Acoustic seabed segmentation from direct statistical clustering of entire multibeam sonar backscatter curves

A fast, simple method is presented to obtain acoustic seabed segmentation from multibeam sonar backscatter data, for situations where processed backscatter curves are already available. Unsupervised statistical clustering is used to classify multibeam sonar backscatter curves in their entirety, with the curves essentially treated as geometrical entities. High variability in the backscatter curves is removed by along-track averaging prior to clustering, and no further preprocessing is required. The statistical clustering method is demonstrated with RESON 8125 multibeam sonar data obtained in two bathymetrically complex environments. These are a sandwave field in Keppel Bay, Queensland, and an area of inter-island sand, reef, seagrass, and rhodolith beds in Esperance Bay, Western Australia. The resulting acoustic charts are visually compelling. They exhibit high spatial coherence, are largely artifact free, and provide spatial context to comparatively sparse grab samples with relatively little effort. Since the backscatter curve is an intrinsic property of the seafloor, the mappings form standalone charts of seafloor acoustic properties. In themselves they do not need ground truthing. Conceptually, use of the full angular backscatter curve should form the primary means of obtaining acoustic seabed segmentation. However, this is dependent on the scale and configuration of seabed backscatter features compared to the dimensions of the averaged swathe used to obtain reliable realisations of the backscatter curve.     

基于合成孔径声学深拖调查的海底浅表层流体活动研究——以SAMS DT6000深拖在琼东南海域调查为例

声学深拖作为一个声学设备搭载平台,主要功能是获取高分辨率的声学数据,精细刻画海底地形地貌特征以及浅层剖面结构,对于研究海底浅表层流体活动系统的类型、形成机制和演化模式有着重要作用.本文介绍的合成孔径声学深拖（Synthetic Aperture Sonar Deep-tow）搭载了合成孔径声呐、浅地层剖面仪以及多波束系统等声学设备,相比于传统的侧扫声呐,合成孔径声呐采用小物理孔径基阵通过信号处理虚拟合成大孔径基阵来获得方位向高分辨率,大大提高了测绘速率,同时结合高分辨率的浅地层剖面和多波束背散射数据,可实现海底浅表层特征的三维立体显示.为查明调查区海底浅层流体活动的声学特征,分析天然气水合物相关的流体渗漏活动性与浅层构造之间的关系,我们利用声学深拖对研究区进行了全覆盖的扫测,获得了高分辨率的合成孔径声呐图像、浅地层剖面资料以及多波束背散射数据,平面上识别出多个呈条带状的海底丘状体,火焰状的流体渗漏,新月形的麻坑构造等流体活动地质构造;浅层剖面上可见气体聚集的声学空白段落,凸起的活跃喷口,以及反射杂乱的柱状浑浊带.通过识别流体活动的特征,我们总结了浅层流体活动演化模式具有周期性：游离气体通过高渗透运移通道上升至海底,首先扩散聚集造成局部沉积物体积膨胀形成丘状体;然后受其各种外界因素影响丘状体崩塌而引起气体渗漏;最后流体逸散剥蚀海底松散沉积物而形成麻坑构造;随着流体排出,喷口重新闭合,流体在地层中再次聚集,聚集的气体又将沉积地层上拱,在麻坑底部又可能生成含气丘状体.海底浅表层蕴藏着丰富的地质信息,这对于研究海底复杂的流体活动有着重要意义.     

Unsupervised fuzzy classification and object-based image analysis of multibeam data to map deep water substrates, Cook Strait, New Zealand

A comprehensive 32 kHz multibeam bathymetry and backscatter survey of Cook Strait, New Zealand (∼8500 km²), is used to generate a regional substrate classification map over a wide range of water depths, seafloor substrates and geological landforms using an automated mapping method based on the textural image analysis of backscatter data. Full processing of the backscatter is required in order to obtain an image with a strongly attenuated specular reflection. Image segmentation of the merged backscatter and bathymetry layers is constrained using shape, compactness, and texture measures. The number of classes and their spatial distribution are statistically identified by employing an unsupervised fuzzy-c-means (FCM) clustering algorithm to sediment samples, independent of the backscatter data. Classification is achieved from the overlay of the FCM result onto a segmented image and attributing segments with the FCM class. Four classes are identified and uncertainty in class attribution is quantified by a confusion index layer. Validation of the classification map is done by comparing the results with the sediment and structural maps. Backscatter (BS) strength angular profiles are used to show acoustic class separation. The method takes us one step further in combining multibeam data with physical seabed data in a complementary analysis to seek correlations between datasets using object-based image analysis and unsupervised classification. Texture within these identified classes is then examined for correlation with typical backscatter angular responses for mud, sand and gravel. The results show a first order correlation between each of the classes and both the sedimentary properties and the geomorphological map.     

Bed diversity in the shallow water environment of Pappas lagoon in Greece

<正>Acoustic classification systems and the Sediment Trend Analysis method were used to identify and map the bed diversity in a very shallow(3.0m),coastal lagoon,Pappas lagoon,Western Greece. Analogue acoustic data,collected by means of a 100 kHz side scan sonar system,were digitized and classified into six acoustic classes using recently developed acoustic classification systems (SonarClass,TargAn).By comparing the acoustic classes to ground truth data consisting of sediment grain size and visual inspection of the lagoon-bed,it is demonstrated that the six acoustic classes correlate well with the predominant surface sediment types and vegetation.Thus the spatial distribution of the classes can be considered to represent the spatial pattern of the sedimentary assemblages of the lagoon.The grain size trend analysis identified three dominant sediment pathways and directional trends which could be related to the predominant wind direction,the sediment influx through the inlets and the sediment supply from a small stream in the southern part of the lagoon.The integration of acoustic and sedimentological data together with advanced data processing systems leads to a better understanding of the sedimentary,morphological and biological processes in a shallow lagoon in different spatial and temporal scales and will therefore be beneficial to both sedimentological and biotic-diversity studies.     

Use of the Jiaolong manned submersible for accurate mapping of deep-sea topography and geomorphology

High-resolution bathymetric side-scan sonar (BSSS) performs the functions of traditional side-scan sonar, while also providing a depth-sounding function that allows simultaneous measurement of seafloor topography and geomorphology. Submarine microtopography and microgeomorphology detection ability and advanced underwater acoustic digital communication are important technical capabilities of the Jiaolong manned submersible. High resolution BSSS achieved accurate detection of seafloor topography and geomorphology at a depth of 7000 m, and successful mapping of local microtopography and microgeomorphology in the Mariana Trench.     

The use of autonomous underwater vehicles to map the variability of under-ice topography

We review the development of autonomous underwater vehicle (AUV) use under sea ice to map the three-dimensional (3-D) structure of the ice underside. The author, after extensive experience in under-ice profiling from submarines using single-beam sonar, carried out the first under-ice sidescan sonar profiling from an AUV in 2002 in the Greenland Sea. This was followed in August 2004 by the first full multibeam sonar experiment, using Kongsberg EM2000 sonar aboard the Autosub-II vehicle off NE Greenland. Two experiments using a small Gavia vehicle deployed through holes in the ice followed in 2007 and 2008, in the Beaufort Sea and off Ellesmere Island. Examples of the 3-D imagery are shown, and the two approaches of using a large vehicle deployed from a ship and a small through-ice vehicle are compared and found to be complementary. The imagery has shown that although first-year (FY) ridges have the familiar shape of a triangular prism made of small ice blocks, multi-year (MY) ridges are found to be broken up by lead formation into a chain of individual large ice blocks rather than a coherent linear feature. New work and future plans are described.     

海洋地球物理研究与海底探测声学技术的发展

海洋地球物理以物理学的思维与方法研究占地球三分之二面积的海洋系统.20世纪地球科学迅猛发展，它的重大进展是海底扩张说与板块构造说的出现和海底大洋的发现，以及前者所引发的地球科学思想革命，从固定论向活动论的思维转变.海底研究对于20世纪地球科学发展的贡献极为巨大，而海洋地球物理是推动海底科学研究的重要原动力.海洋地球物理在20世纪地球科学的发展中有过辉煌的成就，占有十分重要的地位；在新的21世纪里，海洋地球物理研究仍然保持着前沿科学的地位，继续推动着地球科学的进展.目前的海底探测主要还是依赖于声学探测技术.水下声学定位技术是实现水下探测系统精确定位和海底高精度探测的基础.传统性的海洋地震探测技术是研究海底构造与海洋岩石圈深部结构和寻找海底矿产的主力技术，它近年来无论在海上采集技术还是数据处理技术方面都发展得很快.多波束测深、侧扫声呐测图和海底地层剖面测量等则是近数十年快速发展起来探测海底浅部结构信息的技术.这些技术已经在当代海底科学研究、海底资源勘查、海洋工程和海洋开发，以及海洋军事活动等方面发挥出极其重要的作用.     

Modelling distribution of marine benthos from hydroacoustics and underwater video

Broad-scale mapping of marine benthos is required for marine resource management and conservation. This study combines textural derivatives based on bathymetry from multibeam hydroacoustics with underwater video observations to model and map sessile biota between 10- and 60-m water depth over 35 km² in Point Addis Marine National Park (MNP), Vic., Australia. Classification tree models and maps were developed for macroalgae (all types, mixed red algae, Ecklonia, and rhodoliths) and sessile invertebrates (all types, sponges, and ascidians). Model accuracy was tested on 25% of the video observation dataset reserved from modelling. Models fit well for most macroalgae categories (correct classification rates of 67–84%), but are not as good for sessile invertebrate classes (correct classification rates of 57–62%). The poor fit of the sessile invertebrate models may be the combined result of grouping organisms with different environmental requirements and the effect of false absences recorded during video interpretation due to poor image quality. Probability maps, binary single-class maps, and multi-class maps supply spatially explicit, detailed information on the distribution of sessile benthic biota within the MNP and provide information at a landscape-scale for ecological investigations and marine management.     

Submarine tumuli and inflated tube-fed lava flows on Axial Volcano,Juan de Fuca Ridge

A seafloor lava field was mapped within the summit caldera of Axial Volcano, Juan de Fuca Ridge, using SeaMARC I sidescan sonor and submersible observations. By analogy with similar subaerial features, we infer that several volcanic seafloor features here formed by the process of lava flow inflation. Flow inflation occurs within tube-fed lava flows when lava continues to be supplied to the interior of a flow that has ceased advancing, thus uplifting the flow's rigid surface and creating a suite of characteristic surface structures. Inflated lavas require a feeder lava tube or tube system connected to a remote lava source, and therefore we infer that inflated submarine lava flows contain lava tubes. Inflated flow features identified from sidescan sonar images elsewhere on Axial Volcano and within the axial valley of the southern Juan de Fuca ridge suggest that flow inflation is a widespread submarine volcanic process.     

结合遗传算法的LVQ神经网络在声学底质分类中的应用

学习向量量化（Learning Vector Quantization，LVQ）神经网络在声学底质分类中具有广泛应用. 常用的LVQ神经网络存在神经元未被充分利用以及算法对初值敏感的问题，影响底质分类精度. 本文提出采用遗传算法（Genetic Algorithms，GA）优化神经网络的初始值，将GA与LVQ神经网络结合起来，迅速得到最佳的神经网络初始权值向量，实现对海底基岩、砾石、砂、细砂以及泥等底质类型的快速、准确识别. 将其应用于青岛胶州湾海区底质分类识别研究中，通过与标准的LVQ神经网络的分类结果进行比较表明，该方法在分类速度以及精度上都有了较大提高.     

Post-glacial sediment dynamics in the Irish Sea and sediment wave morphology: Data–model comparisons

The irregular seafloor of the narrow Irish Sea on the NW European Shelf has been documented over several decades. From recently collected swath bathymetry data, very large trochoidal, nearly symmetrical sediment waves are observed in many parts of the Irish Sea and appear similar to those described from other continental shelf seas in North America that were covered by glacigenic sediments during the Last Glacial Maximum. Swath multibeam and single beam bathymetry data, backscatter intensity, shallow seismic imagery, video footage and sediment cores from the Irish Sea high sediment waves have been integrated to identify their genesis with reference to present and past hydrodynamic variability. From cross-sectional profiles over asymmetrical sediment waves in the Irish Sea the direction of asymmetry is used to map residual bed stress directions and associated bedload transport paths. Irish Sea peak bed stress vectors were generated using a two-dimensional palaeo-tidal model for the NW European shelf seas and compare well with the observations. Tidally induced bed stresses are modelled to have increased between 7–10 ka BP, to be nearly symmetrical in magnitude and to have reversed in dominant direction on a millennial scale. These environmental conditions during the post-glacial marine transgression are suggested here to help comprehend the construction of the very large sediment waves, with local variations due to differences in sediment grain size, sediment supply, water depth and intensified currents due to seafloor slopes. Model parameterisation using an open ocean boundary with time-dependent tidal changes and the implementation of high-resolution bathymetric information will improve future models of small-scale bed shear stress patterns and improve the predictive value of such modelling efforts.     

一种新型海底沉积物声学原位测量系统的研制及应用

为了能够精确地测量海底表层沉积物的声学参数,自主研制了一种新型海底沉积物声学原位测量系统,与国内外传统的声学原位测量系统相比,该系统能够实时显示声波波形,调整测量参数,其工作方式除了站位式测量之外,还实现了拖行式连续测量,极大地提高了工作效率.根据前期海试情况,对海底仪器结构进行了重新设计,使之可以同时测量海底沉积物及海底海水的声学参数,同时建立了双向数字信道,解决了测量过程中系统信号的干扰问题.该系统的结构分为两部分:甲板控制单元和水下测量单元,整套系统通过主机控制程序进行控制,采用GPS定位系统测定仪器的大地坐标.为了检验系统的稳定性及准确性,分别进行了实验室水槽实验和海上试验.利用水声测量设备对测量系统进行实验室水槽标定分析,实验结果表明系统测量值相对误差仅为0.04%,测量结果具有较高的精度.海上试验在青岛胶州湾和东海海域进行,获得了试验区域海底沉积物声速和声衰减系数的测量数据,将测量数据与他人的研究结果进行对比分析,结果表明测量数据与前人研究结果一致,较为准确.该原位测量系统在站位式测量和拖行式测量中都能够快速准确地测量出沉积物声速和声衰减系数,可以作为海底底质声学测量的调查设备.     

莺歌海盆地中央坳陷带海底天然气渗漏系统初探

地球物理资料表明,莺歌海盆地区麻坑众多,浅层气发育,在高分辨率地震剖面上可以识别出到达海底的地震模糊带,在3.5 kHz的浅层剖面上可见穿透到海底的气体聚集带,在声呐图像可以识别出麻坑.莺歌海盆地不仅在其边缘隆起带沉积层较薄的地区存在海底天然气渗漏现象,而且在中央坳陷带沉积层巨厚的地区也存在海底天然气渗漏,海底天然气渗漏在莺歌海盆地是广泛分布的.莺歌海盆地广泛分布的海底天然气渗漏是全球海底天然气渗漏系统的重要组成部分,海底天然气渗漏系统排出的大量甲烷气体的一部分可能穿越水层到达海面.     

Multi-beam backscatter measurements used to infer seabed habitats

Backscatter from multi-beam sonar (MBS) was used to discriminate ecologically relevant seabed characteristics based on 62 reference sites sampled with georeferenced video, sediment grab and rock dredge between 50 and 500 m water depth. A simple biotope characteristic of soft (unconsolidated) and hard (consolidated) was used to compare the acoustic backscatter data with the data on mega-epifauna and substrate type obtained from video and physical sampling. Substrate type of homogeneous reference sites was predicted by matching the backscatter incidence angle profile (0–70°) to that of a seabed scattering model. Referencing the seabed backscatter to a consistent incidence angle (40°) gave a metric with high spatial resolution (2.4–20 m), which minimised errors of range, incident angle and beam compensation. This simple metric provided a consistent approach to analyse and interpret the data and was strongly correlated with substrate type and faunal functional groups. The high resolution backscatter metric was a closer match to the small spatial scale of seabed patch lengths observed by video (50% <50 m).     

深渊着陆器坐底位置的精确测量和反演计算

深渊着陆器在下潜过程中,由于受到海流的影响,其坐底位置会偏离投放点,偏离量和偏离方向很难估计,而声学水下定位方法在深渊海区不太适用.通过在海面进行气枪十字放炮作业,利用炮点准确位置和时间信息,以及海底地震仪(Ocean Bottom Seismometer,OBS)记录的直达水波到时,对深渊着陆器的坐底位置进行精确测量可以提供有效定位数据.该方法对采集得到的数据进行预处理,使用蒙托卡洛方法结合最小二乘法,对万泉号着陆器实际坐底位置进行了反演校正,得到三个潜次的位置校正偏移量分别为211m、178m、861m,偏移方向各不相同,校正精度为±20m,比以往的定位精度大为提高.此外,该方法测量确定了三个海底标志物的精确位置,为未来万米无人或载人潜器提供了可靠的水下参考坐标.     

The potential impact of bedform migration on seagrass communities in Torres Strait,northern Australia

Seagrass communities in the northwest of Torres Strait are known to disappear episodically over broad areas. Sediment mobility surveys were undertaken within two study areas during the monsoon and trade wind seasons, in the vicinity of Turnagain Island, to find out if the migration of bedforms could explain this disappearance. The two study areas covered sand bank and sand dune environments to compare and contrast their migration characteristics. Repeat multibeam sonar surveys were used to measure dune-crest migration during each season.     

Characterizing uncertainties for quantifying bathymetry change between time-separated multibeam echo-sounder surveys

Changes of bathymetry derived from multibeam sonars are useful for quantifying the effects of many sedimentary, tectonic and volcanic processes, but depth changes also require an assessment of their uncertainty. Here, we outline and illustrate a simple technique that aims both to quantify uncertainties and to help reveal the spatial character of errors. An area of immobile seafloor is mapped in each survey, providing a common ‘benchmark’. Each survey dataset over the benchmark is filtered with a simple moving-averaging window and depth differences between the two surveys are collated to derive a difference histogram. The procedure is repeated using different length-scales of filtering. By plotting the variability of the differences versus the length-scale of the filter, the different effects of spatially uncorrelated and correlated noise can be deduced. The former causes variability to decrease systematically as predicted by the Central Limit Theorem, whereas the remaining variability not predicted by the Central Limit Theorem then represents the effect of spatially correlated noise. Calculations made separately for different beams can reveal whether problems are due to heave, roll, etc., which affect inner and outer beams differently. We show how the results can be applied to create a map of uncertainties, which can be used to remove insignificant data from the bathymetric change map. We illustrate the technique by characterizing changes in nearshore bed morphology over one annual cycle using data from a subtidal bay, bedrock headland and a banner sand bank in the Bristol Channel UK.     

