Observations on the pseudostatic analysis of embankments and slopes with arbitrary seismic force inclination

A simple method for evaluating the seismic stability of embankments and slopes has been developed by modifying the generalized procedure of slices proposed by Janbu. A pseudostatic analysis is carried out. The seismic force is represented through its horizontal and vertical components, acting on the centre of gravity of the soil mass. Defining a nominal state of plastic equilibrium, the safety factor of the slope for a given value of the seismic force is obtained through an iterative procedure that converges very rapidly. Moreover, by assuming a seismic force of intensity varying from zero value (static condition) up to a limit value, depending on the seismicity of the area, one can obtain the “influence lines” of the horizontal and vertical acceleration. Finally, a directional parametric analysis has been carried out considering a seismic force of a given intensity, acting in a direction varying from 0° to 360° with respect to the vertical axis through the gravity centre of the slope. By means of this procedure it is possible to locate a “critical zone” for the slope, i.e., that one within which any seismic force can cause the failure of the slope. Moreover, it appears that it is incorrect to consider only the horizontal acceleration in the stability analyses of slopes under earthquake loading conditions, as the most dangerous direction is usually different from the horizontal one.     

大别山地震波速度剖面的重力拟合及花岗岩带

笔者对穿越大别山造山带的六安—大冶宽角反射地震剖面进行了重力拟合。拟合结果表明严格按宽角反射地震速度换算成的密度剖面所产生的是一个重力高,它反映出大别山是一个穹隆,与实测大别山重力低大相径庭。只有将位于大别山山根上,南北大别之间设置一个从地表直达莫霍界面的巨大低密度体,重力曲线才能得到很好的拟合。这个低密度体应为近北西走向的花岗岩带。它与反射地震剖面上石镇透明反射地震带位置吻合,但宽度远较反射地震透明带为大。重力曲线的拟合进一步说明,在华北陆块与扬子陆块碰撞后的白垩纪时,大别山出现一个伸展期,在这个时期,大别山穹隆形成,并伴随有大规模花岗岩的侵入,超高压变质岩从地壳中下部折返到地表。研究说明,联合应用反射地震、宽角反射地震和重力,进行综合解释是必要的,可以得到更令人信服的地质结论。     

Tectonic style of the Appalachian allochthonous zone of southern Quebec: Seismic and gravimetric evidence

An integrated interpretation of seismic reflection and gravity results yields an image of the structural units (nappes, overthrust sheets and underthrust slabs) of the Cambrian-Ordovician metasedimentary and metavolcanic sequences of the Appalachian province of southern Quebec at depth and the relative position of the underlying Precambrian crystalline basement. The tectonics related to crystalline Precambrian basement may be correlated with four interpretations and combinations of these. A décollement of Paleozoic sedimentary and volcanic piles over a rigid crystalline basement is most probable considering the interpretation of gravity and especially seismic data. The tectonic style at depth is rather unresolved from surface geological information. The elaboration of a two-dimensional model is constrained by physical properties of rocks, maximum depth extents of individual bodies and seismic reflectors in addition to surface geology. Finally, a minimum of 1500 km of shortening and Iapetan closing is suggested.     

三维密度界面反演的一个近似方法

在区域地球物理研究工作中,应用重力资料反演密度界面,尤其是计算莫氏面的深度,已被广大学者所重视。目前某些计算方法,如线性公式法、压缩质面法等已被应用。我们认为,在利用小比例尺区域重力资料反演密度界面时,现有的一些方法各有其一定的局限性。     

惠民凹陷东部区域性走滑断裂构造系统的发现和意义

惠民凹陷东部地区油气勘探程度相对较低,岩浆活动剧烈,深部地质构造复杂。为此,通过岩石物性特征的研究和引起重、磁异常的地质因素分析,将重、磁、震综合地球物理解释技术应用于解决盆地深层地质构造问题,确定了重力和磁力异常在研究区的分布特征,结合地震资料解释发现了近SN向延伸的滨县-高青-博山区域性右旋走滑断裂构造系统和NNW向延伸的白桥-文祖区域性左旋走滑断裂构造系统。这两条区域性走滑断裂构造系统对本区中新生代构造格局具有重要控制作用,并控制了研究区内线性分布的火山活动和幔源CO2气的分布。     

Geophysics of proposed proterozoic sutures in Canada

Sutures have been identified or proposed at several sites in the Canadian Shield. Although many authors have suggested that some form of primitive plate tectonic processes have played a major role in the formation of the Archean crust of the Superior structural province, Proterozoic sutures, resembling Phanerozoic examples, have been proposed only within the younger Churchill and Grenville provinces, and at or near the boundaries between the older Superior and Slave provinces and the younger provinces. Gravity, magnetic, seismic and paleomagnetic data related to three proposed sutures in the latter category have been interpreted and generally support the view that the Shield comprises a mosaic of once separated, but now joined cratonic blocks. The sutures vary in complexity from wide zones, containing distinctive rocks formed at ancient accreting and consuming plate margins, to cryptic sutures marked only by abrupt changes in metamorphism and zones of cataclasis. Paired negative and positive gravity anomalies associated with these sutures have been interpreted in terms of juxtaposed crustal blocks of different density and thickness. Magnetic anomaly patterns change abruptly at structural province boundaries. Available seismic results support the concept of thickened crust in the younger province as derived from gravity modelling. Most authors prefer interpretation of Proterozoic pole positions in terms of one-plate models, but in some cases the interpretation of paleomagnetic data is equivocal and two-plate models have been proposed.     

A Geotransect in the Central Indian Shield,across the Narmada-Son Lineament and the Central Indian Suture

This paper reports on a geotransect in the central Indian shield along a 100 km wide NW-SE corridor between Hirapur and Rajnandgaon. This corridor has been selected based on two seismic profiles—a 235 km long seismic-refraction/wide-angle-reflection profile between Hirapur and Mandla and a 130 km long coincident deep-reflection/refraction profile between Seoni and Kalimati. Since the geologic, gravity, magnetic, and heat-flow data are available up to Rajnandgaon, the second part of the corridor has been extended by another 80 km in the absence of seismic data. From northwest to southeast, the transect corridor covers different tectonic units of the Late Archean to Mesoproterozoic Bundelkhand craton, the Paleoproterozoic to Mesoproterozoic Satpura mobile belt, the Middle Archean to Mesoproterozoic Kotri-Dongargarh mobile belt, and the Neoproterozoic Bastar craton.

The seismic results in the Bundelkhand craton show lower crustal velocity values at a very shallow depth; these data have now been interpreted as a lower-crustal intrusive body that is present throughout the Bundelkhand craton in the lower crust at depths of 23 to 25 km. Combined interpretation of seismic travel times with the gravity data indicate the presence of a local magmatic body at mid-crustal depth in the Satpura mobile belt. The crust-mantle boundary is at depths varying between 40 and 44 km.

The seismic-reflection data set identifies the presence of a suture at the Satpura mobile belt/ Kotri-Dongargarh mobile belt boundary. A well-defined Moho offset and a pattern of adjacent fabrics, each characterized by dips toward each other, mark tectonically imbricated crust on opposite sides of the suture.     

塔里木盆地西南古生代地震地层学特征

塔里木盆地西南地区经历了从海相到陆相的发展历程,沉积了多类岩石,在地震剖面上表现为丰富的地震相类型.分析研究古生界地震相划分及其特征,将有助于解决非井及覆盖区古生代沉积相特征及展布,为塔西南地区石油地质条件的评价提供可靠的基础资料.     

New data on the structure of the Kas Block in the basement of the West Siberian Plate

A new concept concerning the structure and stages of evolution of the Kas Block of the West Siberian Plate is stated in this paper. The Kas Block is traditionally considered to be a subsided western margin of the Siberian Platform. The new concept is based on the results of the interpretation of the geophysical data recently obtained along the reference and regional profiles in this territory. The geological interpretation of the deep dynamic sections obtained by reprocessing of the CDP seismic reflection records has been performed for the first time. The structural features of the Kas Block, as well as the character of its junction with the Siberian Platform and the Paleozoic framework, are discussed. The tectonic scheme of the territory and the scheme of the pre-Late Devonian surface of the Kas Block have been compiled. The Baikalian age of the basement of the Kas Block is substantiated. The Salairian allochthonous ophiolite-basalt tectonic nappe is localized for the first time within the sedimentary cover of the Kas Block. The available information allowed us to reconstruct the development of the western margin of the Siberian continent in the Riphean and Early Paleozoic before and after the Baikalian Orogeny, respectively. The informational background of the geological and geophysical interpretation involves the results of the CDP seismic reflection profiling, including the deep dynamic seismic sections and parameters of the P-wave velocities along the reference 1-SB seismic line and the regional Vostok 10, 12, 15, and 16 seismic lines; the results of the deep seismic and magnetotelluric soundings; the gravity measurements; the magnetic exploration; and the new coherent physical geological models.     

Geodynamic evolution of Jbel Cheid (Northern Tunisian Atlas) from geophysical and geological data

Understanding the formation and the development of salt structures is very important especially because they are of significant economical interest. Detailed understanding of this process will help reservoir prediction and hydrocarbon recovery. In this work, we use a combination of geological observations along with the interpretation of geophysical data (seismic and Bouguer anomaly data) to better constrain the geology of the Jbel Cheid structure. The shape of Triassic body of Jbel Cheid (Northern Tunisian Atlas) structure and its geodynamic evolution have been determined by gravity analyses and 2.5D modeling, correlated with others geophysical data (seismic) and geological observations. Semi-automatic structural analysis was performed before modeling, to identify lateral gravity discontinuities. The complete Bouguer and residual gravity anomaly maps indicate a positive amplitude gravity anomaly over the Triassic evaporitic outcrop (Jbel Cheid) and prominent NE–SW-trending features associated with the boundary of the Triassic rocks and surrounded layers. The seismic profile shows a thickness variation of post-salt layers. Taking into account the 2.5D gravity model, seismic profile and surface data, geodynamic evolution of Jbel Cheid can be subdivided on three stages (reactive, active, and passive) which well correlated to the model proposed by Vendeville (2002).     

High resolution local Moho determination using gravity inversion: A case study in Sri Lanka

The seismic data incorporated in global Moho models are sparse and therefore the interpolation of global Moho depths on a local area may give unrealistic results, especially in regions without adequate seismic information. Gravity inversion is a useful tool that can be used to determine Moho depths in the mentioned regions. This paper describes an interactive way of local Moho depth determination using the gravity inversion method constrained with available seismic data. Before applying inversion algorithms, the Bouguer gravity data is filtered in various stages that reduce the potential bias usually expected in Moho depth determination using gravity methods with constant density contrast assumption. A test area with reliable seismic data is used to validate the results of Moho computation, and subsequently the same computation procedure is applied to the Sri Lankan region. The results of the test area are in better agreement with seismically determined Moho depths than those obtained by global Moho models. In the Sri Lankan region, Moho determination reveals a fairly uniform thin crust of average thickness around 20 km. The overall result suggests that our gravity inversion method is robust and may be suitable for local Moho determination in virgin regions, especially those without sufficient seismic data.     

The Diamantina River ring feature,Winton region,western Queensland

The Diamantina ～120 km-diameter ring feature, a unique feature in western Queensland, is manifested by a near-360° circular drainage pattern, radial creeks and a coincident radiometric K–Th–U pattern. The structure has been studied in the context of an investigation of the nature and origin of Australian circular structures. Geophysical signatures, including total magnetic intensity (TMI), gravity and seismic reflection transect data from the region of the ring feature are examined to help test the origin of the structure. A western subdued TMI arc with a ～110 km diameter is offset by ～30 km eastward from the western rim of the drainage ring. Bouguer anomaly data show a gravity low near the centre of the ring structure, but no outer circular pattern. Two recent seismic transects indicate a moderately reflective to weakly reflective crust below flat lying strata of the Jurassic–Cretaceous Eromanga and Permian–Triassic Galilee basins, and above a usually well-defined ～39–45 km-deep Moho. An approximately ～100 km-wide seismically non-reflective to weakly reflective zone overlapping the Diamantina ring feature separates crust of different seismic reflection character to either side. The nature of the seismic non-reflective crust is unknown. A potential interpretation of the ring structure in terms of asteroid impact cannot be confirmed or rejected given the present state of knowledge, owing to (1) the near-30 km depth of the seismically non-reflective zone along the transects; and (2) the shift of the TMI part ring zone relative to the geomorphic expression of the Diamantina ring feature. A test of the nature and origin of the Diamantina ring feature requires a cored drill hole near the centre of the TMI ring structure.     

重力-地震联合反演解释技术在拉布达林盆地油气资源评价中的应用

内蒙古拉布达林盆地面积大、油气勘探程度很低.由于浅层广泛分布的火山岩的强反射屏蔽作用造成地震勘探对深层成像差,难以评价盆地油气勘探潜力.为了解决盆地主要目的层上库力组底界深度及沉积岩分布不清的问题,对盆地南部二维地震资料开展重力-地震联合反演解释.通过在GM-SYS重磁剖面解释软件中加载地震剖面,建立重力模型,利用地震浅层可靠解释成果做约束条件,消除上覆火山岩的重力干扰,减少了重力反演深层构造界面的多解性,成功地反演了盆地上库力组底界深度、沉积岩厚度,指出巴彦哈达断陷为有利勘探区,在此基础上估算出油气资源量.     

Evidence for and relationship between recent distributed extension and halokinesis in the Farasan Islands,southern Red Sea,Saudi Arabia

This paper presents a structural evolution study of the Farasan Bank using an integration of field work, remote sensing data, and regional Bouguer gravity data interpretation. The architecture of the Farasan Islands is influenced by normal faults parallel to the Red Sea rift axis delineating a series of graben and asymmetric half-graben structures as well as salt domes. Geological data suggest that the negative gravity anomaly over the Farasan Bank reflects thick salt deposits. The gravity data shows a general NW-SE trend with the main negative anomaly coincident with the Farasan Islands. Gravity data together with seismic reflection data suggest that salt diapirs are elongated and parallel to the main rift orientation (NW-SE). This indicates that salt deposition and diapirism was controlled by rift-related basement structures. Forward modeling of gravity data constrained by marine seismic reflection interpretation reveals that the evaporites directly overlie the basement in most places. No pre-evaporite formation was detected. Integration of the modeling with field observations and interpretations suggests that the Farasan Islands are in an area of active extension. Extension and salt diapirism on the flank of the mid-ocean ridge is likely to be synchronous with renewed spreading at 5 Ma.     

巴彦浩特盆地火成岩的综合地球物理解释

采用综合地球物理方法识别巴彦浩特盆地火成岩,首先利用高精度重磁资料和大地电磁测深资料,初步确定火成岩的平面位置和深度,然后利用钻遇火成岩的已知测井资料,建立人工合成地震记录,在地震剖面上标定出火成岩并分析其波组特征,采用BP神经网络进行火成岩的精细识别.研究表明,综合地球物理解释是提高反演解释精度的有效途径.     

海上地震资料处理中的潮汐静校正处理研究及应用

海水因为受月亮引力的作用会产生潮汐现象,在海上地震资料中表现为假断层现象,这种现象会影响地震资料的成像质量,导致错误层位解释及储层预测,因此在地震资料处理中,需要进行潮汐静校正处理。这里讨论了如何应用海上采集的高程值计算出地震波来回通过基准海平面与放炮时海平面之间的薄层水体所经历的时间,再将全部地震数据都校置在同一观察平面,从而实现潮汐静校正处理的方法,并应用于海上某工区中,得到较为理想的效果。     

Structural evolution of the southern Rif Cordillera (Morocco): tectonics and synsedimentary fault processes

This paper attempts to investigate the tectonics of the southern Rif Cordillera. Hydrogeological and oil well data, together with interpretation of seismic reflection lines help to characterize the architecture of the Rharb–Mamora Basin located in the frontal region of the Gibraltar Arc. The facies map constructed from the drilling data exhibits four main types of Pliocene facies: (i) conglomerates; (ii) limestones; (iii) sandstones and sands more or less rich with shelly remains; (iv) clays. The lateral variation of deposits is accompanied by thickening, which can reach a few tens of metres. Thickening of layers and lithofacies variation indicate synsedimentary faulting processes. Two major fault zones have been identified: Kenitra–Sidi Slimane Fault Zone (K2SFZ) and Rabat–Kenitra Fault Zone (RKFZ). In the western coastal area, the geometrical configuration suggests a partition into horsts and grabens in the southern part, and a system of three geological units in the northern part. The analysis and interpretation of the gravity data reveal an important gravity anomaly, referred to as the Kenitra Gravity Anomaly. It corresponds to the Hercynian faults deduced by the seismic reflection line interpretation: K2SFZ and RKFZ. From Larache to El Jadida cities, the Kenitra area represents the hinge between the positive and negative gravity values, with a major negative anomaly in the eastern part of Kenitra. Copyright © 2004 John Wiley & Sons, Ltd.     

三参数小波分频处理在走滑断层识别中的应用

压扭应力场环境下形成的走滑断层一般具有断面陡立,在剖面上呈花状等特点,其构造特征复杂,剖面解释和空间组合难度较大。同一地震剖面的浅、中、深层的资料品质有差异,断点、断面的清晰度也不同。以地质背景为基础,针对不同层段采用不同主频的地震资料进行断层解释,能够增强其可靠性。三参数小波变换具有比傅里叶变换、Morlet小波变换更好的时频聚焦性,可以使断层在不同主频的地震剖面上取得最佳的成像效果,有利于人工解释并提高其可信度。南堡凹陷走滑断层发育,这里采用三参数小波分频处理技术,针对该区块的地震资料进行处理并用于断层解释,取得了较好的效果。     

A high-velocity layer in the lower crust of the North German Basin

A network of deep seismic refraction profiles in Northern Germany consisting of parts of the European Geotraverse (EGT) and additional new Unes is interpreted. The most striking result is the proof of an approximately 10 km thick high-velocity layer in the lower crust. Its P-wave velocity of 6.9-7.5 km s⁻¹ is typical for shield crusts or lower crust in extensional environments intruded by mafic magma. The layer is observed in an area of roughly 150 × 180 km north of the Elbe river and seems to continue north-east, at least up to the Caledonian deformation front at the southern edge of the Ringkøbing-Fyn High. It correlates spatially with an area of high positive gravity anomalies. Here, a Moho topography of several kilometres, which had already been postulated on the basis of gravity inversions and sporadic near-vertical P_MP reflections, could be confirmed by the interpretation of seismic wide-angle records. The termination of the high-velocity lower crust at the Lower Elbe Lineament, which strikes parallel to the Teisseyre-Tornquist Zone, contributes to its definition as a major lineament in the context of central European tectonics.     

中国科学深钻选址地球物理调查与大别-苏鲁岩石圈

本文简要地讨论了大别-苏鲁超高压变质带大陆科学钻探选址新采集的地球物理调查剖面及相关地球物理成果。这些剖面以深反射地震及大地电磁测量为研究岩石圈构造的主要方法，结合地质资料和区域重磁平面图，分析大别—苏鲁地体的深部地质构造与岩石圈主要特征，以及大陆科学钻探靶区选择的地质地球物理依据。同时讨论了大陆科学钻探的靶区(江苏东海县)的地壳构造与地热研究结果，并将深度偏移地震剖面与先导孔岩心钻探结果进行了对比。