation of the 2.5-D Green's function for arbitrary acoustic media

    In acoustic wavefield modelling, the 2.5-D approximation, in which the model parameter is considered to be invariant in the strike direction and the source is assumed to be a point excitation so that the wavefield has 3-D features, is an economical and realistic scheme for seismic full-waveform inversion. On the other hand, according to Parseval's theorem, the least-squares inversion of time-domain seismic data is equivalent to that carried out

  • A new kinematic method for mapping seismic reflectors

    A new kinematic method for mapping seismic reflectors

    This paper presents a modern version of an old technique of common tangent reflection migration. Rather than using the graphical method of swinging arcs and looking for the envelope of touching tangents on widely separated geophones, we use a numerical scheme of searching along each isochron, constructed by a wave‐equation‐based modeling scheme for arbitrary velocity media, to find the common tangent points. The assumption is made that the rece

  • A new method for crosswell reflector imaging

    A new method for crosswell reflector imaging

    Imaging seismic reflectors with crosshole and VSP tomographic data is only occasionally carried out using Kirchhoff-style migration schemes. We introduce a new kinematic method here for tomographically imaging reflectors. The scheme first picks up traveltimes of each reflected event from common-shot gather tomographic data. It then uses the picked times to image the reflector interfaces with a very important principle: that two reflection points ...

  • A synthetic study on crosshole resistivity imaging using different electrode arrays

    A synthetic study on crosshole resistivity imaging using different electrode arrays

    By means of numerical simulations, the possibilities of crosshole resistivity imaging with different survey geometries are investigated for three- and four-electrode arrays. The sensitivity variation for the different arrays and the effectiveness in crosshole resistivity imaging with such data are examined. A comparative analysis was carried out by computation of the sensitivity function and anomaly effect, and a synthetic model was used to test ...

  • An Adaptive Wavenumber Sampling Strategy for 2.5D Seismic-Wave Modeling in the Frequency Domain

    An Adaptive Wavenumber Sampling Strategy for 2.5D Seismic-Wave Modeling in the Frequency Domain

    Simulation of seismic waves from a 3D point-source in a 2D medium may be performed in the frequency-wavenumber domain (called 2.5D modelling). It involves computing the Fourier-transformed Green's function for a number of frequency (ω) and strike direction wavenumber (ky) values and doubly inverse transforming to convert to the traveltime and distance space. Such modeling produces a wavefield with 3D features but the computation becomes pseudo ...

  • An analytical treatment of single station triaxial seismic direction finding

    An analytical treatment of single station triaxial seismic direction finding

    Triaxial seismic direction finding can be performed by eigenanalysis of the complex coherency matrix (or cross power matrix). By splitting the symmetric Hermitian coherency matrix C to D + E (where det(E) = 0 and D is diagonal), we shift unpolarized (or inter-channel uncorrelated) data into D and then E becomes 'random noise free'. Without placing any restrictions on the signal set—P, S, Rayleigh—matrix E has only one non-zero eigenvalue (at ...

  • Composite boundary‐valued solution of the 2.5-D Green’s function for arbitrary acoustic media

    Composite boundary‐valued solution of the 2.5-D Green’s function for arbitrary acoustic media

    Theoretically, the Green’s function can be used to calculate the wavefield response of a specified source and the Fréchet derivative with respect to the model parameters for crosshole seismic full‐waveform inversion. In this paper, we apply the finite‐element method to numerically compute the 2.5-D Green’s function for an arbitrary acoustic medium by solving a composite boundary‐valued problem in the wavenumber‐frequency domain. The composite b

  • Computing the Sensitivity Kernels for 2.5-D Seismic Waveform Inversion in Heterogeneous, Anisotropic Media

    Computing the Sensitivity Kernels for 2.5-D Seismic Waveform Inversion in Heterogeneous, Anisotropic Media

    2.5-D modeling and inversion techniques are much closer to reality than the simple and traditional 2-D seismic wave modeling and inversion. The sensitivity kernels required in full waveform seismic tomographic inversion are the Fréchet derivatives of the displacement vector with respect to the independent anisotropic model parameters of the subsurface. They give the sensitivity of the seismograms to changes in the model parameters. This paper ...

  • Crosshole acoustic velocity imaging with full-waveform spectral data: 2.5-D numerical simulations

    Crosshole acoustic velocity imaging with full-waveform spectral data: 2.5-D numerical simulations

    This paper focuses on the question of full-waveform inversion and the use of crosshole full-waveform spectral data. We examine the differences in effectiveness of each form of data (real, imaginary, amplitude, phase and Hartley spectra) and determine which is best for imaging the velocity distribution. By 2.5-D numerical simulation for three models, we found that, except for the phase data, the monochromatic real, imaginary, amplitude and ...

  • Crosshole resistivity imaging of aquifer properties

    Crosshole resistivity imaging of aquifer properties

    Crosshole resistivity tomography has been trialled at the Bolivar site north of Adelaide to delineate aquifer properties and preferential flow paths associated with artificial recharge and recovery operations. To date only the pre-storage electrical measurements have been carried out, but follow-up monitoring and background subtraction will be conducted at various stages of pumping to delineate storm-water injection and harvesting from the ...

  • Crosshole resistivity tomography using different electrode configurations

    Crosshole resistivity tomography using different electrode configurations

    This paper investigates the relative merits and effectiveness of cross-hole resistivity tomography using different electrode configurations for four popular electrode arrays: pole–pole, pole–bipole, bipole–pole and bipole–bipole. By examination of two synthetic models (a dipping conductive strip and a dislocated fault), it is shown that besides the popular pole–pole array, some specified three- and four-electrode configurations, such as ...

  • Crosshole seismic inversion with normalized full‐waveform amplitude data

    Crosshole seismic inversion with normalized full‐waveform amplitude data

    We investigate a simple scheme for full‐waveform amplitude spectrum inversion of crosshole seismic data with an unknown source wavelet. The method is based on our 2D/2.5D finite‐element method of acoustic‐wave modeling. The normalized amplitude data, defined as the spectral ratio of the original trace amplitude to the average amplitude for the entire common shot gather, are used for full‐waveform inversion in the frequency domain. In essence, t

  • Explicit expressions and numerical calculations for the Fréchet and second derivatives in 2.5D Helmholtz equation inversion

    Explicit expressions and numerical calculations for the Fréchet and second derivatives in 2.5D Helmholtz equation inversion

    In order to perform resistivity imaging, seismic waveform tomography or sensitivity analysis of geophysical data, the Fréchet derivatives, and even the second derivatives of the data with respect to the model parameters, may be required. We develop a practical method to compute the relevant derivatives for 2.5D resistivity and 2.5D frequency-domain acoustic velocity inversion. Both geophysical inversions entail the solution of a 2.5D Helmholtz ...

  • Explicit expressions for the Fréchet derivatives in 3D anisotropic resistivity inversion

    Explicit expressions for the Fréchet derivatives in 3D anisotropic resistivity inversion

    We have developed explicit expressions for the Fréchet derivatives or sensitivity functions in resistivity imaging of a heterogeneous and fully anisotropic earth. The formulation involves the Green’s functions and their gradients, and it is developed from a formal perturbation analysis and by means of a numerical (finite-element) method. A critical factor in the equations is the derivative of the electrical conductivity tensor with respect to t