南海北部海底沙波地貌

南海北部水深８０—２５０ｍ的外大陆架和上大陆坡海底，有大片沙波地貌分布，它们主要是晚更新世末次冰期低海面时遗留下来的滨海沙波、沿岸沙丘、沙堤和沙垄，属残留沙体地貌。它们明显沿着本次冰期时形成的最低海岸线两侧分布，但在现代海底底流作用下，其两翼及沙波谷部发育有次一级的小型沙波或沙纹，它们的形态细小、排列紧密，与残留沙体地貌组成复式海底沙波。这就是流动沙波，是不稳定的地质因素，对海底工程有一定危害，但根据外大陆架实测底流数据，运用计算机数学模型方法计算，表明在细沙和中细沙大面积覆盖区，海底沙波每年均以０．１６６—０．５３４ｍ的速度向深海方向迁移，移动速度相当缓慢，其对海底工程设施不会造成直接危害。     

南海北部海底沙波研究

南海北部水深100~250 m的陆架和陆坡斜坡海底分布着大片沙波地貌,本文采用底形相图参数近似计算和泥沙起动流速2种计算方法,结合海底微地貌的实际形态,研究了沙波的沉积特征及其成因机理。研究认为,区内的沙波为现代沉积地貌;控制沙波形成发育的主要水动力因素是潮流与风暴条件下风海流的叠加;区内泥沙颗粒粒径适宜,海底坡度平缓,有利于海底沙波的发育。     

台湾浅滩大型沙波、潮流结构和推移质输运特征

对2006年8月所获台湾浅滩的底质、水深、多层位流速、流向以及CTD观测数据进行了分析,探讨了台湾浅滩大型沙波区水流的时空分布特征、沉积物输运过程及影响因素,结果表明,台湾浅滩动力作用较强,底质为粗砂,其上发育的沙波波长为325~822 m,波高为14.4~20.3 m,较接近由前人总结的沙波波高、波长的关系式计算出的波高和波长。沙波波峰、波谷、迎水面和背水面流速以12.5 h为周期沿顺时针方向旋转;底部最大流速出现在半日潮周期的同一时段,对应的方向均为偏南方向,且沙波迎水面的流速大于背水面的。在两个半日潮过程中研究区域的沉积物输入量大于输出量,处于淤积状态。在潮流作用下沉积物输运量较少,沙波脊线移动较小,而台湾浅滩有较大规模的沙波运动,故认为改造沙波的主要作用力应该是风暴流。在观测的断面中涨潮推移质输运量大于落潮的,且第一个半日潮涨落潮的输运能力均较第二个半日潮的大,这可能与潮流的流速-时间不对称特征及潮能变化形式有关。     

The Bedload Movement in the Changjiang Estuary

- Sandwaves in the Changjiang estuary were measured with a shallow sediment profiler and an echosounder from 1978 to 1988. The data, together with grain size and bedform of sediment indicates that the bedload movement by rolling and saltation is of great significance to sediment transport and is the principal factor responsible for sandwave and sandbody development in the estuary. The sandwaves were found well-developed, which is related to the tidal range and the velocity of ebb current. However, the further growth is restricted by strong flood current prevailing in the estuary. Because of the significant bedload, the sandbodies shift obviously and frequently, and sometimes the exchange of position occurs between the sandbodies and tidal channels. As a result, ships are regularly forced to change their navigation course.     

长江河口底沙运动规律

于１９８１－１９８８年以浅地层剖面仪和回声测探仪取得沙波观测资料，运用这些资料以及河床表层沉积物和历年河口水文、地形资料，采用水文学、沉积学与泥水沙运动力学相结合的研究方法，分析长江河口底沙的运规律。结果表明：长江河口底沙运动非常频敏，一般有单颗粒滚动、跳跃，沙波及沙体推移等形式；在沙质床面上沙波发育良好，其形成、发展和消失和潮差落潮流速有一定的相关性，因受涨，落潮流改造，沙波难于得到充分发展；沙     

海底沙波迁移过程原位观测简易装置设计与试验

海底沙波的迁移对海底基础工程具有潜在危害,成为人们日益关注的热点。目前对海底沙波迁移过程的研究缺乏原位长期观测数据的支撑,设计一种海底沙波迁移过程的原位观测装置,通过水压差与高程差变化的对应关系来确定沙波的形态变化和迁移过程。并且在室内波浪水槽中对装置观测的可行性和观测结果的准确性进行验证,同时在仰口湾潮滩进行现场观测应用。该装置室内试验测量计算得到沙波表面高度变化5.9 cm,而观测得到沙波表面高度实际变化5.5 cm,两者吻合程度很好。试验结果表明,这套海底沙波迁移过程原位观测装置能对沙波外部形态变化进行测量且准确度较高,为海底沙波迁移的观测提供了新方法。     

Sedimentology of a subtidal,tide-dominated sand body in the Yellow Sea,southwest Korea

Odanam Satoe, a subtidal, tide-dominated sand body in the Yellow Sea, Korea, is linear in plan and asymmetrical in cross-section. It consists of fine- to medium-grained, well-sorted subangular sand. Bedforms consist of high-amplitude (1–2 m) sandwaves on the lower flanks of the gentler-sloping bar surface, and medium-amplitude (0.5-1 m) sandwaves on the sand body trough adjoining the steeper face, the bar crest and shallower parts of the gently sloping bar surface. Bedforms are absent on the relatively steeper bar surface, which is characterized by 2° slopes. Bedform orientation on the gentler slope is oblique by 30° to the bar crest, parallel to the sand-body crest on the crest itself, and opposite to the steeper sand-body face in the trough below the steeper slope of the bar.Bottom current velocity data show that tidal currents are semi-rotary with a flood time—velocity asymmetry over the gentler slope, and ebb time—velocity asymmetry over the steeper slope during most of the tidal cycle. Tidal-current flow parallels bar elongation over the steeper slope, whereas over the gentler slope, tidal-current flow is directed at 30° to the bar crest and changes to normal to the crest one hour prior to low tide. Bedform orientation mapped with side-scan sonar shows agreement with these flow directions.Sand dispersal around the sand body is controlled by time—velocity asymmetry and partial rotary flow directions of tidal currents. This circulation causes not only a trapezoidal mode of grain dispersal, but also westerly migration of the sand body documented from comparative bathymetric surveys in 1964 and 1980.     

Small-scale sediment transport patterns and bedform morphodynamics: new insights from high-resolution multibeam bathymetry

New multibeam echosounder and processing technologies yield sub-meter-scale bathymetric resolution, revealing striking details of bedform morphology that are shaped by complex boundary-layer flow dynamics at a range of spatial and temporal scales. An inertially aided post processed kinematic (IAPPK) technique generates a smoothed best estimate trajectory (SBET) solution to tie the vessel motion-related effects of each sounding directly to the ellipsoid, significantly reducing artifacts commonly found in multibeam data, increasing point density, and sharpening seafloor features. The new technique was applied to a large bedform field in 20–30 m water depths in central San Francisco Bay, California (USA), revealing bedforms that suggest boundary-layer flow deflection by the crests where 12-m-wavelength, 0.2-m-amplitude bedforms are superimposed on 60-m-wavelength, 1-m-amplitude bedforms, with crests that often were strongly oblique (approaching 90°) to the larger features on the lee side, and near-parallel on the stoss side. During one survey in April 2008, superimposed bedform crests were continuous between the crests of the larger features, indicating that flow detachment in the lee of the larger bedforms is not always a dominant process. Assessment of bedform crest peakedness, asymmetry, and small-scale bedform evolution between surveys indicates the impact of different flow regimes on the entire bedform field. This paper presents unique fine-scale imagery of compound and superimposed bedforms, which is used to (1) assess the physical forcing and evolution of a bedform field in San Francisco Bay, and (2) in conjunction with numerical modeling, gain a better fundamental understanding of boundary-layer flow dynamics that result in the observed superimposed bedform orientation.     

Seafloor instability at East Flower Garden Bank,northwest Gulf of Mexico

An increasing volume of evidence suggests that normal faulting and graben formation associated with salt diapirism may be catastrophic. The evidence includes: 1) removal of solid salt from the diapirs by dissolution, 2) the nature of rock outcrops at the crests of domes, 3) seismic profiles showing recent displacement of the seafloor, and 4) changes in coral growth rates at the East Flower Garden Bank. The possibility of catastrophic movement of the seabed is an engineering constraint that must be addressed prior to emplacing structures such as jack-up rigs and production platforms.     

Images of the seabed in the Faroe-Shetland Channel from commercial 3D seismic data

A large number of 3D deep seismic surveys in the Faroe-Shetland Channel gives continuous coverage over most of the region. These surveys were designed primarily to image depths in excess of 4 km, use low frequency sources and are recorded at low temporal sample rates. However, commercial 3D data can generate highly detailed images of the seabed due to the high spatial sample rate, typically 12.5 m. This is particularly true in waters below 200 m. Despite geophysical artefacts, the images reveal that there are a number of sedimentary processes at work adjacent to and within this channel. On the West Shetland Shelf, iceberg scouring and moraines reflect the impact of glaciation. On the West Shetland slope there is clear evidence for down-slope processes, such as debris flows, linear erosion channels, basal fans and (one case) slope failure. Along-slope processes are also active as indicated by the presence of sediment waves and contourite mounds. On the floor of the basin, polygonal cracking can be observed. The most spectacular feature appears to be the Judd Deeps, a system of cliffs approximately 200 m high and 40 km across. Traditionally, seabed investigation has been performed using high-resolution surveys. This study shows that deep exploration data can also provide useful images of the seafloor.     

Sediment movement characteristics of coast and analysis of seabed evolution

The rotary ring flume is used to study the silty sand movement in a periodic alternating current.Characteristics of sediment movement of different coasts in the tidal current are summarized.More detailed analysis of erosion-sedimentation function in a numerical simulation is made.The equilibrium sediment concentration is advanced.Based on the equilibrium sediment concentration,the seabed erosion-sedimentation index is derived and the seabed erosion-sedimentation calculation is analyzed.The seabed erosion-sedimentation index is used to calculate the seabed evolution of Yangshan sea area and a good agreement with field measurements is obtained.     

波浪作用产生沙波的动力机制研究

界面波作用产生的周期沙波现象的动力机制一直没有得到合理圆满的解释。本文针对这种现象通过对浅水弱非线性Boussinesq方程,讨论界面波由于反射效应产生的反射波与入射波的非线性相互作用,得出了一个不受时间变量制约的不传播非线性二阶驻波解。从这个驻波解空间分布上可以看出,这种动力机制作用产生的周期沙波通常是其波形平行于反射壁,并且沙波波长为入射波垂直反射壁面分量的一半。数值模拟结果证明这个非线性二阶波是Genus-2波列的一个子集．水槽实验和海岸高空照片资料上清楚的分辨出这种由反射壁效应产生的沙波的客观存在性。     

Bed shear stresses and the sedimentation of sandy gravels

It is suggested that variations in bed shear stress between the crests and troughs of gravel waves may be the cause of simultaneous deposition of sand and gravel and, coupled with spatial variations in gravel wave steepness, can produce variations in the proportions of sand and gravel in the seabed sediment.     

循环振动导致黄河口潮坪土成分结构变异研究

在黄河三角洲潮坪采集循环振动荷载施加前后长1 m的原状样各一个,在实验室对原状样以10 cm为间隔取样,进行粒度成分分析和微结构观测,研究循环振动荷载导致的海床土成分与结构的变化。发现总体上振动导致土体颗粒长轴呈现竖向排列趋势,深0~40 cm的表层土细粒含量增高,级配变好;深40~70 cm土体细粒含量降低,级配变差;70 cm以下土体受振动影响不明显。现场海床土孔隙水压力监测结果表明,这种海床土的成分结构变异与循环荷载导致土体重复液化及渗流场作用有关。     

Side-scan sonar mosaic of a sand Ridge field: southern Mid-Atlantic Bight

Side-scan sonar coverage of a 1.5 km by 1.5 km area of the inner shelf depicts the morphology of part of a submarine ridge field. The presence of megaripples indicates that ridge sediments are presently reworked by currents. Megaripples occur in the coarser sands of the north-facing ridge flanks. Distribution of megaripples and the ridge asymmetry support the hypothesis that sand ridges respond as large-scale bedforms to south-setting flows. Megaripple crests were observed to be aligned shore-parallel which indicates a pre-survey episode of shore-normal bedload transport.     

Breaking wave forces and velocity fields

A new technique for measuring velocities under breaking wave crests using laser-doppler anemometry has been developed. Results may be obtained at positions up to 4 millimeters from the crest.A wave field is presented for a vertically fronted wave and comparison is made with an appropriate numerical study. The velocities obtained are far in excess of the predictions of linear theory.Measurements have also been made of the forces produced by this wave on a horizontal cylinder. Comparisons with large regular waves indicate that forces due to breaking waves can be up to five times greater for similar wave heights.     

埕岛海区水下岸坡沉积物不稳定运动的区域差异性

埕岛海区水下岸坡是海底不稳定活动的频发地带,存在着海底差异冲蚀、残留岗丘、沉积物块体运动等灾害地质,对海上工程设施造成极大威胁。通过对海区多波束、声呐、浅剖等声学调查资料的分析,结合前人研究成果,对水下岸坡沉积物块体运动的区域差异性进行了研究。认为埕岛水下岸坡区沉积物块体运动极为发育,受海区波浪诱发因素的控制,岸坡上部水深3~6 m的区域中,海底沉积物块体运动沿坡向深水方向水平运动的趋势明显,对海洋工程设施影响较大;岸坡下部,沉积物块体运动主要表现为向深部地层的扰动和区域性的失水塌陷压实作用。     

Geological information from telecommunications cable route surveys of the continental shelf

Several burial assessment surveys using a seabed plough have been carried out along routes on the United Kingdom continental shelf to assess their potential for cable burial. Detailed information on sediment thicknesses and characteristics within the top meter of the seabed was collected using a survey tool called the Plough Surveyor. Geophysical surveys along these routes have enabled correlation with the plough data. Results include the range of tensions, tension trace characteristics, and typical penetration, and these have been correlated with seabed types and thickness of sediment.     

Acoustic evidence of shallow gas accumulations and active pockmarks in the ?zmir Gulf, Aegean sea

Based on high-resolution Chirp seismic, multibeam bathymetry and side scan sonar data collected in the ?zmir Gulf, Aegean Sea in 2008 and 2010, gas-related structures have been identified, which can be classified into three categories: (1) shallow gas accumulations and gas chimneys, (2) mud diapirs, and (3) active and inactive pockmarks. On the Chirp profiles, shallow gas accumulations were observed along the northern coastline of the outer ?zmir Gulf at 3-20 m below the seabed. They appear as acoustic turbidity zones and are interpreted as biogenic gas accumulations produced in organic-rich highstand fan sediments from the Gediz River. The diapiric structures are interpreted as shale or mud diapirs formed under lateral compression due to regional counter-clockwise rotation of Anatolian microplate. Furthermore, the sedimentary structure at the flanks suggests a continuous upward movement of the diapirs. Several pockmarks exist close to fault traces to the east of Hekim Island; most of them were associated with acoustic plumes indicating active degassing during the survey period in 2008. Another Chirp survey was carried out just over these plumes in 2010 to demonstrate if the gas seeps were still active. The surveys indicate that the gas seep is an ongoing process in the gulf. Based on the Chirp data, we proposed that the pockmark formation in the area can be explained by protracted seep model, whereby sediment erosion and re-distribution along pockmark walls result from ongoing (or long lasting) seepage of fluids over long periods of time. The existence of inactive pockmarks in the vicinity, however, implies that gas seepage may eventually cease or that it is periodic. Most of the active pockmarks are located over the fault planes, likely indicating that the gas seepage is controlled by active faulting.