摘要:

AbstractA 2D numerical finite‐difference algorithm accounting for surface topography is presented. Higher‐order, dispersion‐bounded, cost‐optimized finite‐difference operators are used in the interior of the numerical grid, while non‐reflecting absorbing boundary conditions are used along the edges. Transformation from a curved to a rectangular grid achieves the modelling of the surface topography. We use free‐surface boundary conditions along the surface. In order to obtain complete modelling of the effects of wave propagation, it is important to account for the surface topography, otherwise near‐surface effects, such as scattering, are not modelled adequately. Even if other properties of the medium, for instance randomization, can improve numerical simulations, inclusion of the surface topography makes them

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00216.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractAn equivalent source procedure is derived for upward continuation of unevenly spaced gravity and magnetic data. The dipole layer is placed on a topographic relief approximated by a polyhedral surface, the stations being the vertices of the triangular faces. The dipoles have linear magnitudes, being directed along the normal vector over each triangle. The unknown values of the dipole magnitudes at each station are obtained by a suitable modification of the usual integral equation considering the discontinuity of the normal vector at each vertex of the dipole surface. Profile data processing is also studied. A numerical test outlines the accuracy and the limitations of the model for the case of a magnetic field significantly perturbed by a rough topographic relief.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00217.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe superposition integral expressing the field due to a magnetic source body is relatively simple to evaluate in the case of a homogeneous magnetization. In practice this generally requires that any remnant component is uniform and the susceptibility of the body is sufficiently low to permit the assumption of a uniform induced magnetization. Under these conditions the anomalous magnetic field due to a polyhedral body can be represented in an intuitive and physically appealing manner. It is demonstrated that the components of the magnetic field H can be expressed as a simple combination of the potentials due to two elementary source distributions. These are, firstly, a uniform double layer (normally directed dipole moment density) located on the planar polygonal faces of the body and, secondly, a uniform line source located along its edges. In practice both of these potentials (and thus the required magnetic field components) are easily computed. The technique is applicable to polyhedra with arbitrarily shaped faces and the relevant expressions for the magnetic field components are suitable for numerical evaluation everywhere except along the edges of the body where they display a logarithmic singularity.

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00218.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractWe investigate, from a theoretical point of view, the possibility of performing marine two‐level magnetovariational measurements. An apparent resistivity function is denned and calculated after solving the differential equation governing the behaviour of the natural magnetic field variations inside a one‐dimensional earth. In order to generalize the problem, a frequency‐dependent resistivity is assumed to characterize the layers and the distortions caused by the polarization effects are carefully analysed. The computation of three‐layer amplitude and phase diagrams for the apparent resistivity function shows that, in the case of an intermediate polarizable layer, sandwiched between a non‐dispersive overburden and substratum, the H‐type sequence results are the most affected by the dispersion phenomenon as it occurs in magnetotellurics. Finally we consider the problem of the sensitivity of the method, since, in practice, it requires top and bottom sensors separated by a vertical finite distance. It is found that in the higher‐frequency range, due to the strong attenuation of the relative components of the field, the depth of the bottom sensor must be small enough to guarantee detectable signals, well above the full‐scale resolution of the acquisition system. Conversely, in the lower‐frequency range such a depth must be large enough to allow the difference between the top and bottom signals to be above the same recording sens

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00219.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractTrue‐amplitude (TA) migration, which is a Kirchhoff‐type modified weighted diffraction stack, recovers (possibly) complex angle‐dependent reflection coefficients which are important for amplitude‐versus‐offset (AVO) inversion. The method can be implemented using existing prestack or post‐stack Kirchhoff migration and fast Green's function computation programs. Here, it is applied to synthetic single‐shot and constant‐offset seismic data that include post‐critical reflections (complex reflection coefficients) and caustics. Comparisons of the amplitudes of the TA migration image with theoretical reflection coefficients show that the (possibly complex) angle‐dependent reflection coefficients are

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00220.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractWe use a marine seismic dataset to examine the reflections from two gas sands, a lignitic sand and a cineritic bed, by means of their amplitude versus offset (AVO) responses. This offset‐dependent signature is related to specific petrophysical and thus elastic situations or to peculiar interference patterns and may help to distinguish the nature of the amplitude anomalies on the stack sections.The prestack analysis is carried out on seismic data which have undergone an accurate true‐amplitude processing.It is found that the lignitic‐sand reflections exhibit a decreasing AVO while the two‐gas sands show markedly increasing AVO trends. Also the reflections from the cineritic layer show increasing amplitudes with offset that may be due either to the petrophysical nature of the cinerites or to thin‐layer interference or to both.In order to verify the reliability of the actual AVO responses we develop a detailed model from well data and compute a synthetic CMP seismogram. In order to account for mode conversions and thin‐layer effects, the synthetic seismograms are computed using the reflectivity method. The wavelets used in the synthetics are retrieved from actual seismic and borehole data by means of wavelet processing. When finely layered structures are present, the estimation of a reliable wavelet is extremely important to get the correct synthetic AVO response. In particular, the AVO responses of the cineritic layer differ substantially if we make use in the computation of the synthetics of a Ricker wavelet or of a wavelet estimated through wavelet processing.The good match between the observed and modelled data confirms the reliability of the processing sequence and of the final AVO

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00221.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractImproving the accuracy of NMO corrections and of the corresponding interval velocities entails implementing a better approximation than the formula used since the beginning of seismic processing. The exact equations are not practical as they include many unknowns. The approximate expression has only two unknowns, the reflection time and the rms velocity, but becomes inaccurate for large apertures of the recording system and heterogeneous vertical velocities. Several methods of improving the accuracy have been considered, but the gains do not compensate for the dramatic increase in computing time. Two alternative equations are proposed: the first containing two parameters, the reflection time and the focusing time, is not valid for apertures much greater than is the standard formula, but has a much faster computing time and does not stretch the far traces; the other, containing three parameters, the reflection time, like focusing time and the tuning velocity, retains high frequencies for apertures about twice those allowed by the standard equation. Its computing time can be kept within the same limits. NMO equations, old and new, are designed strictly for horizontal layering, but remain reliable as long as the rays travel through the same layers in both the down and up directions.An equation, similar to Dix's formula, is given to compute the interval velocities. The entire scheme can be automated to produce interval‐velocity sections without manual pickin

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00222.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractBased on a Born approximation of a thin sheet integral equation, it is shown that small‐scale surficial heterogeneity significantly distorts the electromagnetic field excited by electric dipoles only when either the source or the receiver are located on the heterogeneities. When a surface heterogeneity is beneath the source, the associated distortion of the electromagnetic field is manifest as a change in the effective electric dipole moment. Hence the magnetotelluric transfer functions and impedance relations remain undistorted in this case. When a surface heterogeneity is beneath the receiver, the electric field is severely distorted, but the magnetic field is only slightly distorted. The impedance tensor is therefore strongly distorted, but the tipper vector is almost unaltered. Since the controlled source tipper is a function of 1D earth conductivity, it is proposed that tipper data should be used in the first stage of 1D interpretation. For a 1D earth, the tipper vector must always point towards the source and, in the near‐field limit, should have unit length. These two necessary conditions must be met by the measured tipper before it is interpreted one dimensiona

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00223.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00224.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00225.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY