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1.
Marek Grad Rolf Mjelde Wojciech Czuba Aleksander Guterch the IPY Project Group 《Geophysical Prospecting》2012,60(5):855-869
Wide‐angle seismic data acquired by use of air‐guns and ocean bottom seismometers (OBS) contain strong direct water arrivals and multiples, generally considered as noise and thus not included in the modelling. However, a recent study showed that standard ray‐tracing modelling of the water multiples recorded off the Bear Island, North Atlantic, provided a reliable estimate of the velocity distribution in the water layer. Here, we demonstrate that including the amplitudes in the modelling provide valuable information about the VP contrast at the seafloor, as well as the VP/VS ratio and attenuation (QP) of the uppermost sediments. The VP contrast at the seafloor is estimated at about 250 m/s, within a precision of approximately ±30 m/s. The VP/VS ratio in the uppermost sedimentary layer is modelled in the range 2.25–2.50 and the QP factor is estimated at 1000 for the water, 30–50 for the uppermost layer and 40–50 for the second sedimentary layer. The values obtained for the sediments suggest a lithology dominated by silty clays, with porosity below average. 相似文献
2.
Elastic rock properties can be estimated from prestack seismic data using amplitude variation with offset analysis. P‐wave, S‐wave and density ‘reflectivities’, or contrasts, can be inverted from angle‐band stacks. The ‘reflectivities’ are then inverted to absolute acoustic impedance, shear impedance and density. These rock properties can be used to map reservoir parameters through all stages of field development and production. When P‐wave contrast is small, or gas clouds obscure reservoir zones, multicomponent ocean‐bottom recording of converted‐waves (P to S or Ps) data provides reliable mapping of reservoir boundaries. Angle‐band stacks of multicomponent P‐wave (Pz) and Ps data can also be inverted jointly. In this paper Aki‐Richards equations are used without simplifications to invert angle‐band stacks to ‘reflectivities’. This enables the use of reflection seismic data beyond 30° of incident angles compared to the conventional amplitude variation with offset analysis. It, in turn, provides better shear impedance and density estimates. An important input to amplitude variation with offset analysis is the Vs/Vp ratio. Conventional methods use a constant or a time‐varying Vs/Vp model. Here, a time‐ and space‐varying model is used during the computation of the ‘reflectivities’. The Vs/Vp model is generated using well log data and picked horizons. For multicomponent data applications, the latter model can also be generated from processing Vs/Vp models and available well data. Reservoir rock properties such as λρ, μρ, Poisson's ratio and bulk modulus can be computed from acoustic impedance, shear impedance and density for pore fill and lithology identification. λ and μ are the Lamé constants and ρ is density. These estimations can also be used for a more efficient log property mapping. Vp/Vs ratio or Poisson's ratio, λρ and weighted stacks, such as the one computed from λρ and λ/μ, are good gas/oil and oil/water contact indicators, i.e., pore fill indicators, while μρ mainly indicates lithology. μρ is also affected by pressure changes. Results from a multicomponent data set are used to illustrate mapping of gas, oil and water saturation and lithology in a Tertiary sand/shale setting. Whilst initial log crossplot analysis suggested that pore fill discrimination may be possible, the inversion was not successful in revealing fluid effects. However, rock properties computed from acoustic impedance, shear impedance and density estimates provided good lithology indicators; pore fill identification was less successful. Neural network analysis using computed rock properties provided good indication of sand/shale distribution away from the existing wells and complemented the results depicted from individual rock property inversions. 相似文献
3.
针对工程中大量存在的平面应变问题,依据平面应变条件和广义胡克定律,基于SMP、Lade-Duncan、AC-SMP和广义Mises强度准则,推导出考虑中主应力及泊松比影响的无黏性土主、被动土压力计算公式,并将其扩展至黏性土,讨论基于各强度准则土压力计算公式的适用范围。结果表明：考虑中主应力对土强度的贡献后,基于各强度准则所计算的主动土压力均小于朗肯主动土压力,被动土压力均大于朗肯被动土压力;主动土压力Pa随着泊松比的增大而减小,被动土压力PP随着泊松比的增大而增大,且泊松比越大,与实测数据更为接近;基于同一强度准则下得到的主、被动土压力适用的内摩擦角范围随着泊松比的增大而增大;基于各强度准则的土压力计算公式均能较好的描述挡土结构上土压力的大小,其中广义Mises强度准则计算结果与实际工程更为吻合,研究成果可为挡土结构上土压力的计算提供一定理论参考。 相似文献
4.
The ultrasonic profiling method of measuring the compressional and shear wave velocities in cylindrical rock samples is extended to measurements in some weathered and fresh granite blocks collected from the Hyderabad (India) region. This possibility of the method provides a means of investigating the elastic properties of the less compact rocks, of which the near-surface formations are particularly important. In this article the important parts of the ultrasonic profiling instrument developed are described and the relevant aspects of the seismic wave fields and identification of the individual waves in the wavetrain responses to longitudinal excitation are considered. Compressional, shear and surface (Rayleigh) wave velocities in some fresh and weathered granites are detailed. The compressional velocities range from 4.8 km/s to 5.5 km/s in fresh granites and lie between 1.1 km/s and 2.5 km/s in weathered granites. Young's modulus and Poisson's ratios computed from the measured velocities are also presented. An empirical relation of the form log E= 4.27 + 2.11 log Vp between Young's modulus E and compressional velocities Vp in the fresh granites studied is deduced. The versatility of the approach is thus demonstrated. 相似文献
5.
利用2009～2016年内蒙古自治区数字地震台网宽频带固定地震台站的远震波形数据,采用接收函数H-k算法获得23个基岩台站下方的莫霍面深度和泊松比,同时,收集并筛选出277个已有探测台阵和流动台站的接收函数研究结果,综合分析给出大兴安岭造山带及两侧邻区莫霍面深度、泊松比的分布特征。研究表明,研究区域的莫霍面在整体上呈现自东向西逐渐加深的特征,莫霍面深度为25.0～42.3km,平均约为33.5km。莫霍面最浅的区域为松辽盆地(深度为27.0～35.0km),最深的区域为大兴安岭重力梯级带以西地区(深度为41.0～42.3km)。研究区域泊松比为0.19～0.33,平均值为0.26,大于全球大陆地壳的平均值。泊松比高值异常区集中在火山岩区及具有较厚沉积层的盆地。台站所处位置的海拔与莫霍面深度之间具有较强的正相关性,艾里补偿模式在研究区成立,莫霍面起伏与区域地形地貌特征间具有显著的镜像关系。大兴安岭地区的莫霍面深度与泊松比间存在显著的反相关关系,而在松辽盆地及周缘地区未发现明显的规律性,这也意味着松辽盆地在构造演化过程中经历了更为复杂的地壳改造过程。 相似文献
6.
Viscoelastic modelling reveals that the interaction of compressional-wave velocity Cp, compressional-wave quality factor Qp, shear-wave velocity Cs, shear-wave quality factor Qs and Poisson's ratio as a function of time intercept τ and ray parameter p, is complicated; however, distinct, potentially diagnostic behaviours are seen for different combinations of viscoelastic parameters. Synthetic seismograms for three viscoelastic reservoir models show that variations in the Poisson's ratio produce visible differences when compared to the corresponding elastic synthetic seismograms; these differences are attributable to interaction of the elastic parameters with Qp and Qs. When the P-wave acoustic impedance contrast is small, viscoelastic effects become more apparent and more useful for interpretation purposes. The corresponding amplitude and net phase spectra reveal significant differences between the elastic and the viscoelastic responses. When P-wave reflectivities are large, they tend to dominate the total response and to mask the Q reflectivity effects. The attenuation effects are manifested as an amplitude decay that increases with both time and ray parameter. The sensitivity of the computed seismic responses for various combinations of viscoelastic parameters suggests the opportunity for diagnostic interpretation of τ-p seismic data. The interpretation of the viscoelastic parameters can permit a better understanding of the rock types and pore fluid distribution existing in the subsurface. 相似文献
7.
We present a method for estimating P- and S-velocities within defined layers (macromodel), using only kinematic properties (i.e. traveltimes) of the wavefield. The method does not require identification of mixed-mode events on prestack or post-stack data. After obtaining a Vp-depth model by coherency inversion, S-velocities are determined by coherency optimization along computed traveltime curves for mixed-mode events on prestack data. Since the method does not involve any dynamic wavefield computations, a simple ray-tracing algorithm is used to solve the forward problem. The simplicity of the scheme, together with the ability to apply it locally, makes it highly suitable for interactive use. Results of this method may be used to detect Poisson's ratio anomalies within or between layers and may serve as an initial model for more complicated elastic inversion algorithms. 相似文献
8.
I. Lerche 《Pure and Applied Geophysics》1987,125(5):717-751
We demonstrate how multiples, generated at the interfaces of plane parallel beds, modify the propagation characteristics of an originally coherent seismic wave. For waves propagating at an angle to the bedding plane we find that theSV andP-waves couple so that neither is a pure mode. TheSH-wave, while modified in its propagation characteristics by multiples, remains a pure mode. The coupling ofSV-multiples into the quasi-P-mode appears weaker than the coupling ofP-wave multiples into the quasi-SV mode; at least this is so for the two simple cases of (a) density fluctuations only and (b) correlatedV
p
andV
s
fluctuations which conserve Poisson's ratio.We also find that the coupling is sensitive to both the angle of propagation and frequency. In addition there is a cut-off angle forP-wave multiples influencing the quasi-SV mode. Propagation angles larger than the cut-off permit theP-multiples to modify the phase of the quasi-SV mode, but not its effective attenuation. No such cut-off effect is found for SV-multiples influencing the quasi-P mode, whose angle-dependent and frequency-dependent phase distortion and effective attenuation are influenced both byP-wave multiples andSV-multiples.In view of the mathematical complexity of the expressions describing the phase, and effective attenuation of modes when allowance is made forP-andS-wave multiples, we strongly advocate numerical coding of the major mathematical formulae. By so doing a systematic study can be undertaken of the frequency and offset dependence of seismic waves as a function of seismic source input and power spectral behavior of the fluctuations in density and elastic constants of beds. It is our opinion that the full mathematical expressions are too involved to permit an analytic, systematic investigation to be given of the phase and attenuation of seismic waves with any degree of sophistication or generality. 相似文献
9.
Kate Hadley 《Pure and Applied Geophysics》1975,113(1):1-23
Summary In a series of triaxial experiments we have measuredV
p
,V
s
and volumetric strain simultaneously in dilating dry and saturated rocks. For the first time these data permit quantitative comparison of seismic velocities or their ratio and dilatant volumetric strain. In air-dry samplesV
p
/V
s
decreases by a few per cent at strains of 10–3; in saturated materials with high pore pressure,V
p
/V
s
increases by a comparable amount. Decreases in seismic velocity ratio are difficult to generate in initially saturated rocks even with low pore pressures and at strain rates of 10–4/sec. A liquid-vapor transition will not produce a significant drop inV
p
/V
s
. If dilatancy and fluid flow are responsible for seismic travel time anomalies prior to earthquakes, our results suggest that such anomalies will occur only in regions where pore fluid source to sink dimensions are of the order of 10 km or more, or in regions where the rocks are not saturated to begin with. 相似文献
10.
We present a numerical solution for the momentum equation of the magnetosheath particles that describes the distribution of the pressure anisotropy of the magnetosheath plasma in the midday meridian plane. The pressure anisotropy is a maximum near the magnetopause subsolar point (p⊥/p\Vert\cong10). The pressure anisotropy is caused by two factors: particles with small pitch angles (V\Vert>V⊥) which travel along the magnetic field lines away from the equatorial plane of the magnetosheath; and particles, after crossing the bowshock, which reach the bulk velocity component directed along the magnetic field lines again, away from the magnetosheath equatorial plane. This velocity increases with increasing distance from the subsolar point of the bowshock, and does not permit particles with large pitch angles (V⊥>V\Vert) to move toward the equatorial plane. 相似文献
11.
Measurements of the changes in phase path of F-region reflections at normal incidence at Kodaikanal (77° 28′E, 10° 14′N, dip 3°N) from February 1991 to February 1993 are used to determine the variation of the equatorial evening F-region vertical drifts (Vz) with season, solar and magnetic activity. It is found that on average, at Kodaikanal, the post-sunset peak in Vz(Vzp) is higher in equinox and local winter months than in local summer. The day-to-day variability in Vzp is highest in summer and lowest in winter. This seasonal trend persists even on magnetically quiet days (Ap\leq14). Vzp is found to increase with 10.7 cm solar flux in all three seasons but tends to saturate for large flux values (>230 units) during local summer and winter months. Magnetic activity [represented by Ap as well as the time-weighted accumulations of ap and ap ()] does not seem to have any statistically significant effect on Vzp, except during equinoctial months of moderate solar activity, when Vzp decreases as magnetic activity increases. 相似文献
12.
The Wenchuan
13.
Azimuthal variation in AVO response for fractured gas sands 总被引：1，自引：0，他引：1
Natural fractures in reservoirs play an important role in determining fluid flow during production, and hence the density and orientation of fractures is of great interest. In the presence of aligned vertical fractures, the reflection amplitude at finite offset varies with azimuth. The effect of natural fractures on the azimuthal AVO response from a gas-sandstone reservoir encased within shale is investigated. A simple expression for the difference in P-wave reflection coefficient from the top of the reservoir parallel and perpendicular to the strike of the fractures is obtained in terms of the normal and tangential compliances, ZN and ZT, of the fractures. This expression is valid for small anisotropy and material contrasts and is compared with the results of numerical modelling. For a given value of ZT, the azimuthal variation in reflection coefficient at moderate offsets is found to increase with decreasing ZN/ZT. For gas-filled open fractures ZN/ZT ≈ 1, but a lower ratio of ZN/ZT may result from the presence of cement or clay within the fractures, or from the presence of a fluid with non-zero bulk modulus. For ZN/ZT = 1 and moderate offsets, the variation with offset of the reflection coefficient from the top of the fractured unit is dominated by the contrast in Poisson's ratio between the gas sand and the overlying shale, the effect of fractures only becoming noticeable as the critical angle for the unfractured sandstone is approached. However, for reflections from the base of the fractured unit, the variation in reflection amplitude with azimuth is much greater at conventional seismic offsets than for the reflection from the top. Azimuthal variations in the strength of the reflection from the top of the reservoir depend only on the variation in reflection coefficient, whereas the raypath is also a function of azimuth for reflections from the base of the fractured unit, leading to stronger, more visible, variations of AVO with azimuth. It follows that an azimuthal variation in AVO due to fractures in the overburden may be misinterpreted as due to the presence of aligned fractures in the reservoir. 相似文献
14.
Published accounts of outburst floods from glacier-dammed lakes show that a significant number of such floods are associated not with drainage through a tunnel incised into the basal ice—the process generally assumed—but rather with ice-marginal drainage, mechanical failure of part of the ice dam, or both. Non-tunnel floods are strongly correlated with formation of an ice dam by a glacier advancing from a tributary drainage into either a main river valley or a pre-existing body of water (lake or fiord). For a given lake volume, non-tunnel floods tend to have significantly higher peak discharges than tunnel-drainage floods. Statistical analysis of data for floods associated with subglacial tunnels yields the following empirical relation between lake volume V and peak discharge Qp : Qp = 46V0.66 (r2 = 0.70), when Qp is expressed in metres per second and V in millions of cubic metres. This updates the so-called Clague–Mathews relation. For non-tunnel floods, the analogous relation is Qp = 1100V0.44 (r2 = 0.58). The latter relation is close to one found by Costa (1988) for failure of constructed earthen dams. This closeness is probably not coincidental but rather reflects similarities in modes of dam failure and lake drainage. We develop a simple physical model of the breach-widening process for non-tunnel floods, assuming that (1) the rate of breach widening is controlled by melting of the ice, (2) outflow from the lake is regulated by the hydraulic condition of critical flow where water enters the breach, and (3) the effect of lake temperature may be dealt with as done by Clarke (1982). Calculations based on the model simulate quite well outbursts from Lake George, Alaska. Dimensional analysis leads to two approximations of the form Qp ∝ Vqf(hi, θ0), where q = 0.5 to 0.6, hi is initial lake depth, θ0 is lake temperature, and the form of f (hi, θ0) depends on the relative importance of viscous dissipation and the lake's thermal energy in determining the rate of breach opening. These expressions, along with the regression relations, should prove useful for assessing the probable magnitude of breach-type outburst floods. 相似文献
15.
The compression wavefield is efficiently converted to shear-wave energy at post-critical angles in areas of high impedance contrast at the sea floor. We have analysed mode-converted shear waves in a data set acquired with a hybrid marine/land geometry in Isfjorden, Svalbard. Through a kinematic 2D ray-tracing modellingV
p/Vs ratios for part of the uppermost 5km of the crust are obtained. Low values (V
p
/V
s
=1.65) are tentatively associated with the section of Devonian sandstones which appears to attain a minimum thickness of 1.5km below 3 km depth about 10km west of Kapp Thorden. 相似文献
16.
A series of time‐lapse seismic cross‐well and single‐well experiments were conducted in a diatomite reservoir to monitor the injection of CO2 into a hydrofracture zone, based on P‐ and S‐wave data. A high‐frequency piezo‐electric P‐wave source and an orbital‐vibrator S‐wave source were used to generate waves that were recorded by hydrophones as well as 3‐component geophones. During the first phase the set of seismic experiments was conducted after the injection of water into the hydrofractured zone. The set of seismic experiments was repeated after a time period of seven months during which CO2 was injected into the hydrofractured zone. The questions to be answered ranged from the detectability of the geological structure in the diatomic reservoir to the detectability of CO2 within the hydrofracture. Furthermore, it was intended to determine which experiment (cross‐well or single‐well) is best suited to resolve these features. During the pre‐injection experiment, the P‐wave velocities exhibited relatively low values between 1700 and 1900 m/s, which decreased to 1600–1800 m/s during the post‐injection phase (?5%). The analysis of the pre‐injection S‐wave data revealed slow S‐wave velocities between 600 and 800 m/s, while the post‐injection data revealed velocities between 500 and 700 m/s (?6%). These velocity estimates produced high Poisson's ratios between 0.36 and 0.46 for this highly porous (～50%) material. Differencing post‐ and pre‐injection data revealed an increase in Poisson's ratio of up to 5%. Both velocity and Poisson's ratio estimates indicate the dissolution of CO2 in the liquid phase of the reservoir accompanied by an increase in pore pressure. The single‐well data supported the findings of the cross‐well experiments. P‐ and S‐wave velocities as well as Poisson's ratios were comparable to the estimates of the cross‐well data. The cross‐well experiment did not detect the presence of the hydrofracture but appeared to be sensitive to overall changes in the reservoir and possibly the presence of a fault. In contrast, the single‐well reflection data revealed an arrival that could indicate the presence of the hydrofracture between the source and receiver wells, while it did not detect the presence of the fault, possibly due to out‐of‐plane reflections. 相似文献
17.
P. Digranes R. Mjelde S. Kodaira H. Shimamura T. Kanazawa H. Shiobara E. W. Berg 《Pure and Applied Geophysics》1996,147(4):611-629
Three component recordings from an array of five ocean bottom seismographs in the northwestern part of the Vøring basin have been used to obtain a 2-D shear-wave (S-wave) velocity-depth model. The shear waves are identified by means of travel-time differences compared to the compressional (P) waves, and by analyzing their particle motions. The model has been obtained by kinematic (travel-time) ray-tracing modelling of the OBS horizontal components.The shear-wave modelling indicates that mode conversions occur at several high velocity interfaces (sills) in the 4–10 km depth range, previously defined by a compressional-wave velocity-depth model using the same data set.An averageV
p
/V
s
ratio of 2.1 is inferred for the layers above the uppermost sill, indicative of both poorly consolidated sediments and a low sand/shale ratio. A significant decrease in theV
p
/V
s
ratio (1.7) below the first sill may in part be atributed to well consolidated sediments, and to a change in lithology to more sandy sediments. This layer is interpreted to lie within the lower Cretaceous sequence. At 5–10 km depthV
p
/V
s
ratios of 1.85 indicate a lower sand/shale ratio consistent with the expected lithologies. The averageV
p
/V
s
ratio inferred for the crust is 1.75, which is consistent with values obtained north of Vøring, in the Lofoten area. An eastward thinning of the crystalline basement is supported by the shear-wave modelling. 相似文献
18.
《Physics of the Earth and Planetary Interiors》1987,46(4):357-368
Poisson's ratio of the lower mantle, calculated from recently published values of seismic wave velocities and extrapolated to atmospheric pressure and room temperature is found to be in the range 0.23 ⩽ ν ⩽ 0.25. These values are compared with the values of Poisson's ratio calculated for binary mixtures of MgSiO3 perovskite and magnesiowüstite with various iron contents. Current values of the experimental error on measured elastic moduli give little hope to be able to discriminate between pyrolite and chondritic lower mantles: both are acceptable if the shear modulus of perovskite is in the upper range of Liebermann et al. estimates. A similar calculation using the seismic parameter φ confirms the results obtained by considering Poisson's ratio and further constrains the value of the shear modulus of perovskite to lie between 1600 and 1700 kilobars for current mantle models to remain plausible. Chemical stratification of the mantle is, therefore, possible but not required by seismological data. 相似文献
19.
Although previous seismic monitoring studies have revealed several relationships between seismic responses and changes in reservoir rock properties, the quantitative evaluation of time‐lapse seismic data remains a challenge. In most cases of time‐lapse seismic analysis, fluid and/or pressure changes are detected qualitatively by changes in amplitude strength, traveltime and/or Poisson's ratio. We present the steps for time‐lapse seismic analysis, considering the pressure effect and the saturation scale of fluids. We then demonstrate a deterministic workflow for computing the fluid saturation in a reservoir in order to evaluate time‐lapse seismic data. In this approach, we derive the physical properties of the water‐saturated sandstone reservoir, based on the following inputs: VP, VS, ρ and the shale volume from seismic analysis, the average properties of sand grains, and formation‐water properties. Next, by comparing the in‐situ fluid‐saturated properties with the 100% formation‐water‐saturated reservoir properties, we determine the bulk modulus and density of the in‐situ fluid. Solving three simultaneous equations (relating the saturations of water, oil and gas in terms of the bulk modulus, density and the total saturation), we compute the saturation of each fluid. We use a real time‐lapse seismic data set from an oilfield in the North Sea for a case study. 相似文献
20.
The relationship between Vp and Vs may be used to predict Vs where only Vp is known. Vp/Vs is also used to identify pore fluids from seismic data and amplitude variation with offset analysis. Theoretical, physical, as well as statistical empirical Vp‐Vs relationships have been proposed for reservoir characterization when shear‐wave data are not available. In published work, the focus is primarily on the Vp‐Vs relationship of quartzitic sandstone. In order to broaden the picture we present Vp‐Vs relationships of greensand composed of quartz and glauconite by using data from the Paleocene greensand Nini oil field in the North Sea. A Vp‐Vs relationship derived from modelling is compared with empirical Vp‐Vs regressions from laboratory data as well as from log data. The accuracy of Vs prediction is quantified in terms of root‐mean‐square error. We find that the Vp‐Vs relationship derived from modelling works well for greensand shear‐wave velocity prediction. We model the seismic response of glauconitic greensand by using laboratory data from the Nini field. Our studies here reveal that brine‐saturated glauconitic greensand can have a similar seismic response to that from oil‐saturated quartzitic sandstone and that oil‐saturated strongly cemented greensand can have a similar amplitude variation with offset response to that from brine‐saturated weakly cemented greensand. 相似文献