地震层析成像方法及其在岩体结构研究中的应用

根据岩体结构理论 ,软弱结构面对岩体变形与破坏具有优先控制作用 ,它与围岩在地震波传播性质 (波速与衰减特性 )上的差别 ,可以利用地震层析成像技术 ,对它进行空间定位 ,进而研究岩体的内部结构特征。针对岩体结构研究的特点 ,本文提出将地震波走时成像方法与地震波衰减成像方法 (振幅与脉冲宽度 )集成到一个处理流程中 ,实现了岩体结构的综合成像方法。     

Crosshole seismic tomography and borehole logging for engineering site characterization in Sikeston, MO, USA

This paper presents a geophysical investigation performed in the median separating the east and west bound lanes of U.S. Highway 60, approximately 15 km west from the town of Sikeston, MO, USA. Two boreholes, drilled at depth of 45 m, approximately located 6 m from the Wahite Ditch Bridge in U.S. Highway 60, were used to carry out the geophysical tests. The objective of the survey was to obtain the density distribution of soil materials and high-resolution compressional and shear wave velocities of the shallow subsurface for computation of elastic engineering properties of the unconsolidated material interposed between the two investigated boreholes.The studied site is located in the New Madrid Seismic Zone (NMSZ), one of the major seismic source zones in the eastern United States. Although no major seismic events have occurred in the New Madrid area since the catastrophic earthquakes of 1811–1812, more intensive studies in this area are required to better understand the local soil effects and the liquefaction potential of unconsolidated earth materials on strong ground motion.Joint interpretation of borehole logs and velocity images obtained by P- and S-wave traveltime inversions outlined shallow anomalies, which were interpreted as caused by variation in relative porosity and compactness of saturated unconsolidated soil materials. In the interval between 17 and 19 m of depth, a velocity anomaly with distinct characteristics is recognized. Analysis of traveltimes of P-wave energy propagated from common source-receiver depth positions at that depth interval delineated a low velocity zone (lower than estimated P-wave velocity in water) in which propagated S-wave energy was negligible. Analysis of porosity logs did not outline any porosity anomaly for that region. The anomaly is interpreted as caused by the presence of a gas-bearing zone extending for 1.5 m (+0.76 m from the top and the bottom of the depth interval), through which propagation of shear energy was almost prevented and compressional energy traveled a very low speed. Relative lower values of shear modulus were computed for the gas-bearing zone; the same depth interval also exhibited relative lower values of bulk modulus, relative higher values of compressibility and relative high porosity. However, the low velocity anomaly is not caused by relative increased porosity but rather to replacement of water by gas in the pore volumes of this zone.The presence of a shallow thin gas-bearing zone is interpreted as hazardous in the occurrence of strong motion in the New Madrid Seismic Zone. Shallow water table, as detected by seismic and log measurements in the area, increases the risk of liquefaction of the sampled soil under strong ground motions.     

Seismic imaging of lithospheric discontinuities and continental evolution

Discontinuities in physical properties within the continental lithosphere reflect a range of processes that have contributed to craton stabilization and evolution. A survey of recent seismological studies concerning lithospheric discontinuities is made in an attempt to document their essential characteristics. Results from long-period seismology are inconsistent with the presence of continuous, laterally invariant, isotropic boundaries within the upper mantle at the global scale. At regional scales, two well-defined interfaces termed H (60 km depth) and L (200 km depth) of continental affinity are identified, with the latter boundary generally exhibiting an anisotropic character. Long-range refraction profiles are frequently characterized by subcontinental mantle that exhibits a complex stratification within the top 200 km. The shallow layering of this package can behave as an imperfect waveguide giving rise to the so-called teleseismic Pn phase, while the L-discontinuity may define its lower base as the culmination of a low velocity zone. High-resolution, seismic reflection profiling provides sufficient detail in a number of cases to document the merging of mantle interfaces into lower continental crust below former collisional sutures and magmatic arcs, thus unambiguously identifying some lithospheric discontinuities with thrust faults and subducted oceanic lithosphere. Collectively, these and other seismic observations point to a continental lithosphere whose internal structure is dominated by a laterally variable, subhorizontal layering. This stratigraphy appears to be more pronounced at shallower lithospheric levels, includes dense, anisotropic layers of order 10 km in thickness, and exhibits horizontal correlation lengths comparable to the lateral dimensions of overlying crustal blocks. A model of craton evolution which relies on shallow subduction as a principal agent of craton stabilization is shown to be broadly compatible with these characteristics.     

Seismic hazard assessment under complex source size distribution of mining-induced seismicity

It is well-documented that a variety of factors controlling the rockmass fracturing process in mines often results in a complexity of mining event size distribution. In such cases, the estimation of the probability functions of source size parameterizations, with the use of presently known distribution models, brings about an unacceptable and systematic over- or underestimation of the seismic hazard parameters. It is, therefore, recommended that the non-parametric, kernel estimators of the event size distribution functions, be applied to stationary hazard studies in mining seismicity.These data-driven estimators, adapted to seismic source size characterization, accurately fit all kinds of data underlying distributions, regardless of their complexity. Recently, the non-parametric approach to size characterization was supported by a special method of uncertainty analysis based on resampling techniques. At present, it is a fully developed method, which provides point and interval estimates of size distribution functions and related hazard parameters. Two examples of its use in studying mining seismic data are presented and discussed in this paper. The analyzed data sets were recorded in two different copper mines in Poland. The smoothed bootstrap test for multimodality, which is a specialized tool for investigating the shapes of probability densities, provided highly significant proof that in both cases the probability densities of source size parameterization were complex thus implied the superiority of the non-parametric estimation to the classic, model-based approach in the studied cases. The data were then used to construct non-parametric, kernel estimates of the source size cumulative distribution function (CDF), the exceedance probability and the mean return period. Furthermore, confidence intervals for these quantities were also estimated. The intervals for CDF were narrow, showing that the procedures of non-parametric estimation and resampling based uncertainty analysis were precise. Due to the fact that the mean return period is very sensitive to values of the CDF, in particular for larger events sizes, the uncertainty of the return period estimates was not insignificant but remained manageable. The point and interval estimates of source size CDF and hazard parameters so obtained were compared with the respective point estimates achieved from the inappropriate in the case of complex magnitude distributions, model-based approach.     

工程地震折射波解释方法研究进展

地震折射波勘探是工程与环境地球物理中应用最为广泛的方法之一。作为一种简洁方便又经济实惠的勘探方法,它可为工程地质提供近地表地层起伏变化和速度横向变化以及潜水面变化资料等。随着工程地质勘查和城市地质勘探工作的发展,工程地震折射波法数据处理与解释变得尤为重要。本文主要介绍了浅层地震折射波法的发展历程、应用条件,并总结和讨论了几种主要折射波解释方法的原理。在此基础上,比较并讨论了各种方法在应用中的优势和不足,阐述了近年来国内外浅层地震折射波法的发展现状及趋势,并着重介绍了作为研究热点的折射层析成像方法。研究表明：当探测深度较浅且分界面足够平整、界面倾角较小时,最方便的解释方法是截距时间法;当勘探目标的深度达到25 m时,t₀差数法最为适用;勘探目标埋深超过25 m时,应当使用广义互换法。随着勘探精度要求越来越高,使用折射波走时层析成像技术可以满足纵横向速度变化的近地表地层,包括大倾角地层、隐伏层、界面起伏层等。     

电阻率层析成像法探测滑坡面正演模拟与反演成像研究

滑坡灾害已成为危及人民生命财产安全的重大问题,确定滑坡体和滑坡面的分布是滑坡治理的关键因素,电阻率层析成像法因对物性差异的界面响应敏感,被广泛应用到滑坡面的探测中。首先,对现有的滑坡地电模型进行概化,得到了4种常见的滑坡的简化模型;其次,针对不同类型的滑坡,采用有限元方法进行了系统的数值正演研究,得到了层析成像法探测滑坡面的响应特征,提出了不同类型滑坡面的快速识别方法。然后,采用最小二乘法开展了反演研究,总结了层析成像法探测滑坡面的反演成像规律,结合数值正演的响应特征,提出了层析成像法探测滑坡面的异常识别特征。最后,在山东沂源西里镇滑坡进行了层析成像法探测,成功探明了滑坡面的分布,表明了层析成像法探测滑坡面是行之有效的,为滑坡的治理方案设计奠定了基础     

Imaging low-velocity anomalies with the aid of seismic tomography

Theoretical considerations (Snell's law) suggest that low-velocity fanomalies are undersampled and therefore should be poorly resolved by inversion schemes based on ray-tracing methods. A synthetic study considering a 40×20 m low-velocity anomaly (300 m/s) placed at the center of a 400×50 m block with gradient background velocity model (from 3000 m/s at the surface to 4000 m/s at the base) indicates that the low ray density in ray-tracing coverage diagrams of tomographic inversion studies can be used as evidence for the existence of low-velocity anomalies. Combined normal incidence seismic reflection images and the velocity models obtained by tomographic inversions of first-arrival travel times form an efficient scheme to resolve low-velocity anomalies such as fracture zones. Furthermore, the velocity models derived from tomographic inversions are used in a wave equation datuming algorithm to account for statics caused by a strongly laterally variable shallow surface (weathering) layer and provide seismic reflection images of fracture zones (low-velocity anomaly) within a granitic pluton.     

Identification of karst features using seismic P-wave tomography and resistivity anisotropy measurements

A football stadium with a capacity of a hundred thousand spectators is under construction over a karst terrain, 10 km west of the old town of Istanbul, Turkey. A large cavity of approximately 30 m³ was detected beneath the sports field through a number of boreholes so that a geophysical survey was required to further investigate a portion of the sports field. We utilized seismic refraction tomography and dc-electrical method with rotated Wenner array to delineate zones with solution voids and cavities. Total core recovery (TCR) was 5–15% from boreholes where zones with low velocities were identified through tomographic inversion, whereas TCR values were above 60% in zones with higher velocities. Both low velocity zones in the tomographic images and increasing resistivity anisotropy with depth appear to indicate that the cavity extends toward the west and south at a depth of approximately 8 m, although the southward and westward extension changes in character.     

Seismic reflection imaging of thin, kimberlite dykes and sills: exploration and deposit characterization of the Snap Lake dyke, Canada

Seismic reflection techniques are, for the first time, used to image a thin, diamondiferous, kimberlite dyke from subcrop to depths greater than 1300 m. Exploration for vertical kimberlite pipes generally utilizes potential field techniques that often fail to reveal subhorizontal or shallow-dipping intrusions. In contrast, seismic reflection methods are well suited for imaging targets with this geometry. Therefore, in order to evaluate seismic reflection as a tool for subhorizontal kimberlite dyke/sill exploration and mine planning, a feasibility study and subsequent seismic survey were undertaken on the diamondiferous Snap Lake dyke (Northwest Territories, Canada). A substantial drilling program has mapped the dyke as a gently dipping sheet that averages 2–3 m in thickness. The detailed structural and composition data available at Snap Lake provides a unique opportunity to test reflection techniques on a well-sampled deposit. The feasibility study involved measuring P-velocities and densities of cores drilled from the kimberlite and host rocks. These data were used to model reflection amplitudes, evaluate resolution limitations, and determine the acquisition parameters for the reflection survey. Two 2-D lines were acquired that provide comparative datasets for different sources (explosive and vibroseis) and ground types (land and lake ice). In addition, the exploration-scale survey incorporated high fold (40–260 nominal) and long offsets (3260 m). The explosive-source profile recorded on land yielded a superb image of the dyke from depths of 60 m to more than 1300 m over a lateral distance of 5700 m. The seismic image correlates well with adjacent drill hole data and adds considerable detail to the topography of the kimberlite sheet determined by drilling. The vibroseis source also imaged the dyke, but only when sources and geophones were on land; the dyke was not imaged beneath the ice due to reverberation and attenuation effects. The frequency response and unusually strong reflection amplitudes from the dyke indicate the importance of tuning effects and multiples for this type of target and acquisition environment. Apparent correlations between reflection amplitudes and dyke structure (e.g., thickness, feathering, 3-D geometry) suggest that seismic reflection data may be valuable for guiding drilling programs. The results demonstrate that, in the appropriate situation, seismic methods have great potential for use in kimberlite exploration, subsurface mapping, and detailed imaging for mine development purposes.     

Surveying and modeling of rock discontinuities by terrestrial laser scanning and photogrammetry: Semi-automatic approaches for linear outcrop inspection

Rock face diagnosis is a monitoring operation that is used to optimize rock-risk treatment works in terms of ensuring that safety requirements are met at the lowest cost. Diagnoses require measuring the location and orientation of rock discontinuities at the surface of the rock mass. These measurements are then entered into a structural model that extrapolates the data collected at the surface to the inner part of the rock mass. Currently, most surveys are empirical and are carried out manually using a compass-clinometer. In addition, they tend to examine only the most highly fractured area of a rock face, even though safety considerations demand an exhaustive study of the whole face. These deficiencies can be overcome by using dense 3D measurement techniques such as terrestrial laser scanning and optical imaging to obtain a more complete 3D model and structural statement. Hence, we have developed a semi-automatic process that allows 3D models to be combined with the results of field surveys in order to provide more precise analyses of rock faces, for example, by classifying rock discontinuities into subsets according to their orientation. Further research is being carried out in order to combine 3D data and 2D digital images as a support for structural survey. Trials carried out in a limestone quarry in the French Alps allowed us to compare data sets obtained using manual surveying methods, the well-known laser scanning method and the lower-cost photogrammetric survey method.     

城市地下孤石的地震波场响应特征及其在地震层析成像中的应用研究

城市浅层地质条件复杂,地下孤石是常见的地质灾害体.为降低城市地下空间开发过程中孤石造成的安全隐患,本文通过构建城市地下孤石的数值模型,分析地震波在含孤石介质中的传播特征,根据地震波的传播特征利用层析成像技术对孤石进行速度成像,并通过工程实例进行验证.研究表明:(1)地震波场的数值模拟是研究地球物理勘探的有效方法,能高效...     

极震区岩体地震动力破坏研究体系框架浅析

极震区岩体地震动力破坏是造成极震区严重的地震地质灾害和次生灾害的根本原因。汶川地震中因此而导致的交通工程结构损坏、中断和次生崩滑流、堰塞湖等地质灾害,不仅加剧了灾害损失,并给震后的灾区恢复重建和对外交通遗留了较多的安全隐患。从岩体稳定、山体稳定和区域稳定的层次,分别对有关极震区岩体地震动力破坏的主要研究成果进行了系统总结,分析了其现状和存在的问题。结合对研究思路和方法论的讨论,构建了极震区岩体地震动力破坏研究的体系框架,提出了极震区岩体地震动力破坏研究的主要内容和方向,以期明晰概念,理清思路,为有关问题的深入研究提供有益的参考。     

极震区岩体地震动力破坏研究体系框架初探

极震区岩体地震动力破坏是造成极震区严重的地震地质灾害和次生灾害的根本原因。汶川地震中因此而导致的交通工程结构损坏、中断和次生崩滑流、堰塞湖等地质灾害,不仅加剧了灾害损失,并给震后的灾区恢复重建和对外交通遗留了较多的安全隐患。从岩体稳定、山体稳定和区域稳定的层次,分别对有关极震区岩体地震动力破坏的主要研究成果进行了系统总结,分析了其现状和存在的问题。结合对研究思路和方法论的讨论,构建了极震区岩体地震动力破坏研究的体系框架,提出了极震区岩体地震动力破坏研究的主要内容和方向,以期明晰概念,理清思路,为有关问题的深入研究提供有益的参考。     

Mantle dynamics of Western Pacific and East Asia: Insight from seismic tomography and mineral physics

Recent results of high-resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of Western Pacific and East Asia. The most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation (corner flow) processes in the mantle wedge. The combination of the two processes may have caused the back-arc spreading in the Lau basin, affected the morphology of the subducting Philippine Sea slab and its seismicity under southwest Japan, and contributed to the formation of the continental rift system and intraplate volcanism in Northeast Asia, which are clearly visible in our tomographic images. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent (a) small mantle plumes, (b) upwellings associated with the slab collapsing down to the lower mantle, or (c) sub-slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab. Combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite + water, we proposed a petrologic model for arc volcanism. Arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water-rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge. A large amount of fluids can be released from hydrated MORB at depths shallower than 55 km, which move upwards to hydrate the wedge corner under the fore-arc, and never drag down to the deeper mantle along the slab surface. Slab dehydration reactions at 120 km depth are the antigorite-related 5 reactions which supply water-rich fluids for forming the volcanic front. Phase A and Mg-surssasite breakdown reactions at 200 and 300 km depths below 700 °C cause the second and third arcs, respectively. Moreover, the dehydration reactions of super-hydrous phase B, phases D and E at 500–660 km depths cause the fluid transportation to the mantle boundary layer (MBL) (410–660 km depth). The stagnant slabs extend from Japan to Beijing, China for over 1000 km long, indicating that the arc–trench system covers the entire region from the Japan trench to East Asia. We propose a big mantle wedge (BMW) model herein, where hydrous plumes originating from 410 km depth cause a series of intra-continental hot regions. Fluids derived from MBL accumulated by the double-sided subduction zones, rather than the India–Asia collision and the subsequent indentation into Asia, are the major cause for the active tectonics and mantle dynamics in this broad region.     

Mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0) and tsunami: Insight from seismic tomography

To clarify the generating mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0) and the induced tsunami, we determined high-resolution tomographic images of the Northeast Japan forearc. Significant lateral variations of seismic velocity are visible in the megathrust zone, and most large interplate thrust earthquakes are found to occur in high-velocity (high-V) areas. These high-V zones may represent high-strength asperities at the plate interface where the subducting Pacific plate and the overriding Okhotsk plate are coupled strongly. A shallow high-V zone with large coseismic slip near the Japan Trench may account for the mainshock asperity of the 2011 Tohoku-oki earthquake. Because it is an isolated asperity surrounded by low-velocity patches, most stress on it was released in a short time and the plate interface became decoupled after the Mw 9.0 earthquake. Thus the overriding Okhotsk plate there was shot out toward the Japan Trench and caused the huge tsunami.     

Spatial predictions of geological rock mass properties based on in-situ interpretations of multi-dimensional seismic data

The article presents a statistical approach to characterize and predict engineering geological conditions in the up to 2000 m deep Faido tunnel and Gotthard base tunnel in Switzerland. Seismic investigations were conducted to improve the technology of interpreting seismic tomographic images. Overall, the goal of this study was to predict spacial maps of geological rock mass properties, such as, uniaxial compressive strength or total fracture spacing, by using up to six seismic features in combination, e.g., compression-wave and shear-wave velocities and dynamic Poisson's ratio. Self-Organizing Mapping (SOM), an artificial intelligent method, was used for the purposes of interpreting multi-dimensional geophysical attributes derived from seismic profiles of tomographic images along tunnel sidewalls. The SOM-method was applied in the Faido tunnel to delineate complex physical relations between the geological and seismic parameters. Then, the method was applied to predict geological properties around a segment of the Gotthard base tunnel with unknown geological–geotechnical conditions. The results illuminate that correlation analyses (pairwise parameter classification) are substantially less powerful than the SOM-method (multi-parameter classification) in order to interpret geological features from seismic in-situ data. Moreover, predicted spatial distributions of the total fracture spacing and the uniaxial compressive strength, for example, corresponded well with drill core and tunnel mapping results. The SOM-approach was a helpful tool for practitioners in predicting zones of instabilities and geological complexity during underground excavation processes of the Gotthard base tunnel. It is suggested to use such an interpretation method as decision support for purposes of sub/surface exploration and long-term geophysical monitoring of large-scale geoengineering projects, such as, disposals of nuclear waste and greenhouse gases or geopower plants for renewable energy (geothermal, biosoils).     

一种快速建立层析反演初始模型的方法

建立近地表层析反演模型时,通常利用射线追踪方法来计算模型初至旅行时,再与观测的初至实际旅行时进行比较,反复修改模型直到计算值与观测值的误差达到最小。因此,初始模型建立的质量直接影响到层析反演的精度、收敛速度和迭代次数。通过模拟近地表模型,利用时间项反演方法快速建立初始模型,提高了层析反演的精度,加快了收敛速度,减少了迭代次数。     

Tomographic imaging of hydrated crust and mantle in the subducting Pacific slab beneath Hokkaido, Japan: Evidence for dehydration embrittlement as a cause of intraslab earthquakes

We estimate detailed three-dimensional seismic velocity structures in the subducting Pacific slab beneath Hokkaido, Japan, using a large number of arrival-time data from 6902 local earthquakes. A remarkable low-velocity layer with a thickness of ~ 10 km is imaged at the uppermost part of the slab and is interpreted as hydrated oceanic crust. The layer gradually disappears at depths of 70–80 km, suggesting the breakdown of hydrous minerals there. We find prominent low-velocity anomalies along the lower plane of the double seismic zone and above the aftershock area of the 1993 Kushiro-oki earthquake (M7.8). Since seismic velocities of unmetamorphosed peridotite are much higher than the observations, hydrous minerals are expected to exist in the lower plane as well as the hypocentral area of the 1993 earthquake. On the other hand, regions between the upper and lower planes, where seismic activity is not so high compared to the both planes, show relatively high velocities comparable to those of unmetamorphosed peridotite. Our observations suggest that intermediate-depth earthquakes occur mainly in regions with hydrous minerals, which support dehydration embrittlement hypothesis as a cause of earthquake in the subducting slab.