共查询到20条相似文献,搜索用时 15 毫秒
1.
Two-dimensional VSP surveys are often conducted to provide structural illumination of the subsurface away from the borehole. The illumination is achieved through offsetting the source with respect to the downhole geophone. This inevitably gives rise to mode-conversions in both downgoing and upgoing wavefields. Migration of mixed-mode wavefields is complex because the velocity profile used for wavefield extrapolation is valid only for a particular propagation mode; the other mode always propagates at a different velocity. It is therefore advisable to separate the wave-types (P-wave and SV-wave) prior to migration. This may be achieved through wavemode filtering, a multichannel process which exploits the relation between propagation velocity, slowness of events at the recording array and particle motion. The necessary information about particle motion is available only if the VSP data are acquired with a three-component downhole geophone assembly. The wavemode filter partitions wave-types at the recording array; it provides no information about the various changes of propagation mode experienced by the energy as it travels from source to geophone. For the purpose of migration, the intermediate modes of propagation must be deduced. Much of the energy arriving at the receivers is P-wave which has followed the P-wave velocity profile from the source. It can therefore be imaged by conventional (Kirchhoff) migration. As an example of SV-wave imaging, a common mode-code is P-wave from source to reflector and SV-wave from reflector to geophone. Migration of such data calls for back-propagation of the geophone array wavefield, at SV-wave velocity, to the point in the subsurface where it is time-coincident with the forward propagated downwave, at P-wave velocity. 相似文献
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SAMUEL H. GRAY 《Geophysical Prospecting》1992,40(5):565-571
Integral migration techniques perform a sum over an aperture of input traces to obtain output at a single point. The length of the aperture is limited by a spatial Nyquist criterion, which typically prohibits imaging very steep dips at very high frequencies without generating severe migration artifacts (migration operator aliasing). For time-domain Kirchhoff migration, this can be a fatal shortcoming. The standard way to address this problem is to interpolate traces spatially before migration. This reduces the trace spacing, thereby increasing the frequency content which can be migrated without aliasing at steep dips. An alternative remedy to the operator aliasing problem is to modify the phase response of the Kirchhoff migration operator. This operator is frequency-selective across the migration aperture: it passes all temporal frequencies of the input traces in the innermost portion of the aperture (referring to the shallow dips), and gradually cuts out the higher frequencies as it approaches the outer portion of the aperture. Thus, while all frequencies of the input data contribute to the shallow-dip portion of the migrated image, only the permissible low frequencies of the input data contribute to imaging the steepest dips. Using a simple realization of a frequency-selective Kirchhoff migration operator, this technique is illustrated on a synthetic data set involving greater than vertical dips. 相似文献
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RAINER TONN 《Geophysical Prospecting》1991,39(1):1-27
Ten methods for the computation of attenuation have been investigated, namely: amplitude decay, analytical signal, wavelet modelling, phase modelling, frequency modelling, rise-time, pulse amplitude, matching technique, spectral modelling and spectral ratio. In particular, we have studied the reliability of each of these methods in estimating correct values of Q using three synthetic VSP seismograms for plane P-waves with different noise contents. The investigations proved that no single method is generally superior. Rather, some methods are more suitable than others in specific situations depending on recording, noise or geology. The analytical signal method has been demonstrated to be superior if true amplitude recordings are available. Otherwise spectral modelling or, in the ‘ noise-free’ case the spectral ratio method, is optimal. Finally, two field VSPs in sediments are investigated. Only in the case of the highest quality VSP can significant information be deduced from the computed attenuation. 相似文献
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R. MJELDE 《Geophysical Prospecting》1992,40(6):605-617
P-wave and S-wave data acquired with vertical seismic profiling (VSP) often include tube waves propagating in the borehole, although considerable efforts are generally made to ensure that these waves are not recorded. However, several theoretical studies have indicated that tube waves could provide important information about the rock formation and thus should not be considered as pure noise. In order to study some of these aspects experimentally, tube waves were acquired by VSP in a well in the Paris Basin both before and after casing. A sparker was used as source inside the borehole, which ensured that the data recorded contained high-amplitude tube waves. It is shown that the casing is an obstacle which prevents the study of formation parameters, and thus further tube-wave acquisitions should be carried out in open holes only. The before-casing tube-wave reflection log is compared to a synthetic log computed from the sonic log. The high resolution of the tube waves is of particular interest, revealing layers that are too thin to be detected in body-wave surveys. It has recently been suggested that the projection of the tube-wave polarization in the horizontal plane can be used to determine directions of stress-induced anisotropy in the rock formation. Strong polarization anomalies are observed in the data sets but are attributed to tool problems rather than any rock-formation feature. 相似文献
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PHILIP M. CARRION 《Geophysical Prospecting》1990,38(7):689-704
The so-called ‘enhanced migration’ which uses diffraction tomography as the ‘repair tool’ for correction of amplitudes (reflection coefficients) of migrated sections is discussed. As with any linearized procedure, diffraction tomography requires knowledge of the initial model. It is suggested that the initial model is taken as the migrated image. It will be demonstrated that diffraction tomography applied to the data residuals improves the amplitudes of the migrated images. Migration is redefined as the reconstruction of the wavefront sets of distributions (reflection interfaces), and the inversion process as tomographic correction of migrated images. 相似文献
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F. AMINZADEH 《Geophysical Prospecting》1989,37(8):893-906
Data from offshore Norway is used to study applications of elastic VSP modelling in detecting shear waves and observing the effects of successive mode conversion in field-recorded VSP data. The shear-wave velocities and densities from log data are used in conjunction with compressional wave velocities determined from surface seismic and log data in the VSP modelling. The time domain non-normal incidence elastic VSP modelling technique of Aminzadeh and Mendel is used as the modelling algorithm. Two surface seismograms are computed first. One is the vertical component and the other is the horizontal component for plane waves that have specified incident angles. A downward continuation method is then applied to generate seismograms at different depth points. The collection of these seismograms constitutes non-normal incidence VSPs. Both vertical and horizontal components of VSP data can be obtained by this procedure. In this paper non-normal incidence VSPs are generated for a 12.5° incident plane wave. The modelling results of layered earth systems of thin layers and thick layers are both compared with field data, and the effect of mode conversions in thin layers is observed. Several events in the field data can be explained by this elastic VSP modelling. Comparison of the model data and field data enabled a probable tube wave or out-of-plane event to be identified, the removal of which significantly improved the final VSP section. This study also shows how the VSP data helped the interpretation of the surface 3D data. 相似文献
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The standard Kirchhoff algorithm can be generalized for migration of pre-stack finite-offset data from variable-velocity media. The concentric ellipses over which the data are spread in constant velocity media become significantly distorted (even multi-valued) in the variable velocity case. The specific shapes can be explicitly defined by kinematic extrapolation of the source and recorded wave fields with the ray equation. The use of Kirchhoff migration with a surface source and a subsurface recorder requires that two sets of Kirchhoff loci be superimposed. For each trace, the first set of loci is computed with the source and the actual recorder position as foci; the second set is computed with the source and the virtual recorder position as foci. This dual procedure explicitly incorporates the primary diffracted energy and the free-surface reflections, respectively. Implementation involves the construction of a virtual medium, lying above the free surface, with a velocity distribution that is the mirror image of the actual distribution below the free surface. Ray-equation extrapolation is performed through the real/virtual boundary. The resulting image is produced in a split form, with all the contributions of the primary reflected and diffracted energy lying in the lower ‘real’ half and all the contributions of the energy that was reflected at the free surface lying in the upper ‘virtual’ half. The final image is produced by folding the split image about the free surface and adding the two halves. A practical advantage is that the origin of various contributions (and artifacts) can be more readily identified (for interpretation or removal) in the split images. The ray-equation pre-stack migration algorithm is very general. It is applicable to all source-recorder geometries and variable velocity media and reduces exactly to the standard Kirchhoff algorithm when applied to zero or finite-offset surface survey data. The algorithm is illustrated by application to VSP data. For the VSP geometry, the algorithm does not require any specific trace spacing (in depth) and can be used for data from deviated as well as vertical holes. 相似文献
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TH.C. KREY 《Geophysical Prospecting》1987,35(2):135-147
Three-dimensional seismic surveys, in general, do not need the same high degree of CDP coverage as 2-D surveys to achieve a certain signal-to-noise ratio after migration. This can be shown theoretically for Kirchhoff migration and laterally uncorrelated noise. More precisely, there exists a formal relationship between the multiplicity of CDP coverage of a 3-D survey and that of a 2-D survey with the same signal-to-uncorrelated-noise ratio. Frequency and aperture are parameters in the corresponding expression. Heuristically the relationship can be obtained by applying the concept of the Fresnel zone. Though the mathematics in this paper refer to laterally uncorrelated noise, the underlying concepts can probably also be used for weakly correlated noise, e.g., for multiple reflections and for the low-frequency remnants of surface waves. 相似文献
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用Biot介质参数说明了波达波夫震电波方程组中弹性动力学 参数的含义,解释了第一类和第二类震电效应的意义,在忽略第一类震电效应条件下将该方 程组与Pride方程组进行比较,说明了二者在描述第二类震电效应方面的异同点. 同时指出 :波达波夫方程组忽略了流体与固体的耦合质量;方程中的黏性耗散项丢掉了一个孔隙度因 子,依据该方程组计算出的弹性波和转换电场的幅度将偏大;边界条件之一存在错误,会影 响对波在界面上的反射透射规律的描述. 相似文献
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结合阿克苏台周边5级以上中强地震和100 km范围内的4级地震,对阿克苏台跨断层形变资料进行分析.分析结果表明,断层的垂直、水平位移形变异常变化与中强地震活动有明显的对应关系.在监控区350 km范围内的中强地震前,垂直形变异常形态表现为次波效应,而水平形变在100 km范围内的4级地震前则出现明显的伸长或缩短的异常特征. 相似文献
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During the last couple of years there has been much research in the area of wavefield separation of borehole seismic data, and several articles have been published on various separation techniques. Methods involving the application of two-dimensional Fourier transformation, the Radon transformation, multi-level median filters or optimal filters, are all suggested as possible approaches to the wavefield separation problem. This paper compares some of these methods commonly used in the industry. The theories of the chosen methods are described to see how they are related. Using the different methods on synthetic and real data, we show how this theoretical relation is reflected in the relatively similar results obtained. We also show how the different filters treat coherent and random noise. 相似文献
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利用266条强震记录,在研究绝对输入能量谱和相对输入能量谱衰减规律的基础上,对由衰减关系所确立的两种输入能量谱进行对比分析,讨论了延性系数对这两种输入能量谱的影响. 研究发现,在弹性情况下,两种输入能量谱在周期0.5~1.0 s范围内相差不大; 在非弹性情况下 两种输入能量谱在周期0.5 s处相差不大. 周期较小时,绝对输入能量谱要大于相对输入能量谱;周期较大时,绝对输入能量谱小于相对输入能量谱. 延性系数对这两种输入能量谱影响均比较大, 对绝对输入能量谱而言,周期小于0.3 s时,随着延性系数的增大, 能量谱升高;周期大于0.3 s时,随着延性系数的增大能量谱降低. 不同延性系数的绝对输入能量谱在分界点0.3 s左右相等. 相对输入能量谱受延性系数影响与绝对输入能量谱相似, 但分界点在0.5 s左右. 与绝对输入能量谱相比, 相对输入能量谱在短周期段受延性系数的影响较大, 特别是当场地较软时更为明显. 相似文献
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WANG Guixian CHEN Zhicongand ZHANG Ren Associate professor Tsinghua University. Associate Professor Tsinghua University Professor Tsinghua University. 《国际泥沙研究》1991,(1)
The fluctuationg backwater region of a reservoir has a twofold property of a river as well as a reservoir and undergoes scouring and deposition alternately. Phys- ical model simulating the fluvial processes in a fluctuating backwater region has to ensure the similarities of both scouring and deposition. This causes some difficulties in model design and the selection of model sediment. This paper describes the model study of the fluctuating backwater region of the Gezhouba Project on the Yangtze River. Plastic particles with specific gravity of 1 .056 are used. The physical. model covers a reach of mountainous river 130 km in length. The experimental results show that the total amount of deposit, its distribution along the reach, the size distribution of the deposit, and lengths of the backwater region for various discharges all agree very well with the measured data in reservoir after 5 years of operation. This fact indicates that the model design, including the selection of model sediment, is quite acceptable. 相似文献
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A. N. BOWEN 《Geophysical Prospecting》1986,34(3):366-382
Wiener ‘spiking’ deconvolution of seismic traces in the absence of a known source wavelet relies upon the use of digital filters, which are optimum in a least-squares error sense only if the wavelet to be deconvolved is minimum phase. In the marine environment in particular this condition is frequently violated, since bubble pulse oscillations result in source signatures which deviate significantly from minimum phase. The degree to which the deconvolution is impaired by such violation is generally difficult to assess, since without a measured source signature there is no optimally deconvolved trace with which the spiked trace may be compared. A recently developed near-bottom seismic profiler used in conjunction with a surface air gun source produces traces which contain the far-field source signature as the first arrival. Knowledge of this characteristic wavelet permits the design of two-sided Wiener spiking and shaping filters which can be used to accurately deconvolve the remainder of the trace. In this paper the performance of such optimum-lag filters is compared with that of the zero-lag (one-sided) operators which can be evaluated from the reflected arrival sequence alone by assuming a minimum phase source wavelet. Results indicate that the use of zero-lag operators on traces containing non-minimum phase wavelets introduces significant quantities of noise energy into the seismic record. Signal to noise ratios may however be preserved or even increased during deconvolution by the use of optimum-lag spiking or shaping filters. A debubbling technique involving matched filtering of the trace with the source wavelet followed by optimum-lag Wiener deconvolution did not give a higher quality result than can be obtained simply by the application of a suitably chosen Wiener shaping filter. However, cross correlation of an optimum-lag spike filtered trace with the known ‘actual output’ of the filter when presented with the source signature is found to enhance signal-to-noise ratio whilst maintaining improved resolution. 相似文献