Shelf habitat distribution as a legacy of Late Quaternary marine transgressions: A case study from a tropical carbonate province

The legacy of multiple marine transgressions is preserved in a complex morphology of ridges, mounds and reefs on the Carnarvon continental shelf, Western Australia. High-resolution multibeam sonar mapping, underwater photography and sampling across a 280 km² area seaward of the Ningaloo Coast World Heritage Area shows that these raised features provide hardground habitat for modern coral and sponge communities. Prominent among these features is a 20 m high and 15 km long shore-parallel ridge at 60 m water depth. This ridge preserves the largely unaltered form of a fringing reef and is interpreted as the predecessor to modern Ningaloo Reef. Landward of the drowned reef, the inner shelf is covered by hundreds of mounds (bommies) up to 5 m high and linear ridges up to 1.5 km long and 16 m high. The ridges are uniformly oriented to the north-northeast and several converge at their landward limit. On the basis of their shape and alignment, these ridges are interpreted as relict long-walled parabolic dunes. Their preservation is attributed to cementation of calcareous sands to form aeolianite, prior to the post-glacial marine transgression. Some dune ridges abut areas of reef that rise to sea level and are highly irregular in outline but maintain a broad shore-parallel trend. These are tentatively interpreted as Last Interglacial in age. The mid-shelf and outer shelf are mostly sediment covered with relatively low densities of epibenthic biota and have patches of low-profile ridges that may also be relict reef shorelines. An evolutionary model for the Carnarvon shelf is proposed that relates the formation of drowned fringing reefs and aeolian dunes to Late Quaternary eustatic sea level.     

Photogrammetric quantitative study of habitat and benthic communities of deep Cantabrian Sea hard grounds

To study the highly complex deep-sea habitats of the Cantabrian Sea and their macro-epibenthic communities a new towed underwater sled was designed to carry out quantitative visual transects based on photogrammetric analysis. The main objective of the study was undertaken to provide a first approach for gaining a better understanding of the correlation between hard substrates, depth and ecology in this region; thereby enabling researchers to determine the extent to which benthic communities depend on physical factors.The results were compared from two areas with different characteristics and methodological problems: one in the central Cantabrian Sea outer shelf (150 m depth), near the head of the Lastres Canyon, and another at the summit of the Le Danois Bank (555 m depth). Two image databases corresponding to two transects were analysed, with every photo being linked to a faunal list and a set of environmental variables. To assess the amount of variation in faunal densities related to the set of habitat environmental characteristics, a redundancy analysis (RDA) was used. The set of environmental variables comprised depth, temperature, salinity, substrate type and seafloor reflectivity. Using the hierarchical classification proposed by EUNIS, three habitats were identified from a Cantabrian Sea shelf visual transect: A4.12—Sponge communities on circalittoral rock (14.5% coverage), A5.35—Circalittoral sandy mud (56.8%) and A5.44—Circalittoral mixed sediments (28.7%). A typical community appeared on the rocky habitat, made up of yellow coral Dendrophyllia cornigera and the cup sponge Phakellia ventilabrum. On Le Danois Bank, three habitats were identified and the cnidarians (Caryophyllia smithii and Callogorgia verticillata) and the sponges (Asconema setubalense, Aplysilla sp., hexactinellids) characterized rocky habitats and patchy rock-sand habitats.This study provided groundtruthing for the existing surficial seafloor features and very valuable information about the autoecology of sessile and vulnerable species, since the scale used was more appropriate for this kind of study than the macroscale of trawling, with several microhabitats being sampled in the same sample. Also, the use of this non-extractive methodology, which does not cause damage or alterations to benthic communities, is particularly necessary in vulnerable ecosystem studies and Marine Protected Areas monitoring.     

Shallow gas accumulations and seepage in the sediments of the Northeast Persian Gulf

Shallow gas accumulations and gas seepage may indicate deeper hydrocarbon reservoirs. Sonar and high resolution seismic recordings were carried out to obtain gas migration and accumulation below seabed in Northeast Persian Gulf. Results revealed some structures and reflectivity effects, for example few pockmarks, gas seepage, and other surface structures in sonar and seismic data. Most of the features observed on seismic data were located at depths between 5 and 10 m (sub-sea bed) that include acoustic blanking and turbidity, gas packets, gas chimneys, and enhanced reflection. Most of the gas is accumulated on young and thin parallel layers, but the presence of acoustic effect in Aghajari Formation indicates that the origin of the gas accumulation in shallow strata may be located in a deeper reservoir.