摘要:

AbstractConventionally, the Fresnel zone and the geometrical spreading factor are investigated separately, because they belong to different theories of wave propagation. However, if the paraxial ray method is used for establishing the Fresnel–Kirchhoff diffraction formula for a laterally inhomogeneous multilayered medium, it can be shown that the normalized geometrical spreading factor is inversely proportional to the area of the first Fresnel zone associated with the reflection point. Therefore, if no diffracting edge cuts the first Fresnel zone, the geometrical optics approximation represents the principal part of the wavefield obtained by Fresnel–Kirchhoff diffraction theory. Otherwise, the geometrical optics approximation has to be corrected by adding edge diffractions. It is also shown that Kirchhoff‐type migration and geometrical spreading factor correction both reduce the first Fresnel zone to a zone with unit

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00153.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe filter for wave‐equation‐based water‐layer multiple suppression, developed by the authors in thex‐t, the linearτ‐p, and thef‐kdomains, is extended to the parabolicτ‐2domain. The multiple reject areas are determined automatically by comparing the energy on traces of the multiple model (which are generated by a wave‐extrapolation method from the original data) and the original input data (multiples + primaries) inτ‐pspace. The advantage of applying the data‐adaptive 2D demultiple filter in the parabolicτ‐pdomain is that the waves are well separated in this domain. The numerical examples demonstrate the effectiveness of such a dereverberation procedure. Filtering of multiples in the parabolicτ‐pdomain works on both the far‐offset and the near‐offset traces, while the filtering of multiples in thef‐kdomain is effective only for the far‐offset traces.Tests on a synthetic common‐shot‐point (CSP) gather show that the demultiple filter is relatively immune to slight errors in the water velocity and water depth which cause arrival time errors of the multiples in the multiple model traces of less than the time dimension (about one quarter of the wavelet length) of the energy summation window of the filter. The multiples in the predicted multiple model traces do not have to be exact replicas of the multiples in the input data, in both a wavelet‐shape and traveltime sense. The demultiple filter also works reasonably well for input data contaminated by up to 25% of random noise. A shallow water CSP seismic gather, acquired on the North West Shelf of Australia, demonstrates the e

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00154.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe gas industry is continuing to concentrate its research and development efforts on new and advanced technology to improve reservoir descriptions through the producing life and development history of heterogeneous gas reservoirs. A very important aspect of this need is the ability to reduce the uncertainty of estimating probable reserves and to lower the operating costs to recover incremental reserves in producing and depleted gasfields. Established methods for reducing uncertainty in heterogeneous reservoir compartments, such as VSP and cross‐well techniques may enhance resolution, but they are currently not economically justifiable in on‐shore gasfields. Continuity logging using guided waves is an alternative approach to analysing inter‐well seismic data to confirm the continuity of heterogeneous gas reservoir compartments; in particular, the continuity of sand and shale stratigraphy in gas reservoirs.The solution of a coupled system of differential equations based on Biot and homogenization theories is adapted to calculate guided seismic waves trapped in low‐velocity layers. The general solution is for a 3D source in a horizontally layered poroviscoelastic medium having isotropic and laterally homogeneous material properties. A unified representation of the medium that includes fluid‐solid interactions and viscoelastic losses is incorporated into the solution. The guided‐wave part of the vector wave field and fluid‐pressure of the complete wave motion in layered dissipative media is verified and used to simulate dispersion and attenuation of guided seismic waves for continuity logging applications. The results of this work suggest that the multimode wave solution is appropriate to simulate guided seismic wave signatures to indicate continuity of layered earth structures in poroviscoelastic reservoirs. In particular, the normal mode information can be used for planning continuity logging surveys and for interpreting the corresponding seismic data. Further, fluid‐pressure waveforms show that maximum amplitude normal modes can be detected at layer interfaces in fluid‐filled porous media, and the corresponding Airy phase wave groups may carry information on the forma

