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本文基于多波束测深和高分辨率多道反射地震数据研究了东沙海域深水巨型水下沙丘的特征.巨型水下沙丘发育在230~830m水深的上陆坡范围内,呈斑块状分布.NW-SE向的近海底流体运动不仅冲蚀地层,形成了三条与水下沙丘间隔分布的冲蚀带,为水下沙丘提供了沉积物来源,同时也为水下沙丘的形成提供了动力源.研究区水下沙丘波长(L)范围55~510m,波高(h)范围1.5~20m,二者呈指数关系分布.沙丘的波长随水深增大而增大,波高则在500~700m水深范围内最大.水下沙丘NE—SW向展布的脊线和几何参数关系是与现今水动力条件相平衡的结果. 相似文献
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本文基于对南海东北部东沙海域近期采集的多道反射地震资料进行重新处理获得新的地震海洋学数据,分析了该海域内孤立波/内孤立波包、沙丘上方和陡坎附近特殊反射结构特征,从而提供了新的海水层与海底相互作用依据.研究结果表明,除之前已发表文章中地震海洋学资料显示存在的第一模态内孤立波/波包和沙丘上方常见的反射样式-披毛状发射外,地震海洋学资料上还发现了第二模态内孤立波、陡坎上方的上抬型波动反射结构样式.在新的地震海洋学数据中,第一模态内孤立波振幅均小于50 m,宽度上都小于5 km,单个内孤立波的最大振幅约为45 m.内孤立波包的内孤立波振幅都相对较小,均小于40 m,并且与之前不同的是,彼此之间振幅相差不大,没有明显的排列规律.此次地震海洋学数据记录到的第二模态内孤立波,形态较为完整,上层和下层反射的振幅相差不大,在30 m左右;中间层大约在水深130 m位置处,垂向结构的整体大小大于200 m.沙丘上方反射结构普遍存在弱反射层,可能是湍流边界层,并且存在特殊反射样式-披毛状反射.但并不是沙丘上方都存在披毛状反射样式,本文分析它出现在地震海洋学资料上可能是受测线与沙丘走向之间夹角的影响.陡坎区域的水层反射结构则表现为上抬型波动,并常常伴随着同相轴连续性的变化.该波动的大小及反射同相轴的连续性可能取决于陡坎的高度/坡度及水层动力的强度,新数据中出现的一个上抬型波动,高度达20~30 m,它的附近水层还存在一个形态不完整的内孤立波.陡坎附近的水层反射也常常出现弱反射带和小的波动. 相似文献
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海洋反射地震通常用于调查、研究海底地质构造,勘探油气与天然气水合物资源.近期研究表明多道反射地震方法也可以对水柱的热盐细结构成像.中尺度涡与内波是重要的物理海洋现象,但是常规的物理海洋调查是在间隔若干公里的离散测站上进行的,水平分辨率较低,因此对中尺度涡的结构与内波的横向分布了解较差.本文利用在大西洋东部、南海采集的地震数据给出了低频反射地震可以对中尺度涡与内波清晰成像的新的证据.反射地震方法较传统海洋观测手段,具有明显的优势,主要体现在高的水平分辨率和短时间内对整个海水剖面进行成像方面．从地震剖面上,能够清楚地观测到中尺度涡、内波造成的反射特征变化,从而有助于改进对能量在不同尺度的海水运动之间传递过程的认识. 相似文献
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海底冷泉活动在全球大陆边缘海域广泛分布.与传统利用高频声学方法探测海底冷泉羽状流不同,本文利用常规多道反射地震(地震海洋学)方法对海底边界附近水体进行成像,结合盆地流体逸散结构特征,圈定活动冷泉流体活动发育位置,分析其地震反射特征和流体活动特征.研究表明,活动冷泉流体渗漏、逸散活动在多道反射地震剖面上一般呈羽状、扫帚状和不规则状几何形态,内部反射杂乱,反射振幅偏弱,但也有振幅增强的情况出现,这可能是由于含有较多泥质和细粒颗粒物悬浮.所造成的.冷泉活动一般与盆地内部泥底辟、流体管道、断层和裂隙、气烟囱、海底麻坑和泥火山等流体逸散结构相关,反映了地层内部流体自深部向浅部运移,在海底渗漏、逸散形成了活动冷泉.但分析结果仍需要进一步实地观测和理论模拟等相关研究确认. 相似文献
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《中国科学:地球科学》2015,(3)
东沙海底峡谷是南海东北部峡谷群中最西边的一条大型峡谷,其研究对于深入理解晚新生代南海东北部峡谷群的成因、沉积输移机理、台西南前陆盆地及台湾造山带的演化等具有重要意义.综合利用多道反射地震和多波束海底测深数据,研究东沙海底峡谷的地貌、沉积特征及成因.结果表明,东沙海底峡谷是一条陆坡限定型峡谷,它发源于东沙群岛东部上陆坡区,自NWW往SEE方向于水深3000 m处汇入台湾浅滩南峡谷,全长约190 km,平均宽度10 km.根据地震层序分析,在峡谷充填沉积物中识别出了11个层序界面,解释为峡谷的古下切侵蚀面.地震相分析表明,该峡谷及有关沉积主要表现为平行上超充填、杂乱充填、丘状发散和迁移波状等地震相类型,分别解释为浊流或其他重力流沉积与半远洋沉积的交互、滑坡或碎屑流及峡谷底部滞留沉积、浊流溢流形成的天然堤,以及发育于天然堤或峡谷口外海底扇上的沉积波.ODP1144孔合成记录层位标定表明,东沙海底峡谷的发育大致始于中更新世约0.90 Ma.东沙海底峡谷最先形成于现今峡谷中游的上段,随着浊流或其他重力流不断的下切侵蚀,峡谷顺陆坡而下逐渐向海盆方向延伸,同时在溯源侵蚀作用下逆坡向陆架破折带方向延伸至现今峡谷头部.上述11个地震层序界面的年代大致可以与全球低海平面期进行对比,表明海平面变化是控制东沙海底峡谷多期下切-充填的重要因素.综合分析认为,东沙海底峡谷的成因与台湾隆升及台西南前陆盆地的发育这一大的区域构造背景有关,但没有证据表明东沙海底峡谷的形成与断裂、岩浆活动等存在直接联系.陆坡重力搬运过程(包括滑坡及浊流)对东沙海底峡谷的形成演化具有重要影响. 相似文献
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通过对南海北部与西部大量反射地震剖面海水层部分进行再处理,与以往地震海洋学主要关注海水层内部的反射结构不同,本文重点对海底附近水体的各种复杂反射地震特征进行分类、分析与总结.与传统对海底边界层的定义不同,我们将海底边界附近的水体称之为海底边界层.本文利用传统地震相分析方法,分析海底边界层各种复杂反射地震结构的几何形态、内部反射结构、连续性、振幅以及视频率特征,结合过去相关的地震海洋学研究成果、海底边界层理论与其它各种海底附近作用/过程,不仅对中尺度涡旋、内孤立波和背风波在地震剖面上的反射地震特征进行了归类与分析,并推断最新发现的一些反射地震特征可能揭示的各种海洋作用/过程,例如不同的地震相特征可能反映了海底湍流边界层,海底沉积物再悬浮,天然气渗漏羽状流和麻坑内部异常上升流相关海底界面作用过程.结果分析表明,地震海洋学方法不仅能够对海洋内波、涡旋等物理海洋现象进行研究,同时也能够对海底附近各种复杂海洋作用/过程进行成像,从而拓展了地震海洋学的研究领域,一定程度上也能为过去不能有效对海底边界面发生的各种冷泉热液活动、生物和沉积等作用过程进行现场观测提供新的探测方法和研究视角. 相似文献
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南海北部东沙海底冲蚀河谷及其成因探讨 总被引：3，自引：0，他引：3
对覆盖南海北部东沙区的50多条多道地震剖面进行了解释.通过剖面解释,发现东沙隆起后,南海北部海底地形发生了较大变化.这其中包括东沙岛在陆架坡折处的隆起,以及沉积地层以较大的幅度向上翘起.向上翘起的地层在很大的范围内遭受海底剥蚀.在东沙隆起区的北侧,海底发育下切沟.本文解释为现代海底在更强烈的剥蚀条件下形成的海底冲蚀地形,称为海底冲蚀沟.海底冲蚀沟分布于东沙岛的北侧,大致与陆架坡折线,以及600m等深线相平行,其宽度一般在20km左右,NE-SW向长度可达200km,本文称为东沙海底冲蚀河谷.无论是冬季还是夏季,东沙岛以北的吕宋海峡都有黑潮分支从太平洋贯入南海.这一黑潮分支在冬季演化为太平洋-印度洋贯通流,在夏季演化为吕宋海峡次表层流.东沙海底冲蚀谷的分布与太平洋-印度洋贯通流,以及吕宋海峡次表层流的位置完全一致.本文认为,黑潮分支在遇到南海北部陆坡的阻挡后,黑潮水体由陆坡向陆架堆积,并因水深变浅,水体通过空间变小而使流速加大,从而有能力对海底产生剥蚀.东沙区海底地层的剥蚀,以及东沙海底冲蚀谷的发育都是黑潮分支在东沙陆架区形成的强底流和海底长期耦合作用的结果. 相似文献
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麻坑通常是由海底流体逸散形成的一种海底表面的残留地貌.本文所研究的区域主要位于南海的西北部,包括琼东南盆地的西南部和中建南盆地的中北部.该区域海底表面主要覆盖细粒沉积物,并且底流活动强烈,底流水道发育,为麻坑的形成和发育提供了有利条件.麻坑发育的主要因素是流体逸散系统,本文通过反射地震资料在本区发现了断层、气烟囱、泥火山、泥底辟、声空白/声混浊、强反射等多种与流体逸散相关的指示特征.麻坑的几何特征随着区域的不同会有较大变化.首先本区麻坑的尺寸跨度较大,与底流水道伴生的麻坑和广乐隆起的麻坑表面宽度通常是几十米到数百米,其他区域麻坑的表面宽度通常可达数千米,被称为巨型麻坑.其次本区麻坑的表面形态复杂多样,有圆形、椭圆形、新月形、环形等.各种不同的形态有可能指示了麻坑的不同发育阶段,新月形麻坑可能发育程度较低,其次是环形,圆形则相对较为成熟.据此推断发育程度较低的新月形麻坑有可能是活动麻坑,而地震海洋学的资料也倾向于证实这一点. 相似文献
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以往通过地震海洋学方法发现的大多是第一模态内孤立波,本文在台湾岛以南恒春海脊处捕捉到了两个大振幅凸型第二模态内孤立波.利用叠前偏移剖面得到内孤立波的视相速度约为0.7 m·s-1,其视传播方向沿地震测线从S到N,总体上满足较大最大振幅的波视相速度也较大的规律.地震海洋学剖面得到的振幅曲线观测值和理论方程计算出的第二模态内波振幅垂向变化理论值之间具有较好的一致性.通过叠前偏移剖面观察该内孤立波细结构的变化,发现在采集过程中,出现反射同相轴后翼变陡、分叉、合并的现象.由于密跃层中心偏离水层中心以及背景剪切流的影响,两个内孤立波尾部发育了高频内波,使得能量耗散增强.此外,第二模态内孤立波的生成和传播特征受到水体层结、黑潮、背景剪切流以及海底地形等多个因素的影响. 相似文献
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Monitoring surface change on a Namib linear dune 总被引：1，自引：0，他引：1
Ian Livingstone 《地球表面变化过程与地形》1989,14(4):317-332
In tackling the apparently intractable problem of linear dune initiation and maintenance there has been a move away from large-scale deductive models to smaller-scale field studies of individual dunes. This paper reports a study of surface change on a large, complex linear dune in the Namib Desert, southern Africa. The dune surface responds to a markedly seasonal wind regime. In summer westerly winds erode sand from the west flank of the dune and deposit it on the easterly lee side of the dune crest. In winter this pattern is reversed. Easterly winds erode sand from the east slope and deposit it on the west slope. The crest therefore moves back and forth some 15 m each year returning at the end of a year's cycle to its position at the beginning. The position of the base of the dune appears to remain fixed, even though sand is moving throughout the dune system. The dune does extend northward along some resultant of the westerly and easterly winds. Despite relatively high levels of activity, especially at the dune crest, there is no evidence of the breakdown of the linear dune form. The conclusion must therefore be that linear dunes can be maintained in bimodal wind regimes and are not necessarily related to unidirectional parallel regimes as others have suggested. 相似文献
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Early Holocene dune field development in Dalarna,central Sweden: A geomorphological and geophysical case study 
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下载免费PDF全文 Bonäsheden, Sweden's largest continuous dune field, situated in the county of Dalarna, central Sweden, has been investigated using LiDAR (light detection and ranging) remote sensing, ground penetrating radar as well as by field observations and luminescence dating. The use of LiDAR in conjunction with geographic information system (GIS) software proved to be efficient in mapping the inactive dune field and classifying the dune morphology, especially when slope raster images were used. The dunes have formed mostly by winds from the northwest (NW) and are of a transverse type. Still other dune types, such as parabolic dunes, and transverse dunes with a deviating orientation are present. Also, there seems to be different generations of dunes, suggesting a complex palaeowind environment with a change from predominantly north‐westerly winds to more westerly winds. Luminescence dating finally allows us to have an absolute chronology of the development of the Bonäsheden dune field, revealing formation of the dune field closely following the de‐glaciation of this part of Sweden (c. 10.5 ka). The well preserved transverse shape of the majority of the dunes suggests rapid stabilization by vegetation, although sand drift still seems to have been active on a noticeable scale for at least 1500 years and also, occasionally and patchy, as coversand deposition during the Late Holocene. A simple model is proposed for the dune field development of Bonäsheden based on our findings. This model is a useful addition since the majority of present day dune field models focus on the formation of parabolic dunes or large unvegetated dune fields. Our results suggest that most models cannot adequately simulate the formation of such small dune fields as that of Bonäsheden, with apparently rapidly fixated transverse dunes in a previously glaciated, now vegetated area. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
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Dune bedforms and salt‐wedge intrusions are common features in many estuaries with sand beds, and yet little is known about the interactions between the two. Flow visualization with an echosounder and velocity measurements with an acoustic Doppler current profiler over areas of flat‐bed and sand dunes in the highly‐stratified Fraser River estuary, Canada, were used to examine the effect of dunes on interfacial mixing. As the salt‐wedge migrates upstream over the flat‐bed, mixing is restricted to the lower portion of the water column. However, as the salt‐wedge migrates into the dune field from the flat bed, there is a dramatic change in the flow, and large internal in‐phase waves develop over each of the larger dunes, with water from the salt‐wedge reaching the surface of the estuary. The friction Richardson number shows that bed friction is more important in interfacial mixing over the dunes than over the flat‐bed, and a plot of internal Froude Number versus obstacle (dune) height shows that the salt‐wedge flow over the dunes is mainly supercritical. Such bedforms can be expected to cause similar effects in interfacial mixing in other estuaries and sediment‐laden density currents, and may thus be influential in fluid mixing and sediment transport. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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Deposits of late‐Holocene beach sand buried conifer forests episodically emerge on beaches of the Oregon coast. Simultaneously, sand dunes buried late‐Holocene forests growing on marine terraces landward of the beaches. Dune ramps, up to 60 m in elevation, connected the beach and dune deposits. The average age of wood samples from stumps rooted on the shore platforms is 3·07 ± 1·45 ka. The average age of wood and charcoal samples embedded in forest soil on the marine terraces is 3·27 ± 1·46 ka. Between 1994 and 2006, winter storm waves exposed more than 4·5 km2 of late‐Holocene forest soil on shore platforms at 19 localities. Rooted stumps without soil were uncovered at an additional 14 localities. Once exposed, wave action eroded the soil rapidly (one to two years). The intact forest soil and roots on the shore platforms must have been nearly continuously buried, protected and preserved prior to recent exposure. The late‐Holocene buried forest provides the basis for a conceptual model of coastal evolution. A three stage reversal of erosion and sand supply must have occurred: (1) wave erosion switched to seaward advancement of forests, (2) forest growth and soil development switched to burial beneath beach and dune sand and (3) burial and preservation switched to wave erosion, truncation of dune ramps and landward retreat of sea cliffs. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
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Mustafa Al-Marzooq Mamadou S. Diallo Vincent Etienne Thierry Tonellot SanLinn I. Kaka 《Geophysical Prospecting》2019,67(7):1825-1837
The Rub Al Khali region in Saudi Arabia is characterized by the presence of sand dunes separated by salt flats, also called Sabkhas. In general, the elevation of dunes in this region varies between 90 and 250 m above sea level. The presence of these sand dunes, along with the rapidly changing surface topography poses challenges for seismic data acquisition and processing. The high contrast in acoustic impedance between the dune base and the underlying formation often results in amplification of seismic waves that are recorded at stations located on the surface of sand dunes. Attempts to address the issue using conventional surface-consistent amplitude scaling methods without reducing these amplification effects generally fail, thus compromising the suitability of the processed data for amplitude versus offset analysis. In this study, we propose a new reference site technique to reduce the effects of sand dune amplification, enabling the production of data sets that are suitable for amplitude versus offset processing. The proposed technique uses a deterministic approach to derive surface-consistent, frequency-dependent de-amplification functions for shots and stations located on the dunes only. Two-dimensional synthetic and field data examples show that the technique significantly reduces the effects of sand dune amplification. 相似文献
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The idealized model of Besio et al. (On the formation of sand waves and sand banks. Journal of Fluid Mechanics 2006; 557: 1–17) is used to predict the wavelength of tidal dunes (sand waves) generated by tidal currents in estuaries and shallow seas. The predictions are then analysed and a formula is proposed to estimate the wavelength of tidal dunes as a function of the parameters of the problem. The wavelength of the dunes is found to increase when the water depth is increased and/or the strength of the tidal current is decreased. On the other hand, the size of the bottom material (if medium sand is considered) and the tidal ellipticity are found to have a relatively small influence on the length of the bottom forms. The formula proposed provides results which are consistent with field observations of different authors. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
17.
Elda Miramontes Gwenael Jouet Estelle Thereau Miguel Bruno Pierrick Penven Charline Guerin Pascal Le Roy Laurence Droz Stephan J. Jorry F. Javier Hernández-Molina Antoine Thiéblemont Ricardo Silva Jacinto Antonio Cattaneo 《地球表面变化过程与地形》2020,45(6):1469-1482
Evidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high-resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex-shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. However, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small-scale sedimentary features generated by internal waves are superimposed on large-scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd 相似文献