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1.
A new numerical approach to the solution of waves propagating in a fluid-saturated medium, using Biot's theory as a foundation, has important implications for oil reservoir management and earthquake prediction. A numerical scheme is developed using an exponential transformation that explicitly treats the petrophysical and fluid properties of the medium within the framework of a generalized model. The scheme accounts for wave dissipation and velocity modifications. The numerical solution is used to perform numerical experiments to study the dynamic behaviour of waves in a fluid-saturated medium at well-logging frequencies (15 kHz). The results from the numerical experiments indicate that the degree of saturation by a high-viscosity fluid (HVF) such as oil, the temperature and the porosity of a medium strongly influence the spectral power distribution, frequency content and the velocity of waves propagating through the medium. An increase in HVF saturation causes enhanced attenuation of the low-frequency components, and increases the seismic velocity. An increase in porosity, however, enriches the low-frequency components and decreases the seismic velocity. A spectral quantification procedure is suggested and used to obtain information about the petrophysical and fluid properties of the medium from the spectral characteristics of the transmitted waveform. The procedure involves segmentation of the energy or power distribution of the transmitted waveforms into specified energy bands. The energy or power in these bands is then estimated. The extracted quantification variables are found to have strong correlations with the degree of HVF saturation, and the temperature and the porosity of the medium.  相似文献   

2.
Summary Microphones and seismographs were co-located in arrays on Skidaway Island, Georgia, for the launchings of Apollo 13 and 14, 374 km to the south. Simultaneous acoustic and seismic waves were recorded for both events at times appropriate to the arrival of the acoustic waves from the source. Significant comparisons of the true signals are (1) the acoustic signal is relatively broadband compared to the nearly monochromatic seismic signal; (2) the seismic signal is much more continuous than the more pulse-like acoustic signal; (3) ground loading from the pressure variations of the acoustic waves is shown to be too small to account for the seismic waves; (4) the measured phase velocities of both acoustic and seismic waves across the local instrument arrays differ by less than 6 per cent and possibly 3 per cent if experimental error is included. It is concluded that the seismic waves are generated by resonant coupling to the acoustic waves along some 10 km of path on Skidaway Island. The thickness of unconsolidated sediment on the island is appropriate to a resonant ground wave frequency of 3.5 to 4 Hz, as observed. Under appropriate conditions, ground wave observations may prove more effective means of detecting certain aspects of acoustic signals in view of the filtering of wind noise and amplification through resonance.  相似文献   

3.
Seismic wave propagation through the earth is often strongly affected by the presence of fractures. When these fractures are filled with fluids (oil, gas, water, CO2, etc.), the type and state of the fluid (liquid or gas) can make a large difference in the response of the seismic waves. This paper summarizes recent work on methods of deconstructing the effects of fractures, and any fluids within these fractures, on seismic wave propagation as observed in reflection seismic data. One method explored here is Thomsen's weak anisotropy approximation for wave moveout (since fractures often induce elastic anisotropy due to non-uniform crack-orientation statistics). Another method makes use of some very convenient crack/fracture parameters introduced previously that permit a relatively simple deconstruction of the elastic and wave propagation behaviour in terms of a small number of crack-influence parameters (whenever this is appropriate, as is certainly the case for small crack densities). Then, the quantitative effects of fluids on these crack-influence parameters are shown to be directly related to Skempton's coefficient B of undrained poroelasticity (where B typically ranges from 0 to 1). In particular, the rigorous result obtained for the low crack density limit is that the crack-influence parameters are multiplied by a factor  (1 − B )  for undrained systems. It is also shown how fracture anisotropy affects Rayleigh wave speed, and how measured Rayleigh wave speeds can be used to infer shear wave speed of the fractured medium in some cases. Higher crack density results are also presented by incorporating recent simulation data on such cracked systems.  相似文献   

4.
Summary The coherence of atmospheric acoustic-gravity waves has been measured in the period range 10–100 s at the Large Aperture Microbarograph Array in south-eastern Montana. The acoustic-gravity waves observed were signals generated by presumed nuclear explosions. The decrease of coherence with increasing distance between pairs of microbarographs is less rapid in the direction of wave propagation than transverse to it. Variation of direction of arrival over a small range of azimuth (±5°) explains the spatial behaviour of coherence in the direction normal to the wave propagation; variation of phase velocity of ±10 ms-1 explains the behaviour along the direction of wave propagation. Both effects may be due to inhomogeneities in the atmosphere; the velocity variation may be due to the presence in the signal of several normal modes of acoustic- gravity waves, each travelling at a slightly different phase velocity in the range 300–330 ms-1.  相似文献   

5.
Amplitude measurements of the transverse component of SKS waves, the so-called splitting intensity, can be used to formulate a non-linear inverse problem to image the 3-D variations of upper mantle anisotropy. Assuming transverse isotropy (or hexagonal symmetry), one can parametrize anisotropy by two anisotropic parameters and two angles describing the orientation of the symmetry axis. These can also be written as two collinear pseudo-vectors. The tomographic process consists of retrieving the spatial distribution of these pseudo-vectors, and thus resembles surface wave vectorial tomography. Spatial resolution results from the sensitivity of low-frequency SKS waves to seismic anisotropy off the ray path. The expressions for the 3-D sensitivity kernels for splitting intensity are derived, including the near-field contributions, and validated by comparison with a full wave equation solution based upon the finite element method. These sensitivity kernels are valid for any orientation of the symmetry axis, and thus generalize previous results that were only valid for a horizontal symmetry axis. It is shown that both lateral and vertical subwavelength variations of anisotropy can be retrieved with a dense array of broad-band stations, even in the case of vertically propagating SKS waves.  相似文献   

6.
1960-2016年秦岭—淮河地区热浪时空变化特征及其影响因素   总被引：4，自引：3，他引：1  
基于134个气象站点1960-2016年逐日最高温和相对湿度数据,辅以趋势分析、空间分析和相关分析等方法,对秦岭—淮河地区热浪时空变化特征进行分析,探讨了赤道东太平洋海温异常与热浪变化的相关关系。结果表明：①近57年秦岭—淮河地区热浪呈现“非线性、非平稳和阶段性”的变化过程,年代变化可分为3个阶段：1960-1972年热浪呈现东西分异,分界线大致位于112°E,以东地区热浪异常偏多,以西地区则“高低交替”波动;1973-1993年热浪维持“低位波动”,并在20世纪80年代中期呈现快速增加;1994-2016年,关中平原、秦巴山区、巫山山区和四川盆地热浪维持“高位波动”,黄河下游、淮河平原和长江下游热浪则经历从“相对偏多”向“相对偏少”的转变;②在影响因素方面,最高温波动变化是秦岭—淮河地区热浪频次年代变化的主导因素,相对湿度变化的影响相对较弱;③近57年来关中平原热浪年代变化与赤道太平洋西部海温异常关系更为密切,长江流域与东部海温异常关系更为密切;对于黄河下游和秦巴山区的热浪变化与不同分区赤道太平洋海温异常关系均较弱。  相似文献   

7.
Velocity estimation remains one of the main problems when imaging the subsurface with seismic reflection data. Traveltime inversion enables us to obtain large-scale structures of the velocity field and the position of seismic reflectors. However, as the media currently under study are becoming more and more complex, we need to know the finer-scale structures. The problem is that below a certain range of velocity heterogeneities, deterministic methods become difficult to use, so we turn to a probabilistic approach. With this in view, we characterize the velocity field as a random field defined by its first and second statistical moments. Usually, a seismic random medium is defined as a homogeneous velocity background perturbed by a small random field that is assumed to be stationary. Thus, we make a link between such a random velocity medium (together with a simple reflector) and seismic reflection traveltimes. Assuming that the traveltimes are ergodic, we use 2-D seismic reflection geometry to study the decrease in the statistical traveltime fluctuations as a function of the offset (the source–receiver distance). Our formulae are based on the Rytov approximation and the parabolic approximation for acoustic waves. The validity and the limits are established for both of these approximations in statistically anisotropic random media. Finally, theoretical inversion procedures are developed for the horizontal correlation structure of the velocity heterogeneities for the simplest case of a horizontal reflector. Synthetic seismograms are then computed (on particular realizations of random media) by simulating scalar wave propagation via finite difference algorithms. There is good agreement between the theoretical and experimental results.  相似文献   

8.
Generalized Born scattering of elastic waves in 3-D media   总被引：1，自引：0，他引：1  
It is well known that when a seismic wave propagates through an elastic medium with gradients in the parameters which describe it (e.g. slowness and density), energy is scattered from the incident wave generating low-frequency partial reflections. Many approximate solutions to the wave equation, e.g. geometrical ray theory (GRT), Maslov theory and Gaussian beams, do not model these signals. The problem of describing partial reflections in 1-D media has been extensively studied in the seismic literature and considerable progress has been made using iterative techniques based on WKBJ, Airy or Langer type ansätze. In this paper we derive a first-order scattering formalism to describe partial reflections in 3-D media. The correction term describing the scattered energy is developed as a volume integral over terms dependent upon the first spatial derivatives (gradients) of the parameters describing the medium and the solution. The relationship we derive could, in principle, be used as the basis for an iterative scheme but the computational expense, particularly for elastic media, will usually prohibit this approach. The result we obtain is closely related to the usual Born approximation, but differs in that the scattering term is not derived from a perturbation to a background model, but rather from the error in an approximate Green's function. We examine analytically the relationship between the results produced by the new formalism and the usual Born approximation for a medium which has no long-wavelength heterogeneities. We show that in such a case the two methods agree approximately as expected, but that in a media with heterogeneities of all wavelengths the new gradient scattering formalism is superior. We establish analytically the connection between the formalism developed here and the iterative approach based on the WKBJ solution which has been used previously in 1-D media. Numerical examples are shown to illustrate the examples discussed.  相似文献   

9.
The subsurface imaging using conventional seismic reflection technique is challenging in areas where high velocity rocks such as basalts are underlain by low velocity rocks. The seismic image quality worsens in the presence of intercalated sediments within the basaltic layers. In the recent years, the multicomponent seismic exploration technique has drawn great attention because it reduces the ambiguity in seismic imaging, enlarges the S-wave information, and improves the prediction and identification of reservoir fluids. Improvements in sub-basalt imaging techniques could hold highly significant geologic implications such as resource exploration and identifying permanent geochemical trapping potential (such as for carbon sequestration studies). In this article, we examine the possibility of utilizing mode-converted (P-SV) waves for sub-basalt imaging as well as likely complicacies one may expect in such processes.  相似文献   

10.
Analysis of three‐dimensional (3D) seismic data from the headwall area of the Storegga Slide on the mid‐Norwegian margin provides new insights into buried mass movements and their failure mechanisms. These mass movements are located above the Ormen Lange dome, a Tertiary dome structure, which hosts a large gas reservoir. Slope instabilities occurred as early as the start of the Plio‐Pleistocene glacial–interglacial cycles. The 3D seismic data provide geophysical evidence for gas that leaks from the reservoir and migrates upward into the shallow geosphere. Sediments with increased gas content might have liquefied during mobilization of the sliding and show different flow mechanisms than sediments containing less gas. In areas where there is no evidence for gas, the sediments remained intact. This stability is inherited by overlying strata. The distribution of gas in the shallow subsurface (<600 m) may explain the shape of the lower Storegga headwall in the Ormen Lange area.  相似文献   

11.
This paper presents the development of a 2.5-D simulation technique for acoustic wave propagation in media with variable density and velocity. A comparative study of the 2-D and 2.5-D responses of a model reveals the spatially and temporally damped nature of the 2.5-D acoustic wave equations. The simulated results for constant and variable density models show that the density variation affects only the reflectivity of the layer. The computational cost for variable density models is 2.17 and 2.26 times that for constant density models for the 2.5-D and 2-D cases, respectively. Furthermore, the 2.5-D computational cost in the time domain is only about 10–15 per cent more than that for two dimensions, so this modest increase in computational cost can avoid the exorbitant 3-D computational cost.
Snapshots for a crosshole geometry were computed at various times in order to study the effect of heterogeneity on the amplitude and shape of the wave front. Extensive analysis of an oil-bearing reservoir with and without the inclusion of a gas zone was performed using a point source as well as multiple sources. In addition, the effects of the thickness of a low-velocity layer (oil-bearing) and of the location of the source have been studied. It is concluded from the numerical response that the waveguide action of the low-velocity layer depends on its thickness in terms of the dominant wavelength. Trapping of waves was not observed when the source was outside the low-velocity layer. Furthermore, the presence of heterogeneity in the low-velocity layer contributes considerably to the leakage of energy in the adjacent layers due to scattering/diffraction. It was found that, in the 2.5-D numerical simulation, the stability condition and the requirement of the number of grid points per wavelength to avoid grid dispersion are the same as for the 2-D case.  相似文献   

12.
Volcano seismology often deals with rather shallow seismic sources and seismic stations deployed in their near field. The complex stratigraphy on volcanoes and near‐field source effects have a strong impact on the seismic wavefield, complicating the interpretation techniques that are usually employed in earthquake seismology. In addition, as most volcanoes have a pronounced topography, the interference of the seismic wavefield with the stress‐free surface results in severe waveform perturbations that affect seismic interpretation methods. In this study we deal predominantly with the surface effects, but take into account the impact of a typical volcano stratigraphy as well as near‐field source effects. We derive a correction term for plane seismic waves and a plane‐free surface such that for smooth topographies the effect of the free surface can be totally removed. Seismo‐volcanic sources radiate energy in a broad frequency range with a correspondingly wide range of different Fresnel zones. A 2‐D boundary element method is employed to study how the size of the Fresnel zone is dependent on source depth, dominant wavelength and topography in order to estimate the limits of the plane wave approximation. This approximation remains valid if the dominant wavelength does not exceed twice the source depth. Further aspects of this study concern particle motion analysis to locate point sources and the influence of the stratigraphy on particle motions. Furthermore, the deployment strategy of seismic instruments on volcanoes, as well as the direct interpretation of the broad‐band waveforms in terms of pressure fluctuations in the volcanic plumbing system, are discussed.  相似文献   

13.
Summary The displacement response of an elastic half space to a plane pressure wave is examined in order to establish the conditions under which sources of this type can contribute significantly to the long-period seismic noise field. The study is restricted to pressure waves which propagate at velocities well below the seismic wave velocities characteristic of the half space. The numerical studies indicate that pressure waves with amplitudes of 100 μbar or more can contribute significantly to the long-period vertical background noise observed at the surface, provided that the detectors are located on sections of alluvial fill or poorly to moderately indurated sandstones and shales whose thicknesses are greater than about a kilometre. These same waves can also create significant tilt noise on long-period horizontal seismographs located at or near the surface, regardless of the rock type. The seismic disturbances created by pressure waves decay rapidly away from the surface. Therefore, it appears that it may be possible to eliminate the effects of atmospherically generated noise by placing the detectors at moderate depths.  相似文献   

14.
Elastic scattered waves from a continuous and heterogeneous layer   总被引：3，自引：0，他引：3  
Elastic scattering from a continuous and laterally unbounded heterogeneous layer has been formulated using the Born approximation. A general solution of the scattered wave equation for the above-stated medium has been given in terms of a Fourier integral over plane waves. Far-field asymptotic expressions for weak elastic scattering by a finite, continuous and inhomogeneous layer have been presented which agree with earlier results. For perturbations of the two elastic parameters and the density having the same form of spatial variation, the spectrum of plane waves scattered from a heterogeneous layer is expressed as a product of an 'elastic scattering factor'and a 'distribution factor'. As in earlier results for small-scale heterogeneity, the scattering pattern depends on various combinations of perturbations of elastic parameters and density. In order to show the general characteristics of the elastic wave scattering, some scattering patterns have been given.  相似文献   

15.
Non-linear elastic response of rocks has been widely observed in laboratory, but very few seismic studies are reported in the literature, even though it is the most natural environment where this feature could be observed. Analytic solutions to the non-linear wave propagation phenomena are not readily available, and there is a need to use approximated techniques. It is clear that when a seismic wave propagates through a homogeneous non-linear elastic media, it will be perturbed by the non-linearity. This perturbation can be treated as a source of scattering, spreading the energy of the primary wave in space and time, contributing to the seismic coda. This is in some sense similar to the effect of heterogeneities. The properties of the coda due to the non-linearity depend on the amount of non-linearity and the seismic moment. Using a perturbation approach we calculate the amplitude of the scattered waves, and show that it can describe reasonably well the main features of real seismic codas.  相似文献   

16.
We present the results of Rayleigh wave and Love wave phase velocity tomography in the western United States using ambient seismic noise observed at over 250 broad-band stations from the EarthScope/USArray Transportable Array and regional networks. All available three-component time-series for the 12-month span between 2005 November 1 and 2006 October 31 have been cross-correlated to yield estimated empirical Rayleigh and Love wave Green's functions. The Love wave signals were observed with higher average signal-to-noise ratio (SNR) than Rayleigh wave signals and hence cannot be fully explained by the scattering of Rayleigh waves. Phase velocity dispersion curves for both Rayleigh and Love waves between 5 and 40 speriod were measured for each interstation path by applying frequency–time analysis. The average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station-triplets. We find that empirical Green's functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross-correlation without explicit knowledge of the source distribution. The average traveltime uncertainty is less than 1 s at periods shorter than 24 s. We present Rayleigh and Love wave phase speed maps at periods of 8, 12, 16,and 20 s. The maps show clear correlations with major geological structures and qualitative agreement with previous results based on Rayleigh wave group speeds.  相似文献   

17.
Summary. The Backus-Gilbert method has been extended to the estimation of the seismic wave velocity distribution in 2-D or 3-D inhomogeneous media from a finite set of travel-time data. The method may be applied to the inversion of body wave as well as surface wave data. The problem of determining a local average of the unknown velocity corrections may be reduced to a choice of a suitable δ-ness criterion for the averaging kernel. For 2-D and 3-D inhomogeneous media the simplest criterion is to minimize a sum of 'spreads' over all the coordinates. The use of this criterion requires the solution (the averaged velocity corrections) to be represented as a sum of functions, each of which depends only on one coordinate. This is a basic restriction of the method. In practice it is possible to achieve good agreement between the solution and a real velocity distribution by a reasonable choice of the coordinate system.
Numerical tests demonstrate the efficiency of the method. Some examples of the application of the method to the inversion of real seismological data for body and surface waves are given.  相似文献   

18.
Summary. The ability to locate the hypocentres of earthquakes occurring along ridge crests and fracture zones accurately is a prerequisite to solving several problems associated with seafloor tectonics and oceanic crustal formation. Such resolution has rarely been achieved in the past, often because the seismic networks deployed were inadequate for the task. We demonstrate that sea surface receivers are not useful in such studies, and that the minimum acceptable size for a seafloor network is five receivers, of which at least one must be capable of detecting shear waves unambiguously.  相似文献   

19.
Summary. Using a single scattering approximation, we derive equations for the scattering attenuation coefficients of P- and S -body waves. We discuss our results in the light of some recent energy renormalization approaches to seismic wave scattering. Practical methods for calculating the scattering attenuation coefficients for various earth models are emphasized. The conversions of P - to S -waves and S- to P -waves are included in the theory. The earth models are assumed to be randomly inhomogeneous, with their properties known only through their average wavenumber power spectra. We approximate the power spectra with piecewise constant functions, each segment of which contributes to the net, frequency-dependent, scattering attenuation coefficient. The smallest and largest wavenumbers of a segment can be plotted along with the wavevectors of the incident and scattered waves on a wavenumber diagram. This diagram gives a geometric interpretation for the frequency behaviour associated with each spectral segment, including a 'transition' peak that is due entirely to the wavenumber limits of the segment. For regions of the earth where the inhomogeneity spectra are concentrated in a band of wavenumbers, it should be possible to observed such a peak in the apparent attenuation of seismic waves. We give both the frequency and distance limits on the accuracy of the theoretical results.  相似文献   

20.
Summary. The response of a stratified elastic medium can be conveniently characterized by the Green's tensor for the medium. For coupled seismic wave propagation ( P—SV or fully anisotropic), the Green's tensor may be constructed directly from two matrices of linearly independent displacement solutions. Rather simple forms for the Green's tensor can be found if each displacement matrix satisfies one of the boundary conditions on the seismic field. This approach relates directly to 'reflection matrix' representations of the seismic field.
For a stratified elastic half space the Green's tensor is used to give a spectral representation for coupled seismic waves. By means of a contour integration a general completeness relation is obtained for the 'body wave' and 'surface wave' parts of the seismic field. This relation is appropriate for SH and P–SV waves in an isotropic medium and also for full anisotropy.  相似文献   

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