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00155.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractContrasts in elastic parameters can be estimated directly from seismic data using offset‐dependent information in the PP reflection coefficient. A linear approximation to the PP reflection coefficient including three coefficients is fitted to the data, and relative contrasts in various elastic parameters are obtained from linear combinations of the estimated coefficients. Linearized elastic parameter sections for the contrasts in P‐wave impedance, P‐wave velocity, density, plane‐wave modulus, and the change in bulk modulus and shear modulus normalized with the plane‐wave modulus are estimated. If the average P‐ to S‐wave velocity ratio is known, linearized parameter sections including the contrast in the average P‐ to S‐wave velocity ratio and a fluid factor section can be computed. Applied to synthetic data, visual comparison of the estimated and true elastic parameter sections agree qualitatively, and the results are confirmed by an analysis of the standard deviation of the estimated parameters. The parameter sections obtained by inversion of a shallow seismic anomaly in the Barents Sea are promising, but the reliability is uncertain because neither well data nor regional tre

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00156.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractA publicly available and maintained electromagnetic finite‐difference time domain (FDTD) code has been applied to the forward modelling of the response of 1D, 2D and 3D geophysical targets to a vertical magnetic dipole excitation. The FDTD method is used to analyse target responses in the 1 MHz to 100MHz range, where either conduction or displacement currents may have the controlling role. The response of the geophysical target to the excitation is presented as changes in the magnetic field ellipticity.The results of the FDTD code compare favourably with previously published integral equation solutions of the response of 1D targets, and FDTD models calculated with different finite‐difference cell sizes are compared to find the effect of model discretization on the solution. The discretization errors, calculated as absolute error in ellipticity, are presented for the different ground geometry models considered, and are, for the most part, below 10% of the integral equation solutions.Finally, the FDTD code is used to calculate the magnetic ellipticity response of a 2D survey and a 3D sounding of complicated geophysical targets. The response of these 2D and 3D targets are too complicated to be verified with integral equation solutions, but show the proper low‐ and high‐frequency re

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00157.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.tb00158.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.tb00159.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractWe consider the problem of computing the most probable location of a target based on radar measurements of the subsurface. Our algorithm makes use of the maximum likelihood estimator (MLE), which represents a correlation between the measured data and synthetic data generated for the object of interest at different locations. Previous studies assume a plane‐wave acquisition geometry and target object(s) embedded in a uniform background. In this paper, a generalization of the MLE method is presented which is valid for discrete point sources (and receivers) and a 2D model (i.e. a 2.5D acquisition geometry). Within this formulation the treatment of a non‐uniform background model is also possible. We concentrate on geotechnical ground investigations and assume that the characteristic dimensions of the target object are in the range 1–2λ, (λ being the wavelength). The potential of the method is demonstrated employing cross‐hole radar data acquired in a controlled field experiment. The MLE result is also compared with the image obtained employing a full reconstruction method such as diffraction t

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00160.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.tb00161.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractTechniques to reduce the time needed to carry out 3D resistivity surveys with a moderate number (25 to 100) of electrodes and the computing time required to interpret the data have been developed. The electrodes in a 3D survey are normally arranged in a square grid and the pole‐pole array is used to make the potential measurements. The number of measurements required can be reduced to about one‐third of the maximum possible number without seriously degrading the resolution of the resulting inversion model by making measurements along the horizontal, vertical and 45° diagonal rows of electrodes passing through the current electrode. The smoothness‐constrained least‐squares inversion method is used for the data interpretation. The computing time required by this technique can be greatly reduced by using a homogeneous half‐space as the starting model so that the Jacobian matrix of partial derivatives can be calculated analytically. A quasi‐Newton updating method is then used to estimate the partial derivatives for subsequent iterations. This inversion technique has been tested on synthetic and field data where a satisfactory model is obtained using a modest amount of computer time. On an 80486DX2/66 microcomputer, it takes about 20 minutes to invert the data from a 7 by 7 electrode survey grid. using the techniques described below, 3D resistivity surveys and data inversion can be carried out using commercially available field equipment and an inexpensive m

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1996.tb00162.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY