Seafloor Geomorphology and Processes on the Western Continental Margin of India based on Multibeam Acoustic Backscatter Data Analysis

An assessment of the multibeam sonar data of the central Western Continental Margins of India has been carried out to evaluate the seafloor geomorphology and processes by examining the geomorphological attributes e.g., slope, sediments, structures, etc. associated with geomorphic features. The variation in relief and the features located in the region have been mapped and interpreted collectively by utilizing several geospatial mapping tools. The backscatter strength across the area, apparently congruent with the local relief, has helped to examine the sediment movement on the seafloor. The prominent features found in the region include faults, pockmarks, mounds, submarine terraces, and submerged fossil reefs. Several areas with varying topography engender comparable fractal dimension at short scale breaks, and the probability density functions (PDFs) utilizing backscatter data depicting overlapping classes. The present study highlights how fractals and scale break parameters can be utilized to determine the seafloor processes and associated sedimentological dynamics in a complex geographical environment with strong bottom currents, seasonal upwelling, and faulted structure. The role and impact of the various geomorphic processes on the reworking of sediment movement and the overall progression of the seafloor morphology has been revealed for the first time in this part of the ocean bottom.     

黄东海间遗传连通性多物种案例分析与启示

黄东海是太平洋重要的边缘海,拥有丰富的海洋生物多样性资源,但在气候变化及人类活动的双重胁迫下,黄东海面临生物多样性衰退等问题,亟需加强保护。保护区网络被认为可有效防止或减缓人类活动和气候变化对生物多样性的负面影响,但连通性作为保护区网络的重要参数却很难直接测量。因此,本研究希望通过梳理已发表的黄东海间遗传连通性文献来揭示黄东海区域内的保护区是否形成网络。通过搜集已发表的137篇文献,共获得175个研究案例。超过半数的案例(65.71%)认为黄东海之间具有显著的遗传分化,这也意味着跨生态区群体间具有较低的遗传连通性。在总结影响因素时发现长江冲淡水、生境异质性、空间距离、进化过程、生活史(较短的浮游幼体期)、洋流等6个因素阻碍了黄东海群体间的遗传连通性,而洋流、生活史(较长的浮游幼体期)等2个因素促进了遗传连通性。进一步分析发现黄东海群体间的遗传连通性与空间距离呈显著的负相关,而与浮游幼体期呈正相关。因此,根据本研究结果,建议未来保护区网络建设应针对黄、东海生态区分别构建网络,在网络设计时应考虑保护对象的扩散能力及保护区间的空间距离。本研究不仅为构建涵盖黄东海的海洋保护区网络提供理论基础,...     

OBS海洋环境信号分析与应用

为研究海洋环境信号在OBS(Ocean Bottom Seismograph)原始数据中的规律及应用,根据OBS原始数据的波形及频谱特征,将研究区划分为5个时间段,依次为旧涌浪阶段、风浪渐强阶段、风浪全盛阶段、风浪消退阶段和新涌浪阶段。结合海洋天气预报,认为上述现象是由偏南风风浪对海流的影响造成的。参考野外地震数据采集记录班报,得到各阶段的时长和距离,计算风浪渐强、全盛和消退阶段OBS附近海流的平均速度。结果表明:OBS原始资料中浅海海洋环境噪音增强的主要因素是风浪,且风浪引起的噪音信号的波形变化特征是渐进式的;OBS可用于接收某种特殊阶段(如台风、海啸等)的噪音信号,并根据噪音信号的波形特征、频谱变化规律和持续时间估算该阶段的海流速度变化。     

Explaining island‐wide geographical patterns of Caribbean fish diversity: A multi‐scale seascape ecology approach

Geographical patterning of fish diversity across coral reef seascapes is driven by many interacting environmental variables operating at multiple spatial scales. Identifying suites of variables that explain spatial patterns of fish diversity is central to ecology and informs prioritization in marine conservation, particularly where protection of the highest biodiversity coral reefs is a primary goal. However, the relative importance of conventional within‐patch variables versus the spatial patterning of the surrounding seascape is still unclear in the ecology of fishes on coral reefs. A multi‐scale seascape approach derived from landscape ecology was applied to quantify and examine the explanatory roles of a wide range of variables at different spatial scales including: (i) within‐patch structural attributes from field data (5 × 1 m² sample unit area); (ii) geometry of the seascape from sea‐floor maps (10–50 m radius seascape units); and wave exposure from a hydrodynamic model (240 m resolution) for 251 coral reef survey sites in the US Virgin Islands. Non‐parametric statistical learning techniques using single classification and regression trees (CART) and ensembles of boosted regression trees (TreeNet) were used to: (i) model interactions; and (ii) identify the most influential environmental predictors from multiple data types (diver surveys, terrain models, habitat maps) across multiple spatial scales (1–196,350 m²). Classifying the continuous response variables into a binary category and instead predicting the presence and absence of fish species richness hotspots (top 10% richness) increased the predictive performance of the models. The best CART model predicted fish richness hotspots with 80% accuracy. The statistical interaction between abundance of living scleractinian corals measured by SCUBA divers within 1 m² quadrats and the topographical complexity of the surrounding sea‐floor terrain (150 m radius seascape unit) measured from a high‐resolution terrain model best explained geographical patterns in fish richness hotspots. The comparatively poor performance of models predicting continuous variability in fish diversity across the seascape could be a result of a decoupling of the diversity‐environment relationship owing to structural degradation leading to a widespread homogenization of coral reef structure.     

Contourite identification along Italian margins: The case of the Portofino drift (Ligurian Sea)

A brief review of the published evidence of current deposits around Italy is the occasion to test the robustness of matching bottom current velocity models and seafloor morphologies to identify contourite drifts not yet documented. We present the result of the regional hydrodynamic model MARS3D in the Northern Tyrrhenian and Ligurian Sea with horizontal resolution of 1.2 km and 60 levels with focus on bottom current: data are integrated over summer and winter 2013 as representative of low and high intensity current conditions.The Eastern Ligurian margin is impacted by the Levantine Intermediate Water (LIW) with modeled mean velocity of bottom current up to 20 cm s⁻¹ in winter 2013 and calculated bottom shear stress exceeding 0.2 N m⁻² in water depth of 400–800 m. By crossing this information with seafloor morphology and geometry of seismic reflections, we identify a sediment drift formerly overlooked at ca 1000 m water depth. The Portofino separated mounded drift has a maximum thickness of at least 150 m and occurs in an area of mean current velocity minimum. Independent evidence to support the interpretation include bottom current modelling, seafloor morphology, seismic reflection geometry and sediment core facies. The adjacent areas impacted by stronger bottom currents present features likely resulted from bottom current erosion such as a marine terrace and elongated pockmarks.Compared to former interpretation of seafloor morphology in the study area, our results have an impact on the assessment of marine geohazards: submarine landslides offshore Portofino are small in size and coexist with sediment erosion and preferential accumulation features (sediment drifts) originated by current-dominated sedimentary processes. Furthermore, our results propel a more general discussion about contourite identification in the Italian seas and possible implications.     

Regional-scale seafloor mapping and geohazard assessment. The experience from the Italian project MaGIC (Marine Geohazards along the Italian Coasts)

Recent developments in seafloor imaging and mapping techniques greatly improved our capability of identifying marine geohazards affecting continental margins. Geomorphic features can be detected in great detail by high-resolution multibeam imaging and regarded as geohazard indicators; the most common include slide scars and deposits, canyon headscarps and steep erosional flanks, fault-related seafloor unevenness, mud volcanoes, pockmarks, gravity flow deposits, erosional scours and bedforms indicating sediment mobility at diverse temporal/spatial scale. These processes are widespread on Italian continental margins and are potential indicators of geohazard for human settlements and infrastructures in the offshore and coastal zones. The national Project MaGIC (Marine Geohazards along the Italian Coasts) aims at documenting potential geohazards based on the acquisition of high-resolution multibeam bathymetry and on the production of maps of the geohazard-related geomorphic features for most of the Italian continental margins. With reference to this issue, we discuss some of the most frequent problems dealing with reconnaissance, interpretation and cartographic representation of geohazard-related geomorphic features at a regional scale.     

“数字海底”--海底数据集成管理与可视化研究

针对目前"数字海底"建设中存在海底调查数据集成管理与三维可视化表达不足等问题,本文采用GIS技术,以黄河水下三角洲埕北海域为研究对象,利用地理空间数据集成理论与三维建模方法,建立了埕北海域三维海底空间数据库,实现了研究区域地形数据、地层数据、钻孔数据、表层沉积物数据的有效组织管理与可视化,并在此基础上,采用ArcGIS Engine 10.0,结合.NET平台,在Visual Studio 2010开发环境下,利用C#语言进行了二次开发,实现了基于C/S (Client/Server)架构的三维海底虚拟仿真系统的开发,设计了一套面向埕北海域的三维海底虚拟仿真原型系统。     

The first network of marine protected areas (MPAs) in the high seas: The process, the challenges and where next

Marine protected areas (MPAs) are increasingly being established to protect and rebuild coastal and marine ecosystems. However, while the high seas are increasingly subject to exploitation, globally few MPAs exist in areas beyond national jurisdiction. In 2010 a substantial step forward was made in the protection of high seas ecosystems with 286,200 km² of the North-East Atlantic established as six MPAs. Here a summary is presented of how the world's first network of high seas marine protected areas was created under the OSPAR Convention, the main challenges and a series of key lessons learned, aiming to highlight approaches that also may be effective for similar efforts in the future. It is concluded that the designation of these six MPAs is just the start of the process and to achieve ecological coherence and representativity in the North-East Atlantic, the network will have to be complemented over time by additional MPA sites.     

Phase-reversed seabed reflections in seismic data: examples related to mud volcanoes from the South Caspian Sea

The sediment–water interface is usually marked by an increase in acoustic impedance and is therefore displayed in a seismic section as a positive polarity reflection. Here, we use the term “seabed phase reversal” to describe areas of seafloor which are instead expressed as a negative polarity reflection in seismic data. We describe in detail a number of examples of seafloor phase reversals and use a simple one-dimensional geophysical model to test the hypothesis that they are the result of the presence of gas within the seafloor sediment. Our examples are all related to seismically imaged mud volcanoes located within the South Caspian Sea. Sections of phase-reversed seafloor at the summit area of these volcanoes have been mapped to reveal the existence of seafloor mud pools (salses) and recently erupted mud flows which show a strong similarity to smaller-scale features at onshore volcanoes in Azerbaijan. Synthetic geophysical modelling indicates that under the physical conditions likely to occur when the seabed sediment is gas-bearing, the seafloor will be expressed as a strong negative polarity reflection. Unlike other indicators of seafloor gas, such as pockmarks, which merely record the transient expulsion of fluids from sedimentary basins, seafloor phase reversals indicate the presence of gas in marine sediment at the time of survey acquisition. They therefore are of significance to engineering and site survey operations as well as the identification of biological communities and gas flux calculations.     

Investigation of the relationship between seafloor echo strength and sediment type—a case study in Jiaozgou Bay,China

In recent years, increasing attention is being given to investigating the relationships between seafloor echo strength and sediment type. This aspect was assessed in a course of a marine survey conducted in Jiaozhou Bay near the city of Qingdao, China, where various sediment types are known to exist. Multi-beam echo data and sediment samples were acquired, processed and analyzed. The findings demonstrate that multi-beam echo strengths are strongly related to seabed types such as rock, sand and clay. Thus, echo strength increases progressively with the increase in sand content in gravely, sandy, and clayey sediments. However, echo strength is more sensitive to changes in sand content in fine-grained sediment and less sensitive in coarse-grained sediment. Also, high shell content of sediments greatly enhances the acoustic reflection. These findings help to predict seafloor sediment types by analyzing echo strength.     

多波束测深海底混响仿真研究

海底混响是海洋混响的重要组成部分,采用模拟仿真进行验证分析,是仿真技术的一项重要应用。采用单元散射模型,研究在单发射阵元下,分布在同一直线上多个接收阵元接收的海底回波,忽略声波传播的相位起伏,只考虑振幅起伏,将传播损失、声吸收系数、海底反射损失、海底沉积层密度等参数带入海底混响仿真数学模型,仿真海底混响,使其更加接近海底的实际情况。     

中国东部边缘海潮波系统形成机制的模拟研究

以三维高精度潮波数值模拟为基础,动用系统分析方法和地理信息系统技术,对影响中国东部边缘海潮波系统的因素,包括入射潮波、科氏力、海区地形、岸线形状及底摩擦等,进行了模拟试验。结果表明,它们对潮波系统的模式有不同程度的作用,其中科氏力的有无、岸线形态的变化和水下地形的巨变化对潮波系统有重要影响。研究认为,中国东部边缘海潮波系统是一个海－潮相互作用的整体;在一定的入射潮波条件下,该区的海区条件、特别是岸     

Distribution of benthic marine invertebrates at northern latitudes ― An evaluation applying multi-algorithm species distribution models

Different techniques of species distribution modeling were applied to evaluate the distribution of eight benthic marine species in Icelandic waters. The species examined were Symplectoscyphus tricuspidatus, Stegopoma plicatile (both Hydrozoa), Prionospio cirrifera, Amphicteis gunneri (both Polychaeta), Desmosoma strombergi, Eurycope producta (both Isopoda), Andaniella pectinata and Harpinia crenulata (both Amphipoda). Information on 13 environmental variables (temperature mean, temperature mean SD, temperature minimum, temperature maximum, salinity mean, salinity mean SD, oxygen content, particulate organic carbon, seasonal variation index, bottom roughness, sediment thickness, acidification) and records of occurrences of these eight species was collated in an ArcGIS project. Modeling methods applied were MARS, TreeNet, and MaxENT. According to area under the receiver operating curve (AUC) model assessment values, models with moderate to outstanding discriminatory power were found for all species. There was a good overlap in the overall pattern of prediction for most species independent on the modeling technique. Among the three applied techniques MARS seemed to generalize most whereas TreeNet predictions very precisely reflected information from the training data set. The distribution of the selected benthic invertebrate species in Icelandic waters could be linked to a variety of environmental factors related to oceanography, seabed topography and human impact. Their multivariate interactions acted as a structuring force of species distribution, instead of just their one by one individual influence. The selected predictors varied between the different models for the same species. They substituted each other in different models. The expected distribution of the examined species was mapped for a seascape of known environmental settings. Such maps will serve as excellent references in future impact studies and enable the detection of changes in the distribution of benthic marine invertebrates.     

Seafloor acoustic remote sensing with multibeam echo-sounders and bathymetric sidescan sonar systems

This paper examines the potential for remote classification of seafloor terrains using a combination of quantitative acoustic backscatter measurements and high resolution bathymetry derived from two classes of sonar systems currently used by the marine research community: multibeam echo-sounders and bathymetric sidescans sonar systems. The high-resolution bathymetry is important, not only to determine the topography of the area surveyed, but to provide accurate bottom slope corrections needed to convert the arrival angles of the seafloor echoes received by the sonars into true angles of incidence. An angular dependence of seafloor acoustic backscatter can then be derived for each region surveyed, making it possible to construct maps of acoustic backscattering strength in geographic coordinates over the areas of interest. Such maps, when combined with the high-resolution bathymetric maps normally compiled from the data output by the above sonar systems, could be very effective tools to quantify bottom types on a regional basis, and to develop automatic seafloor classification routines.     

基于浅地层剖面的海底浅表层沉积物物理性质参数反演技术研究

海底浅表层（小于1 m）沉积物的物理性质,如粒度、孔隙度、密度等是海洋沉积学研究和海洋工程地质分析的重要内容,目前主要基于有限的海底取样或原位测试获取这些沉积物的物理性质。浅地层剖面是基于声学信号（频率几千赫兹）在沉积物中的传播得到可反映沉积地层结构的数据,其中的一些声学参数,如海底反射系数、波阻抗等与沉积物物理性质密切相关。如何充分而有效地利用浅地层剖面资料,反演得到剖面覆盖区海底浅表层沉积物的物理性质参数,极具科学意义和应用价值,且基于声学属性反演沉积物物理性质是当前研究的热点。为此,本文基于渤海LD16-3CEPA至LD10-1PAPD路由段的浅地层剖面数据和海底表层沉积物的实测物理参数,利用Biot-Stoll模型建立研究区海底反射系数和沉积物物理性质之间的关系,并基于浅地层剖面数据计算得到的海底反射系数,反演了研究区海底浅表层沉积物的孔隙度、密度、平均粒径等物理性质参数。其中反演的孔隙度、密度、平均粒径与实测孔隙度、密度、平均粒径基本相符,偏差度基本都在20%的偏差范围内,表明该反演方法在该区的应用是可行的。     

Multisource multibeam backscatter data: developing a strategy for the production of benthic habitat maps using semi-automated seafloor classification methods

The establishment of multibeam echosounders (MBES) as a mainstream tool in ocean mapping has facilitated integrative approaches towards nautical charting, benthic habitat mapping, and seafloor geotechnical surveys. The bathymetric and backscatter information generated by MBES enables marine scientists to present highly accurate bathymetric data with a spatial resolution closely matching that of terrestrial mapping, and can generate customized thematic seafloor maps to meet multiple ocean management needs. However, when a variety of MBES systems are used, the creation of objective habitat maps can be hindered by the lack of backscatter calibration, due for example, to system-specific settings, yielding relative rather than absolute values. Here, we describe an approach using object-based image analysis to combine 4 non-overlapping and uncalibrated (backscatter) MBES coverages to form a seamless habitat map on St. Anns Bank (Atlantic Canada), a marine protected area hosting a diversity of benthic habitats. The benthoscape map was produced by analysing each coverage independently with supervised classification (k-nearest neighbor) of image-objects based on a common suite of 7 benthoscapes (determined with 4214 ground-truthing photographs at 61 stations, and characterized with backscatter, bathymetry, and bathymetric position index). Manual re-classification based on uncertainty in membership values to individual classes—especially at the boundaries between coverages—was used to build the final benthoscape map. Given the costs and scarcity of MBES surveys in offshore marine ecosystems—particularly in large ecosystems in need of adequate conservation strategies, such as in Canadian waters—developing approaches to synthesize multiple datasets to meet management needs is warranted.     

A geomorphological seabed classification for the Weddell Sea,Antarctica

Sea floor morphology plays an important role in many scientific disciplines such as ecology, hydrology and sedimentology since geomorphic features can act as physical controls for e.g. species distribution, oceanographically flow-path estimations or sedimentation processes. In this study, we provide a terrain analysis of the Weddell Sea based on the 500 m × 500 m resolution bathymetry data provided by the mapping project IBCSO. Seventeen seabed classes are recognized at the sea floor based on a fine and broad scale Benthic Positioning Index calculation highlighting the diversity of the glacially carved shelf. Beside the morphology, slope, aspect, terrain rugosity and hillshade were calculated and supplied to the data archive PANGAEA. Applying zonal statistics to the geomorphic features identified unambiguously the shelf edge of the Weddell Sea with a width of 45–70 km and a mean depth of about 1200 m ranging from 270 m to 4300 m. A complex morphology of troughs, flat ridges, pinnacles, steep slopes, seamounts, outcrops, and narrow ridges, structures with approx. 5–7 km width, build an approx. 40–70 km long swath along the shelf edge. The study shows where scarps and depressions control the connection between shelf and abyssal and where high and low declination within the scarps e.g. occur. For evaluation purpose, 428 grain size samples were added to the seabed class map. The mean values of mud, sand and gravel of those samples falling into a single seabed class was calculated, respectively, and assigned to a sediment texture class according to a common sediment classification scheme.     

Shallow-water imaging multibeam sonars: A new tool for investigating seafloor processes in the coastal zone and on the continental shelf

Hydrographic quality bathymetry and quantitative acoustic backscatter data are now being acquired in shallow water on a routine basis using high frequency multibeam sonars. The data provided by these systems produce hitherto unobtainable information about geomorphology and seafloor geologic processes in the coastal zone and on the continental shelf.Before one can use the multibeam data for hydrography or quantitative acoustic backscatter studies, however, it is essential to be able to correct for systematic errors in the data. For bathymetric data, artifacts common to deep-water systems (roll, refraction, positioning) need to be corrected. In addition, the potentially far greater effects of tides, heave, vessel lift/squat, antenna motion and internal time delays become of increasing importance in shallower water. Such artifacts now cause greater errors in hydrographic data quality than bottom detection. Many of these artifacts are a result of imperfect motion sensing, however, new methods such as differential GPS hold great potential for resolving such limitations. For backscatter data, while the system response is well characterised, significant post processing is required to remove residual effects of imaging geometry, gain adjustments and water column effects. With the removal of these system artifacts and the establishment of a calibrated test site in intertidal regions (where the seabed may be intimately examined by eye) one can build up a sediment classification scheme for routine regional seafloor identification.When properly processed, high frequency multibeam sonar data can provide a view of seafloor geology and geomorphology at resolutions of as little as a few decimetres. Specific applications include quantitative estimation of sediment transport rates in large-scale sediment waves, volume effects of iceberg scouring, extent and style of seafloor mass-wasting and delineation of structural trends in bedrock. In addition, the imagery potentially provides a means of quantitative classification of seafloor lithology, allowing sedimentologists the ability to examine spatial distributions of seabed sediment type without resorting to subjective estimation or prohibitively expensive bottom-sampling programs. Using Simrad EM100 and EM1000 sonars as an example, this paper illustrates the nature and scale of possible artifacts, the necessary post-processing steps and shows specific applications of these sonars.     

南海东沙群岛以北海底沙波稳定性分析

利用在南海东沙群岛北部至西北海域为工程目的所采集的海底地形测量、底质取样、海底底流测量资料,以及以往的区域地质、地球物理资料,采用沉积学、构造学和海洋水动力学综合研究方法,对该区海底沙波形成及稳定性进行了综合分析。认为该区海底沙波是在目前水动力环境下所形成,为今生,海底表层沉积物为晚更新世地层受到冲刷改造的再沉积。沙波形成主要受潮流底流的水动力作用控制,并与构造抬升有关。沙波具有一定的活动性,主要由SE向NW方向移动,且活动性逐渐减弱。北区段海底沙波稳定,中区段海底沙波较稳定,它们对海底工程建设不会造成影响;而南区段海底沙波较为活动,当沙波发生较大规模移动时,对海底工程建设会造成一定影响。     

Horizontal and vertical distributions of <Superscript>137</Superscript>Cs in seabed sediments around the river mouth near Fukushima Daiichi Nuclear Power Plant

Investigations including a bathymetric survey, sonic prospecting, and vibrocoring were performed to understand the horizontal and vertical distribution of ¹³⁷Cs in seabed sediments in shallow seas with depths less than 30 m near the Fukushima Daiichi Nuclear Power Plant. Especially, features of ¹³⁷Cs distributions in deeper sections of the seabed sediments were studied to evaluate the vertical heterogeneity of ¹³⁷Cs distribution in the seabed sediments in shallow seas. The distribution area of the seabed sediments was less than half of the investigation area, and the locations of the seabed sediments were divided into flat and terrace-like seafloors based on their topographical features. The thicknesses of the seabed sediment layers were mostly <2 m. The ¹³⁷Cs inventories in the seabed sediments varied from 13 ± 1 to 3,510 ± 26 kBq m^?2, and continuous distributions of ¹³⁷Cs at depths greater than 81 cm were observed. The ¹³⁷Cs distributions were not uniform; however, the ¹³⁷Cs inventories tended to be larger near the base of the steeper ascending slopes than in the terrace-like seafloors themselves. Based on the relationship between the ¹³⁷Cs inventories and mean shear stress, features of the seafloor topography were inferred to be significant control factors governing the horizontal and vertical distribution of ¹³⁷Cs in the seabed sediments. Rapid changes and multiple peaks in the vertical profile of the ¹³⁷Cs distributions suggest that they are related to pulse input caused by heavy-rain events. Change in the ¹³⁷Cs inventories with depth in this study are larger than those reported in previous studies, indicating earlier results of ¹³⁷Cs inventories per unit in seabed sediments in shallow seas, especially near the river mouth, which drains a radiologically highly-contaminated basin, were underestimated.