摘要:

AbstractThe Bjøirnøya West Basin lies between latitudes 73° and 74°, longitudes 16°E and 18°E, contains at least 8 km of sediments deposited from the Late Jurassic, and is of considerable interest for hydrocarbon exploration. The Cenozoic extensional tectonics in the basin can be clearly seen from seismic data with normal faulting and from subsidence curves with rapid subsidence. The extension occurred during the Late Palaeocene with active extension lasting about 6 million years (m.y.) followed by thermal cooling. The tectonic subsidence within the study area shows a three‐phase development: phase 1, synrift (58–52 Ma (million years before the present day)), is characterized by rapid subsidence; phase 2, postrift (52–5 Ma), by slow subsidence with occasional uplift; and phase 3 (5–0 Ma), by rapid subsidence. An adaptive finite‐element model, with consideration of the radiogenic heat production in the lithosphere, has been used to model the subsidence and heat flow. The modelling of subsidence shows the β‐factor distribution varying from 1.9 to 3.5 with an average of 2.4 for the uniform lithospheric extension. The heat‐flow modelling predicts a rapid increase of heat flow during the Early Palaeocene. The maximum heat flow at about 52 Ma, which could be as much as 3.0 hfu (10−6cal/cm2/s), was followed by a decrease in heat flow. A plate‐weakening model has been proposed to explain the rapid subsidence for the last 5 m.y. by flexure of the elastic lithosphere which is weakened by a decrease in elastic thickness caused by an increase of the temperature gradient in the lithosphere. The plate‐weakening model predicts a heat‐flow increase at 5 Ma of up to 2.0 hfu. Our study, using quantitative modelling of the tectonic subsidence, provides a partial (if not a full) understanding of the tectonic development and thermal evoluti

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00211.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractA series of model experiments was performed in an ultrasonic laboratory to study the radiation of downhole sources in a variety of formations. Three models were used in the experiments. They were a Lucite model, a Lucite model with a free glass pipe in the centre, and a glass‐cased soil model. In addition, a finite‐difference modelling technique was used to simulate the wave propagation in these models and the results of the laboratory and numerical experiments are compared. In the Lucite borehole model the waveforms recorded in the experiment agree very well with the finite‐difference synthetics. The snapshots of the wavefield from the finite‐difference simulation show the radiation pattern of the P‐ and S‐waves in the Lucite formation. These patterns are consistent with the theoretical calculations. In the Lucite model with the free glass pipe, the finite‐difference synthetics are also in good agreement with the experimental observations, especially for the conical P‐wave arrival. The angle between the wavefront of the conical P‐wave and the borehole axis, observed from the snapshot, agrees with the theory. In the cased soil model, the arrival time of the finite‐difference synthetics is in good agreement with the laboratory measurements. The relative amplitudes of the P‐wave and the Mach wave are not correctly modelled because intrinsic attenuation is not included in the finite‐difference calculation. The Mach cone angle from the snapshot agrees with the theoretical prediction. Finally, a finite‐difference method was used to simulate Mach‐wave propagation in a formation with two horizontal layers. In the case of two slow layers, the Mach‐wave generated in the first layer is reflected back from and transmitted through the boundary and another Mach wave is generated at the second layer when the Stoneley wave travels into the second layer. In the case of a formation having one slow and one fast layer, the Mach wave generated in the slow layer is reflected back at the boundary and leaked into the fast layer. There is n

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00212.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe Fourier pseudospectral method has been widely accepted for seismic forward modelling because of its high accuracy compared to other numerical techniques. Conventionally, the modelling is performed on Cartesian grids. This means that curved interfaces are represented in a ‘staircase fashion‘causing spurious diffractions. It is the aim of this work to eliminate these non‐physical diffractions by using curved grids that generally follow the interfaces.A further advantage of using curved grids is that the local grid density can be adjusted according to the velocity of the individual layers, i.e. the overall grid density is not restricted by the lowest velocity in the subsurface. This means that considerable savings in computer storage can be obtained and thus larger computational models can be handled.One of the major problems in using the curved grid approach has been the generation of a suitable grid that fits all the interfaces. However, as a new approach, we adopt techniques originally developed for computational fluid dynamics (CFD) applications. This allows us to put the curved grid technique into a general framework, enabling the grid to follow all interfaces. In principle, a separate grid is generated for each geological layer, patching the grid lines across the interfaces to obtain a globally continuous grid (the so‐called multiblock strategy).The curved grid is taken to constitute a generalised curvilinear coordinate system, where each grid line corresponds to a constant value of one of the curvilinear coordinates. That means that the forward modelling equations have to be written in curvilinear coordinates, resulting in additional terms in the equations. However, the subsurface geometry is much simpler in the curvilinear space.The advantages of the curved grid technique are demonstrated for the 2D acoustic wave equation. This includes a verification of the method against an analytic reference solution for wedge diffraction and a comparison with the pseudospectral method on Cartesian grids. The results demonstrate that high accuracies are obtained with few grid points and without extra computational costs as compared with Cartesian

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00213.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractThe five‐electrode resistivity configuration of Barker, who introduced the concept of offset resistivity measurements, is used in a profiling mode in the search for lateral variations of resistivity down to depths of a few tens of metres. Theoretical computations show that plotting simple pseudosections of offset measurements over faults is sufficient to reveal the fault position. The method is subsequently applied in the field for locating a buried fault in coal measures. Although offset pseudosections are often all that is required, apparent resistivity pseudosections are examined and compared to collations of 1D inversions of the sounding profile data and to computations over 2D models. It is shown that apparent restivity pseudosections may present a rather misleading pictur

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00214.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

AbstractWe present a discrete modelling scheme which solves the elastic wave equation on a grid with vertically varying grid spacings. Spatial derivatives are computed by finite‐difference operators on a staggered grid. The time integration is performed by the rapid expansion method. The use of variable grid spacings adds flexibility and improves the efficiency since different spatial sampling intervals can be used in regions with different material properties. In the case of large velocity contrasts, the use of a non‐uniform grid avoids spatial oversampling in regions with high velocities. The modelling scheme allows accurate modelling up to a spatial sampling rate of approximately 2.5 gridpoints per shortest wavelength. However, due to the staggering of the material parameters, a smoothing of the material parameters has to be applied at internal interfaces aligned with the numerical grid to avoid amplitude errors and timing inaccuracies. The best results are obtained by smoothing based on slowness averaging. To reduce errors in the implementation of the free‐surface boundary condition introduced by the staggering of the stress components, we reduce the grid spacing in the vertical direction in the vicinity of the free surface to approximately 10 gridpoints per shortest wavelength. Using this technique we obtain accurate results for surface waves in transversely isotropic

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1994.tb00215.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY