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1.
A combined reflection/refraction (wide-angle) seismic survey was conducted on the continental shelf north-west of Britain, using a conventional streamer with an airgun source, and static ocean-bottom seismometers (OBS) to record wide-angle energy. The shallow structure down to a basaltic layer was reasonably well imaged on the stacked reflection section. The basalts, however, proved to be opaque to the conventional reflection method and prevented the imaging of deeper horizons, where an important velocity inversion was anticipated. This paper reports on the processing, modelling and interpretation of the densely sampled wide-angle OBS data that were coincident with the reflection profile. Eleven OBS instruments were deployed along a 75 km line and recorded signal from a powerful 149 litre (9100 in.3) airgun array fired every 50 m. Data processing was performed using a standard industrial reflection seismic software package prior to first-arrival picking. Processing steps included geometry definition, trace summation and display of the data using various scaling algorithms. An initial model was constructed from 1D velocity-time profiles digitized every 4 km along the stacked section. First arrival traveltime modelling rapidly converged to a detailed model of the structure of the top 5 km of the crust. Modelling revealed the existence of a buried low-velocity Mesozoic sedimentary basin, of a prominent basement horst and of a normal fault penetrating to the basement. 相似文献
2.
VSP资料上下行波场发育丰富.本文在分析VSP直达波、上行反射波、下行反射波传播路径及其照明范围的基础上,指出了常规VSP波动方程偏移方法缺陷,进而通过修改波场延拓方式,提出了上下行反射波联合成像方法,并在高频近似下分析了该方法的成像原理.该方法不需要进行VSP上下行反射波场分离,能够同时对VSP资料中的一次反射波、自由表面多次波、层间多次波进行成像,比常规成像剖面具有更宽的成像范围和更好的成像效果.该方法能够对下行一次反射波进行成像,从而可以实现常规偏移方法难以处理的高陡倾角构造成像.模拟资料和实际资料处理证明了本文方法的正确性. 相似文献
3.
地球物理技术在天然气水合物预测中的应用 总被引：1，自引：0，他引：1
Based on the sensitivity of geophysical response to gas hydrates contained in sediments, we studied the prediction of gas hydrates with seismic techniques, including seismic attributes analysis, AVO, inverted velocity field construction for dipping formations, and pseudo-well constrained impedance inversion. We used an optimal integration of geophysical techniques results in a set of reliable and effective workflows to predict gas hydrates. The results show that the integrated analysis of the combination of reflectivity amplitude, instantaneous phase, interval velocity, relative impedance, absolute impedance, and AVO intercept is a valid combination of techniques for identifying the BSR （Bottom Simulated Reflector） from the lower boundary of the gas hydrates. Integration of seismic sections, relative and absolute impedance sections, and interval velocity sections can improve the validity of gas hydrates determination. The combination of instantaneous frequency, energy half attenuation time, interval velocity, AVO intercept, AVO product, and AVO fluid factor accurately locates the escaped gas beneath the BSR. With these conclusions, the combined techniques have been used to successfully predict the gas hydrates in the Dongsha Sea area. 相似文献
4.
根据Ecker的水合物沉积物的三种微观模式,计算含水合物沉积层和含游离气沉积物的弹性模量,分析对比了水合物的不同微观模式、不同水合物饱和度以及不同游离气饱和度对沉积物弹性模量的影响;从纵横波分离的弹性波动方程出发,采用交错网格空间有限差分方法模拟地震波在海底天然气水合物沉积地层的传播,得到纵、横波的海底地震（OBS）共接收点道集。数值算例表明,当水合物作为流体的一部分或胶结颗粒骨架时,仅纵波记录上存在BSR;当水合物胶结颗粒接触,纵、横波记录上均存在BSR。并且,OBS会接收到上行纵波和上行横波在海底界面形成的转换波,干扰横波记录上BSR的识别。 相似文献
5.
2016年6月在南黄海海域实施了海底地震仪(OBS)的二维深地震探测.本文详细分析了在该次地震探测中获得的浅水水域OBS数据的特征,提出了噪声的组合压制方法.研究表明,浅海水域的OBS数据在系统时间、能量及子波等方面存在明显差异,海底多次波干扰严重、有效频段中陷波问题突出,原始台站记录信噪比低、品质差、大炮检距的有效震相难以识别和拾取.本文提出的噪声组合压制处理技术与流程,主要由基于统计子波反褶积的子波整形、基于多项式插值的t-x域线性噪声压制和采用自动搜索的海底多次波压制等三部分组成.净化处理之后,反射/折射震相的波组特征清晰,信噪比得到有效改善与较大提高,可识别震相的范围较常规处理平均扩大60%以上.本文完善了浅水区OBS数据处理的步骤与流程,将为后续地壳结构研究提供可靠的基础数据,可更好的服务于地壳深地震及油气资源的探测. 相似文献
6.
到目前为止,已经探明的天然气水合物储量要远小于预测的水合物储量.这种现状不同程度地反映出我们对天然气水合物赋存规律认识不足,和对天然气水合物勘探方法认识不足.本文根据作者多次参加水合物地球物理调查国际航次的认识及文献资料,综述了海域天然气水合物勘探方面一些有效的地球物理技术方法,以利于我国海域天然气水合物的勘探工作以及勘探方法的创新.本文指出,地震勘探是目前进行天然气水合物勘探最常用、也是最重要的方法.地震方法主要包括传统的单道、多道地震方法、高分辨地震方法、深拖多道地震探测方法、海底地震仪方法、多道-多分量海底地震电缆方法、海底地震检波器方法等.此外,根据水合物发育区特有的海底地形地貌特征和水体异常特征,根据水合物发育所需要的温度-压力场特征、电磁特征和含水合物地层的剪切模量特征发展的多波束方法、旁扫声纳方法、海底热流探测方法、海底电磁方法以及海底重力测量方法等都在海域天然气水合物勘探中有着很好的效果. 相似文献
7.
结合汶川地震断裂带动态监测,利用快速响应探测系统,开展了断层带浅部结构人工地震探测.针对地震断裂带动态监测条件下的复杂波场和低信噪比的情况,在f-k波场分离的基础上,分别利用了折射波共中心点成像、面波速度反演、反射波叠加成像方法,进行了浅层断层和构造成像处理,并对处理结果进行了综合解释,给出了断裂带浅部断层分布和速度特征.为汶川地震龙门山断裂带东北端动态监测提供了基础结构信息,所发展的断裂带快速响应探测技术对于地震应急动态监测具有重要意义. 相似文献
8.
叠前深度偏移是复杂构造成像的重要手段.本文基于波场分裂理论,首先给出了波场延拓的一般方程,即一个上下行波的耦合方程组.通常所用的波场延拓方程就是该耦合方程组的特例.根据平方根算子的近似,推导了一种新的高精度混合法偏移方法,用分裂法即可进行计算.通过对一个横向强烈变速模型的叠后偏移及Marmousi复杂构造模型的叠前偏移计算,说明了方法的有效性,精度较高.采用MPI并行编程实现并行计算,提高了计算效率.该方法可用于对横向强烈变速的复杂构造的精确成像. 相似文献
9.
R. Pillet D. Rouland G. Roult D. A. Wiens 《Physics of the Earth and Planetary Interiors》1999,110(3-4):211-234
Direct earthquake-to-station Rayleigh and Love wave data observed on high gain broadband records are analyzed in order to improve the lateral resolution of the uppermost mantle in the southwest Pacific region. We used data of nine permanent Geoscope and Iris stations located in the southern hemisphere and nine other stations as part of two temporary networks, the first one installed in New Caledonia and Vanuatu (hereafter named Cavascope network) by ORSTOM and the EOST from Louis Pasteur University in Strasbourg (France) and the second one installed in the Fiji, Tonga and Niue islands (hereafter named Spase network) by Washington University in St. Louis (USA). In order to collect more significant details on the surficial structures, we included the analysis of short period waves down to 8 s. A multiple frequency filtering technique has been used to recover phase velocities of Rayleigh and Love waves for selected earthquakes with magnitude greater than 5.5 and with known centroid moment tensor (CMT). About 1100 well-distributed seismograms have been processed in the period range 8–100 s and corrections for topography and water depth have been applied to the observed phase velocities. The geographical distribution of phase velocity anomalies have then been computed using the tomographic method developed by Montagner [Montagner, J.P., 1986a. Regional three-dimensional structures using long-period surface waves. Ann. Geophys. 4 (B3), 283–294]. Due to a poor knowledge of dense, well-distributed, crustal thickness values and corresponding velocity models, we did not perform or speculate on the construction of an S-wave 3D velocity model; therefore, we limited this study to the interpretation of the phase velocity distribution. The location of phase velocity anomalies are well determined and the deviations are discussed within the framework of the geological context and compared with other tomographic models. At long periods, from 40 s to 100 s, our results agree well with most of previous studies: the tomographic imaging shows a large contrast between low and high phase velocities along the Solomon, New Hebrides and Fiji–Tonga trenches. The lowest phase velocity anomalies are distributed beneath northern and southern Fiji basins and the Lau basin (corresponding to the volume situated just above the dipping slabs), whereas the highest values are displayed beneath the Pacific plate and the eastern part of Indian plate downgoing under the North Fiji basin. At shorter periods, our results show that the phase velocity distributions are well correlated with the large structural crustal domains. The use of local temporary broadband stations in the central part of the studied area gives us the opportunity to observe surface waves showing well-dispersed trains, allowing extended velocity measurements down to 8 s although aliasing due to multipaths become important. The continental regions (Eastern Australia, New Guinea, Fiji islands and New Zealand) show low velocities which are likely due to thick continental crust, whereas the Tasmanian, D'Entrecasteaux, and the Northern and Southern Fiji basins are characterized by higher velocities suggesting thinner oceanic crust. Additional analysis including the anisotropic case and S-wave velocity inversion with depth is in progress. 相似文献
10.
南海含天然气水合物地层地震反射特征及可能分布研究 总被引：4，自引：0，他引：4
天然气水合物是当前能源与环境领域的研究热点之一,南中国海是天然气水合物赋存的有利区域。天然气水合物的存在改变了沉积地层的声学特征,这一性质使多道地震勘探成为发现海洋天然气水合物的主要手段。本文首先根据地震成像结果定性分析南海可能含天然气水合物沉积地层的地震反射特征,初步确定天然气水合物存在的可能性并指出地震成像关键技术。在无井条件下,构建虚拟井进行波阻抗反演得到定量的地层速度参数进一步证实这一可能性,最后将反演获得的速度场与ODP184 航次在此区域获得的地球物理、地球化学信息综合分析,可以确定此区域天然气水合物的存在及其空间展布。 相似文献
11.
J. L. MARI 《Geophysical Prospecting》1989,37(3):257-277
Seismic attenuation introduces modifications in the wavelet shape in vertical seismic profiles. These modifications can be quantified by measuring particular signal attributes such as rise-time, period and shape index. Use of signal attributes leads to estimations of a seismic-attenuation log (Q-log). To obtain accurate signal attributes it is important to minimize noise influence and eliminate local interference between upgoing and downgoing waves at each probe location. When tube waves are present it is necessary to eliminate them before performing separation of upgoing and downgoing events. We used a trace-by-trace Wiener filter to minimize the influence of tube waves. The separation of upgoing and downgoing waves was then performed in the frequency domain using a trace-pair filter. We used three possible methods based on signal attribute measurements to obtain g-log from the extracted downgoing wavefield. The first one uses a minimum phasing filter and the arrival time of the first extremum. The two other methods determine the Q-factor from simple relations between the amplitudes of the first extrema and the pseudo-periods of the down-going wavelet. The relations determined between a signal attribute and traveltime over quality factor were then calibrated using field source signature and constant-Q models computed by Ganley's method. Q-logs thus obtained from real data are discussed and compared with geological information, specifically at reservoir level. Analysis of the tube wave arrivals at the level of the reservoir showed a tube wave attenuation that could not be explained by simple transmission effects. There was also a loss of signal coherence. This could be interpreted as tube wave diffusion in the porous reservoir, followed by dispersion. If this interpretation can be verified, tube wave analysis could lead to further characterization of porous permeable zones. 相似文献
12.
The source mechanism of a large (Ms ? 7.2) earthquake that occurred in the oceanic plate at the junction of the Tonga—Kermadec trench systems with the aseismic Louisville ridge is found by inverting long-period vertical-component Rayleigh waves recorded by the IDA network. The solution is an almost-pure normal fault, on a plane striking roughly parallel to the trench axis, with seismic moment of 1.7 × 1027 dyn cm, and thus is among the ten largest documented shallow normal-fault earthquakes. A point-source depth of 20 km for the event is resolved by modeling teleseismic body waves; the actual rupture may have extended deeper, to 30 or 40 km. The earthquake was a multiple event, consisting of two sources separated by 16 s. A rupture velocity of 3.5 km s?1 is inferred. The earthquake can be interpreted as tensional failure in the shallow portion of the downgoing plate caused by the gravitational pull of the slab. The Louisville ridge may be creating a local degree of decoupling of the oceanic plate from the overriding plate, and/or a zone of extension within the slab, which could enhance the effect of the gravitational forces in the shallower part of the downgoing plate. In particular, the earthquake could be associated with the break-up of the leading seamount of the ridge, which is currently right at the trench. Alternatively, the earthquake may have been caused by stresses associated with the bending of the plate prior to subduction. 相似文献
13.
Sensitivity analysis and application of time‐lapse full‐waveform inversion: synthetic testing and field data example from the North Sea,Norway 
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下载免费PDF全文 Time‐lapse refraction can provide complementary seismic solutions for monitoring subtle subsurface changes that are challenging for conventional P‐wave reflection methods. The utilization of refraction time lapse has lagged behind in the past partly due to the lack of robust techniques that allow extracting easy‐to‐interpret reservoir information. However, with the recent emergence of the full‐waveform inversion technique as a more standard tool, we find it to be a promising platform for incorporating head waves and diving waves into the time‐lapse framework. Here we investigate the sensitivity of 2D acoustic, time‐domain, full‐waveform inversion for monitoring a shallow, weak velocity change (?30 m/s, or ?1.6%). The sensitivity tests are designed to address questions related to the feasibility and accuracy of full‐waveform inversion results for monitoring the field case of an underground gas blowout that occurred in the North Sea. The blowout caused the gas to migrate both vertically and horizontally into several shallow sand layers. Some of the shallow gas anomalies were not clearly detected by conventional 4D reflection methods (i.e., time shifts and amplitude difference) due to low 4D signal‐to‐noise ratio and weak velocity change. On the other hand, full‐waveform inversion sensitivity analysis showed that it is possible to detect the weak velocity change with the non‐optimal seismic input. Detectability was qualitative with variable degrees of accuracy depending on different inversion parameters. We inverted, the real 2D seismic data from the North Sea with a greater emphasis on refracted and diving waves’ energy (i.e., most of the reflected energy was removed for the shallow zone of interest after removing traces with offset less than 300 m). The full‐waveform inversion results provided more superior detectability compared with the conventional 4D stacked reflection difference method for a weak shallow gas anomaly (320 m deep). 相似文献
14.
近年来,背景噪声成像方法在恢复高频面波信号及获取近地表速度结构方面得到了广泛的应用,本文将该方法应用于准噶尔盆地南缘的呼图壁背斜地区的呼图壁储气库.采用储气库及其周边区域22个台站记录的连续背景噪声数据的垂直分量,通过噪声互相关方法获得了台站对之间的瑞利面波经验格林函数,并进一步提取了0.5~1.5 s的基阶瑞利面波群速度频散曲线.首先根据区域平均频散曲线得到了该地区地下数百米的平均一维横波速度结构,然后利用基于面波射线路径追踪的面波频散直接成像方法得到该地区深度为500 m以上的三维横波速度结构.反演结果显示该地区沉积层较厚,整体横波速度值较小（0.4~0.9 km·s-1）.储气库在地表投影区域的横波速度值较小,这可能是由于抽注水、气引起的沉积岩石裂隙所导致.储气库东北和东南方向均有明显的相对高速区,推测是区域地下水位和地形起伏综合作用的结果.本研究获得的近地表三维速度结构为呼图壁储气库地区的上覆地层物性研究、区域微震精定位、场地效应的评估和去除浅层影响的深部介质成像等研究提供了重要基础. 相似文献
15.
Data from a recently acquired sea-bed logging deep-water survey are analysed for resistive bodies at depths below mudline shallower than about 300 m. A model consistent with known methane hydrate properties is found to explain near-offset structures over an offset scale of a few hundred metres observed in the data. The lateral near-seabed resolution of the sea-bed logging method was determined to less than 100 m for source frequencies of up to 10 Hz. The importance of accurate hydrate maps to improve data processing is demonstrated by placing synthetic reservoirs below hydrates and observing their effects on reference model processing. The phase is shown to be less perturbed by shallow resistors than the amplitude, which is an important quality control of standard anomaly maps. While patchy shallow resistors can generally be mapped with simple normalized magnitude-versus-offset and phase-versus-offset difference analyses, large area distributions of hydrates over kilometres are hard to distinguish from deeper structures using controlled-source electromagnetic data only, short of conducting a full 3D inversion of a sufficiently large survey. Beyond, the study confirms the applicability of controlled-source electromagnetic techniques in general to map shallow resistive structures for drilling hazards and possible future exploration of methane hydrates as an energy source. 相似文献
16.
本文采用反射映像（即U/D成像）原理对大地电磁资料进行成像,将地面观测的波场分解成上行波和下行波,并分别向下延拓,利用上、下行波的时间一致性原理确定地下电性界面的位置.对TE和TM模式的MT响应分别处理可以得到两个深度剖面,两剖面的一致性便为实际反射界面的最佳估计.理论和实际资料的计算表明,MT偏移技术是比较有效的,它可获得地下界面直观的映像.和常规MT反演方法相比较,该方法具有算法简单等优点,并且还能获得表示地下地质构造真实映像的MT深度剖面. 相似文献
17.
南海琼东南盆地是天然气水合物重要远景区之一.由于盆地大部分地区海底地形平缓、地层近于水平,增加了利用地震反射剖面识别似海底反射（BSR,bottom simulating reflector）的难度,从而影响了对水合物的评价.为了进一步开展琼东南盆地水合物调查研究,本文在研究海域进行了海洋可控源电磁探测试验,将自主研发的10台接收机以500 m的间距,投放至水深约为1360 m的海底,完成了一条4.5 km剖面的电磁数据采集.通过对采集的数据进行处理与二维（2D）反演,获得了研究剖面海底的电阻率断面图.反演结果显示,研究区海底60~330 mbsf（meter bottom of seafloor）的地层中,存在多个横向不连续分布的高阻异常体,电阻率介于2~10 Ωm之间;在海底330 mbsf之下,横向上发育了电阻率为2~4 Ωm的3个高阻体.根据研究区热力学条件,本文估算了生物成因气与热成因天然气的水合物稳定带（GHSZ,gas hydrate stability zone）厚度,结合高阻体的分布特征推断了地震剖面上BSR的位置.在此基础上,对反演的电阻率断面进行解释,推断了研究区水合物的分布及游离气运移通道.研究表明,勘探区具有形成天然气水合物矿藏的地质与地球物理条件,其成藏模式可能属于"断层、裂隙输导的下生上储型",水合物的气源为生物成因气. 相似文献
18.
Improved estimates of the amount of subsurface gas hydrates are needed for natural resource, geohazard, and climate impact assessments. To evaluate gas hydrate saturation from seismic methods, the properties of pure gas hydrates need to be known. Whereas the properties of sediments, specifically sands, and hydrate‐bearing sediments are well studied, the properties of pure hydrates are largely unknown. Hence, we present laboratory ultrasonic P‐wave velocity and attenuation measurements on pure tetrahydrofuran hydrates as they form with reducing temperatures from 25°C to 1°C under atmospheric pressure conditions. Tetrahydrofuran hydrates, with structure II symmetry, are considered as proxies for the structure I methane hydrates because both have similar effects on elastic properties of hydrate‐bearing sediments. We find that although velocity increased, the waveform frequency content and amplitude decreased after the hydrate formation reaction was complete, indicating an increase in P‐wave attenuation after hydrate formation. When the tetrahydrofuran hydrate was cooled below the freezing point of water, velocity and quality factor increased. Nuclear Magnetic Resonance results indicate the presence of water in the “pure hydrate” samples above the water freezing point, but none below. The presence of liquid water between hydrate grains most likely causes heightened attenuation in tetrahydrofuran hydrates above the freezing point of water. In naturally occurring hydrates, a similarly high attenuation might relate to the presence of water. 相似文献
19.
The January 18, 2010, shallow earthquake in the Corinth Gulf, Greece (M w 5.3) generated unusually strong long-period waves (periods 4–8 s) between the P and S wave arrival. These periods, being significantly longer than the source duration, indicated a structural effect. The waves were observed in epicentral distances 40–250 km and were significant on radial and vertical component. None of existing velocity models of the studied region provided explanation of the waves. By inverting complete waveforms, we obtained an 1-D crustal model explaining the observation. The most significant feature of the best-fitting model (as well as the whole suite of models almost equally well fitting the waveforms) is a strong velocity step at depth about 4 km. In the obtained velocity model, the fast long-period wave was modeled by modal summation and identified as a superposition of several leaking modes. In this sense, the wave is qualitatively similar to P long or Pnl waves, which however are usually reported in larger epicentral distances. The main innovation of this paper is emphasis to smaller epicentral distances. We studied properties of the wave using synthetic seismograms. The wave has a normal dispersion. Azimuthal and distance dependence of the wave partially explains its presence at 46 stations of 70 examined. Depth dependence shows that the studied earthquake was very efficient in the excitation of these waves just due to its shallow centroid depth (4.5 km). 相似文献
20.
基于高频震源的海洋短排列、高分辨率多道地震探测方法,具有主频高、频带宽的特点,是海底浅层地质研究,特别是海洋天然气水合物调查等方面的重要手段.针对这种地震探测方法不能有效控制电缆沉放深度、虚反射干扰严重且规律性差以及速度分析精度低带来的成像效果差等问题,研究了基于虚反射走时识别与模拟计算的电缆沉放深度计算方法,获得了检波点的准确沉放深度,为虚反射的有效压制提供了保障;采用基于相似性原理的排列长度放大技术,有效地解决了排列长度较短引起的速度谱能量团不聚焦的问题,提高了速度分析灵敏度,确保了地震资料成像处理精度.实际资料的最终成像处理结果表明,地震波组的信噪比和分辨率高,速度结构合理,BSR特征清晰,为海洋天然气水合物的识别和研究提供了高品质资料. 相似文献