摘要:

AbstractThe theoretical behaviour of guided waves in cross‐hole seismic surveys is examined with dispersion curves and synthetic seismograms. The modelling suggests, and field observations confirm, that the dominant energy in many cross‐hole seismic surveys propagates as guided waves, as well as body waves, with the energy tied to an interface, or combination of interfaces, whenever there are interfaces with significant velocity contrasts continuous between the wells. The dispersion and waveforms of such modal solutions carry detailed information about the internal structure, particularly the stress‐aligned structure of fluid‐filled inclusions in the immediate neighbourhood of the waveguide. Since the information content can be controlled to some extent by the choice of frequency and source type and the level of source and receiver in the two wells, such guided waves in cross‐hole surveys may be important for monitoring changes in nearly horizontal reservoirs during hydrocarbon production processes. Initially, we investigate the basic properties of propagation in isotropic strata, and then examine the behaviour of guided waves in the stress‐aligned fluid‐fill

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00569.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractA processing method is presented to attenuate the surface ghost using marine twin streamer data. It is an extension to the dephase and sum method which corrects for the phase of the ghost in both streamer outputs and then adds them in an attempt to fill the notches in the amplitude spectrum. The method presented corrects both the phase and the amplitude effect of the surface ghost by combining both signals as a weighted sum.This method is applicable to all types of twin streamer data, ranging from deep exploration data to very shallow high‐resolution surveys. Both the synthetic and real data examples shown are of the high‐resolution type, using frequencies above 2 kHz and short streamers (active sections of the order of one metre).Both the dephase and sum method and the weighted sum method are applied to synthetic high‐resolution data and the results are compared. This has been done for noise‐free data, data with a high noise level and data with strong geometrical spreading on the ghost reflections. From these test results it can be concluded that in general the weighted sum method gives better results. The improvement in the signal‐to‐noise ratio appears to be the same, due to the additive character of both methods. In the case of high‐resolution twin streamer data recorded in shallow water, the delay time of the ghost reflection can be of the same order of magnitude as the traveltime of the primary. For this situation, geometrical spreading can have a considerable effect on the amplitude of the ghost reflection. If no correction can be made for the spreading function, it might be better to use the dephase and sum method.Both methods are also applied to a real data set recorded in a high‐resolution survey. Because the ghost delay is of the same order of magnitude as the arrival time of the primary, the ghost reflection is strongly affected by geometrical spreading. Since the spreading function of the source is unknown, it cannot be corrected for. This causes the result of the weighted sum method to be less reliable compared to the case where no spread

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00570.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractPrevious studies of radiation from point sources in fluid‐filled boreholes have most often been based on far‐field, stationary phase analysis. In these papers, the explicit contribution of the borehole itself acting as a waveguide has not been properly considered, with a few exceptions. In general, these studies accurately describe S‐wave radiation in high‐velocity rocks such as granites and limestones and P‐wave radiation in most rocks, and experiments have confirmed this. However, tube waves directly influence the external wavefield and in fact create a shear‐wave ‘wake’ outside the borehole due to constructive interference of tube‐wave emission if a velocity condition is met. This constructive interference or wake is generated when the tube‐wave velocity is greater than the shear‐wave velocity. When this happens, a tube‐wave complex pole invalidates the mathematical assumptions for stationary phase analysis and the stationary phase predictions do not agree with experimentally derived radiation patterns. Shales at shallow depths and other soft sediments characteristically have tube‐wave velocities greater than shear‐wave velocities. Because the tube‐wave is of relatively high amplitude compared to body waves generated directly by the source, these secondary shear waves can be the highest amplitude arrivals on receiver arrays.The shape and properties of these secondary shear waves are calculated and shown to have identical properties to Mach waves of aerodynamics and seismology. For instance, these waves are geometrically conical and the aperture of the cone and the moveout velocity can be calculated. This paper also demonstrates the important effect that casing has on the Mach waves and provides predictions about when these waves are likely to be observed. Finally, evidence of Mach waves in data sets is examined and it is shown how these waves have been confused with receiver borehole tube waves.It is possible, though rare, that the tube‐wave velocity of the borehole is greater than the compressional‐wave velocity of the surrounding medium. In this case secondary compressional or compressional Mach waves would be generated although t

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00571.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractIt is useful to be able to calculate synthetic primary reflection sequences from which to generate synthetic seismic sections which can be used for testing new processing algorithms. However, these synthetic reflection sequences should closely match real properties found in recent studies. Using the ARMA(1,1) model resulting from such studies to describe the correlation (or spectral) structure of the sequences, and by matching moments up to fourth order (since the sequences are non‐Gaussian in practice), realistic sequences can be generated. A simple scheme is provided which also eliminates the necessity of throwing away large numbers of simulated values at start‐up. The procedure is illustrated on three real sequences and is seen to reproduce all the important featu

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00572.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractAn approximation to plane‐wave propagation through a composite material is examined using a physical model with oriented but randomly distributed penny‐shaped rubber inclusions within an isotropic epoxy resin matrix. A pulse transmission method is used to determine velocities of shear and compressional waves as a function of angle of incidence and crack density. The experimental and theoretical results of Hudson were compared and limitations within the crack parameters used in this study have been determined. Results from both polarized shear waves (S1, S2) compare favourably with the theory for a composite with up to 7% crack density, but theory and experiment diverge at higher crack densities. On the other hand, compressional‐wave velocities at low crack densities (1% and 3%) compare favourably with the theory. It is also shown that the velocity ratioVp/Vsfor two extreme cases, i.e. propagation normal and parallel to the cracks, as a function of crack density and porosity, has a strong directional depen

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00573.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractA quantitative AVO algorithm suitable for media with slow lateral parameter variations is developed. The method is based on a target‐oriented inversion scheme for estimation of elastic parameters in a locally horizontally stratified medium. The algorithm uses band‐limited PP reflection coefficients in the τ‐pdomain to estimate P‐ and S‐wave velocities, densities and layer thicknesses. To obtain these reflection coefficients, a pre‐processing involving the Radon transform and multiple attenuation is necessary. Furthermore, a macromodel for the velocities above the target zone must be found prior to the inversion.Various inversion tests involving synthetic data with white Gaussian noise and modelling errors that are likely to occur in conjunction with real data have been performed. In general, the inversion algorithm is fairly robust, since it is able to reproduce the main features of the reference model: main interface locations and relative contrasts in the three unknown layer parameters are recovered.From a test combining the effect of source directivity, one thin layer and 20% white Gaussian noise, it was found that neglect of the source directivity in the inversion caused the largest errors in the estimates. This indicates that it is very important either to eliminate the source directivity in a preprocessing step, or to take the directivity into account in the present algorithm. Despite these problems it was concluded that the inversion algorithm was able to reproduce the main features of the ref

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00574.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractConventional velocity analysis can handle a horizontally stratified medium well. There is no indication, though, that it will be as successful when applied to a more complicated geological structure. In fact, a small angle of incidence may transform to a wide‐angle reflection event for a dipping interface. In this case, conventional velocity analysis may lead to large errors and thus cannot be applied. Seismic tomography is attractive as it is virtually free from any restrictions imposed on the velocity distribution in the model space or on the setup of a seismic experiment. It is important, however, to recall that seismic tomography yields results of inferior quality compared to medical tomography. This paper investigates the reason for this and how to suppress a significant blurring of seismic tomograms. Unlike medical tomography, one cannot provide full angular coverage of the model space in a typical seismic experiment: the sources and the receivers cannot surround an unknown object inside the earth to provide a complete spectrum of view angles. Incomplete angular coverage may lead to the occurrence of large inaccuracies in the computed tomograms especially when the initial model is poorly chosen. We demonstrate a method of suppressing the adverse effects related to an incomplete angular recording. This is ‘compensation tomography’ which can be used efficiently in the case of a limited angular aperture. Numerical experiments illustrate the t

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00575.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY