摘要:

AbstractA new method to trace rays in irregular grids based on Fermat's principle of minimum time is introduced. Besides the usual transmitted and reflected waves, refracted, diffracted and converted waves can also be simulated. The proposed algorithm is fast and stable, and respects the reciprocity principle between source and receiver better than procedures adopting the shooting method. It is particularly suited to form part of a traveltime inversion procedure. The use of irregular grids allows adaptation of the earth discretization to the available acquisition geometry and ray distribution, to obtain more stable and reliable tomographic images.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00172.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe significant development in acoustic full waveform logging during the last ten years has made it increasingly possible for log analysts to determine the physical properties of a rock formationin situ.Parallel to the methods applied to a single wavetrain during seismic processing, the new techniques, used for sets of wavetrains, have been successfully tested with acoustic full waveforms. Instantaneous characteristics analysis is included in this group of methods. This approach, leading to qualitative and quantitative interpretation, reveals the influence of small changes in physical properties on acoustic full wavetrains.Applications of complex acoustic waveform analysis for the determination of inhomogeneous zones are presented. Colour diagrams of instantaneous characteristics are used for the detection of fractured regions and slow formations with increased attenuation of acoustic waves.Results of the interpretation of individual acoustic full waveforms, based on cross‐correlation and spectral analysis, using the IDNP and IDNS computer programs, e.g. velocities of compressional waves, shear and Stoneley waves, are presented. Since the bulk density of the rocks was known, it was possible, using the velocities of P‐ and S‐waves obtained, to calculate the dynamic elastic moduli. We used the interpretation of acoustic full wavetrains to calculate porosity. The sonic porosity is compared to the porosity obtained from other logs and to that obtained from core sample analysis.The examples of acoustic full wavetrains were recorded in the Miocene sulphur‐bearing limestones in central Poland. Field measurements were made using the domestic prototype equipment for well log recordings in shallow bo

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00173.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractWe investigate the magnetotelluric response of two‐dimensional bodies, characterized by the presence of low‐frequency dispersion phenomena of the electrical parameters. The Cole‐Cole dispersion model is assumed to represent the frequency dependence of the ‘impedivity’ complex function, defined as the inverse of Stoyer's ‘admittivity’ complex parameter. To simulate real geological situations, we consider three structural models, representing a sedimentary basin, a geothermal system and a magma chamber, assumed to be partially or totally dispersive. From a detailed study of the frequency and space behaviours of the magnetotelluric parameters, taking known non‐dispersive results as reference, we outline the main peculiarities of the local distortion effects, caused by the presence of dispersion in the target media. Finally, we discuss the interpretative errors which can be made by neglecting the dispersion phenomena. The apparent dispersion function, which was defined in a previous paper to describe similar effects in the one‐dimensional case, is again used as a reliable indicator of location, shape and spatial extent of the dispersive bodies. The general result of this study is a marked improvement in the resolution power of the magne

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00174.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractA new 3D wavefield modelling approach based on dynamic ray tracing is presented. This approach is called wavefront construction, and it can be used in 3D models with constant or smoothly varying material properties (S‐ and P‐velocity and density) separated by smooth interfaces. Wavefronts consisting of rays arranged in a triangular network are propagated stepwise through the model. At each time step, the differences in a number of parameters are checked between each pair of rays on the wavefront. New rays are interpolated whenever this difference between pairs of rays exceeds some predefined maximum value. A controlled sampling of the wavefront at all time steps is thus obtained. Receivers are given multiple‐event values by interpolation when the wavefronts pass them. The strength of the wavefront construction method is that it is robust and effi

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00175.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe numerical tracing of short ray segments and interpolation of new rays between these ray segments are central constituents of the wavefront construction method. In this paper the details of the ray tracing and ray‐interpolation procedures are described. The ray‐tracing procedure is based on classical ray theory (high‐frequency approximation) and it is both accurate and efficient. It is able to compute both kinematic and dynamic parameters at the endpoint of the ray segments, given the same set of parameters at the starting point of the ray. Taylor series are used to approximate the raypath so that the kinematic parameters (new position and new ray tangent) may be found, while a staggered finite‐difference approximation gives the dynamic parameters (geometrical spreading).When divergence occurs in some parts of the wavefront, new rays are interpolated. The interpolation procedure uses the kinematic and dynamic parameters of two parent rays to estimate the initial parameters of a new ray on the wavefront between the two rays. Third‐order (cubic) interpolation is used for interpolation of position, ray tangent and take‐off vector from the source) while linear interpolation is used for the geometrical spreading

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00176.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractA tomographic imaging technique combined with coherence inversion is proposed for constructing a near‐surface model from refraction events. A model obtained from coherence inversion serves as a good background model for the tomographic reconstruction. A simultaneous iterative reconstruction technique (SIRT) algorithm was used for this purpose. This is a simple algorithm and can be easily adapted to irregular acquisition geometry and limited angular aperture. Using synthetic data it was shown that the proposed procedure can be used for determination of local velocity anomalies in a shallow subsurface. The technique was also tested on a real data se

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00177.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThis work presents a new modelling scheme for the simulation of electromagnetic radio waves, based on a full‐field simulator. Maxwell's equations are modified in order to include dielectric attenuation processes, such as bound‐ and free‐water relaxation, ice relaxation and the Maxwell–Wagner effect. The new equations are obtained by assuming a permittivity relaxation function represented by a generalized Zener model. The convolution integral introduced by the relaxation formulation is circumvented by defining newhiddenfield variables, each corresponding to a different dielectric relaxation. The equations are solved numerically by using the Fourier pseudospectral operator for computing the spatial derivatives and a new time‐splitting integration algorithm that circumvents the stiffness of the differential equations. The program is used to evaluate the georadar electromagnetic response of a Japanese burial site, in particular, a stone coffin‐lik

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00178.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractA deterministic near‐surface correction procedure is developed for multicomponent VSP data, shot using directional sources and recorded using three‐component receivers. The method is capable of removing unwanted effects of acquisition such as unequal source strengths or misorientations, but may also remove near‐surface multiples and anisotropy. This is of considerable benefit for obtaining accurate and consistent estimates of subsurface anisotropy from different source combinations. Application of the technique is illustrated using a dataset from the Romashkino reservoir in Russia, where three or four different directional sources are used at the same source locations. The technique corrects for the large discrepancies which exist between the estimates obtained using different source combinations. Application of the technique to three wells in the survey region reveals a nearly isotropic subsurface, except for a few isolated zones of moderate to high (2 to 8%) anisotropy which lie close to the expected depth for the reservoir. Although there is no significant correlation with the production figures for each well, theqS1polarization azimuth within the reservoir does vary at each well location, suggesting that this may be a more sensitive indicator of reservoir Dr

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00179.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00180.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00181.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY