海底沉积物的声波频散研究技术

声学测量是调查海底沉积物的重要手段，有着广泛的应用。目前地震、测深都是利用回波到达时间差来探测目的物－几何声学。即使是反映９０年代世界水平的Ｓｅａｂｅａｍ、Ｃｈｉｒｐ也只给不同波速振幅、掠射角或不同频率声波的反射率，而未提供对这些属动力学范畴参数的处理分析方法。

使用偶极声波频散分析进行机械损伤探测和各向异性评价

同地球物理和岩石物理学一样，井眼附近的机械损伤和地层各向异性的声波特性会为钻井和完井工程产生重要的新信息。泥岩的损伤是井眼稳定性问题的预兆。在弱胶结地层，应力导致的机械损伤表明，用选择性垂直射孔会减少出砂危险。探测井眼附近的损伤并把它从未开采的地层中区分出来，将有助于地球物理学家和岩石物理学家描述储层。即使在井眼没有损伤或钻井诱导裂缝常规指示器的情况下，井眼附近的变化也可由声波探测到。各向异性分析不仅帮助地球物理学家进行深度带和AVO分析，而且也帮助完井工程师进行与应力方向一致的定向射孔以使出砂问题最小化、优化开采。具有先进频率域处理技术的交叉偶极声波测井能独特地描述井眼周围地层的机械特性。常规处理方法是在时间域进行，并得到地层各向同性或各向异性信息。我们最近展示了交叉偶极声波数据的频率域处理(即时差一频率分析或频散分析)，它可以把固有各向异性从应力引起的各向异性中区分出来。频散分析扩展了常规的时基相似处理技术来得到更完整的地层描述。钻井过程及其相关的井眼应力集中可明显地影响井眼周围环境，导致横波时差径向变化。交叉偶极声波资料的时差一频率分析显示低频信号进入地层2—3个井眼直径，意味着该岩石是非蚀变的；高频信号进入地层一个井眼半径，可探测到机械损伤区域。均质模型在高频处的差异表明有损伤存在。偶极频散曲线分析可评价机械损伤带的深度。全世界大量的泥岩和储集岩的现场实例显示可从交叉偶极频散曲线中得到非常重要的额外信息。这些数据的频散分析可以识别：1)各向同性和各向异性(固有的和应力诱导的)；2)井眼周围的机械损伤。另外，我们将显示新的频率域处理技术是如何补充常规声波测井处理结果的。这项评价新技术的关键是宽频带采集交叉偶极声波资料和先进的频率域处理技术。

地震资料解释中应用测井数据需注意的几个问题

测井资料作为连接地震与地质的桥梁和纽带,在地震资料构造解释、反演以及储层预测中得到了广泛应用。通过理论及实际测井数据分析,探讨了测井数据的频散特性、不同仪器测量结果的差异性等,指出了地震资料解释中应用测井数据应该注意的几个问题:①地震数据与声波测井数据之间存在由速度频散引起的差异,层位标定中应进行必要的匹配校正,地震反演中不能过分追求反演结果与测井数据的一致性;②测井数据并非真值,同样存在一定的误差,应用中不可将测井数据绝对化;③基于测井数据的统计回归分析应注意数据选取的系统性、数据分布的均匀性以及数据量的充分性。

Laboratory study of fluid viscosity induced ultrasonic velocity dispersion in reservoir sandstones

Ultrasonic velocities of a set of saturated sandstone samples were measured at simulated in-situ pressures in the laboratory.The samples were obtained from the W formation of the WXS Depression and covered low to nearly high porosity and permeability ranges.The brine and four different density oils were used as pore fluids,which provided a good chance to investigate fluid viscosity-induced velocity dispersion.The analysis of experimental observations of velocity dispersion indicates that（1）the Biot model can explain most of the small discrepancy（about 2–3%）between ultrasonic measurements and zero frequency Gassmann predictions for high porosity and permeability samples saturated by all the fluids used in this experiment and is also valid for medium porosity and permeability samples saturated with low viscosity fluids（less than approximately 3 mP·S）and（2）the squirt flow mechanism dominates the low to medium porosity and permeability samples when fluid viscosity increases and produces large velocity dispersions as high as about 8%. The microfracture aspect ratios were also estimated for the reservoir sandstones and applied to calculate the characteristic frequency of the squirt flow model,above which the Gassmann’ s assumptions are violated and the measured high frequency velocities cannot be directly used for Gassmann’s fluid replacement at the exploration seismic frequency band for W formation sandstones.

Wideband dipole logging based on segmen linear frequency modulation excitation

A wideband dipole signal is required for dipole dispersion correction and nearborehole imaging. To obtain the broadband flexural wave dispersion, we use a nonlinear frequency modulation （NLFM） signal and propose a segment linear frequency modulation （SLFM） signal as the dipole excitation signal to compensate for the excitation intensity. The signal-to-noise ratio （SNR） of the signal over the entire frequency band is increased. The finite-difference method is used to simulate the responses from a Ricker wavelet, a linear frequency modulation （LFM） signal, an NLFM signal, and an SLFM signal in two borehole models of a homogeneously hard formation and a radially stratified formation. The dispersion and radial tomography results at low SNR of the sound source signals are compared. Numerical modeling suggests that the energy of the flexural waves excited by the Ricker wavelet source is concentrated near the Airy phase. In this case, the dispersion is incomplete and information regarding the formation near or far from the borehole cannot be obtained. The LFM signal yields dispersion information near the Airy phase and the high-frequency range but not in the low-frequency range. Moreover, the information regarding the formation far from the borehole is not accurate. The NLFM signal extends the frequency range of the flexural waves by compensating for the excitation intensity and yields information regarding the formation information, but it is not easy to obtain. The SLFM signal yields the same results as the NLFM signal and is easier to implement. Consequently, the dipole detection range expands and the S-wave velocity calculation accuracy improves.

Joint inversion method of formation shear-wave anisotropy from logging-while-drilling acoustic data

Most sedimentary formations with fine layers can be characterized as transversely isotropic media.The evaluation of shear-wave anisotropy is critical in logging-while-drilling(LWD)applications.We developed a joint method to simultaneously invert formation shear-wave anisotropy and vertical shear velocity using LWD monopole and dipole dispersion data.Theoretical analysis demonstrates that formation shear-wave anisotropy significantly aff ects the dispersion characteristics of Stoneley and formation flexural waves.The inversion objective function was constructed based on the change in dispersion characteristics and was weighted by the spectra of multipole waves.Numerical results using synthetic examples demonstrate that the joint inversion method can not only alleviate the non-uniqueness problem but also help improve the accuracy of the inversion results.The comparison of diff erent signal-to-noise ratio inversion results proved that the weighted inversion method is more accurate and stable.

Processing Seismic Ambient Noise Data to Obtain Reliable Surface Wave Dispersion Measurements

Ambient noise tomography is a rapidly emerging field of seismological research. This paper presents the current status of ambient noise data processing and its development history over the past several years, with the intention to explain and justify this development through salient examples. The ambient noise data processing procedure can be divided into four principal phases： ① single station data preparation; ② cross- correlation and temporal stacking; ③ measurements of dispersion curves （ performed with frequency-time analysis for both group and phase speeds） ; ④ quality control, including SNR analysis and selection of the acceptable measurements. In addition, we provide a specific solution for a better use of the seismic station data to ambient noise study.

Electrostatic Waves in Weakly Magnetized Quantum Plasmas

By using the quantum magnetohydrodynamic model, the electrostatic waves in weakly magnetized quantum plasmas are investigated. The electrons are treated as a quantum and magnetized species, while the ions are classical unmagnetized ones. The general dispersion relations are derived. It is shown that, both the high frequency electron waves （Langmuire wave and upper-hybrid wave） and the low frequency ion acoustic wave can propagate when the plasmas are cold.

The Body Wave Velocity Structure in the Upper Crust of Fujian Estimated by Noise Records

In this paper,the dispersion curves of the Rayleigh wave and Love wave were extracted from the seismic noise records of 25 broadband stations of the Fujian Seismic Network, and inverted for the lithosphere velocity structure. Furthermore,the velocity model was verified by the seismic explosion observations. Our results indicate that the resolution of the lithosphere velocity structure obtained by this method is good in the shallow part,but in the deep part,inversion accuracy for the wave velocity structure is low,which is caused mainly by the small inter-station distance chosen in the paper. Thus the wave dispersion curves have high accuracy in the short-period part,but the warp of the wave dispersion curve in long-period part is large. Considering the results from both the noise inversion and the traditional inversion,we finally present a new velocity model,and the theoretical travel time calculated with the new model matches the explosion travel time very well.