The potential impact of shell fragment distributions on high-frequency seafloor backscatter

There has been much recent activity on developing and testing high-frequency seafloor volume scattering models and on the related high-resolution characterization of seafloor volumes. This paper will address another plausible volume scattering mechanism that has not yet received much attention-distributions of broken shell fragments in sediment (commonly referred to as shell hash). The shell fragments are modeled as nonaggregating spherical scatterers, and the spatial distribution of the shell pieces is determined using the Percus-Yevick packing factor. Computer simulations of the multiple scattering and a single scattering model are used to study the power backscattered by shell hash sediments as a function of the volume of scatterers and the frequency of the incident wave (10-100 kHz). Parameter values for simulations are obtained by stereological analysis of x-ray computed tomography scans of sediment cores.     

An inverse method for reconstructing the density and sound speed profiles of a layered ocean bottom

A standard inverse problem in underwater acoustics is the reconstruction of the ocean subbottom structure (e.g., the density and sound speed profiles) from an aperture- and bandlimited knowledge of the reflection coefficient. In this paper we describe an inverse solution method due to Candel et al. [12] which is based on the scattering of acoustic plane waves by a one-dimensional inhomogeneous medium. As a consequence of applying the forward scattering approximation to a local wave representation of the acoustic field, they obtain an expression for the reflection coefficient in the form of a nonlinear Fourier transform of the logarithmic derivative of the local admittance. Inversion of this integral transform enables the recovery of the admittance profile via the numerical integration of two first-order differential equations which require as reflection data a single impulse response of the medium. Separate recovery of both the density and sound speed profiles requires two impulse responses for two different grazing angles. In this case, four differential equations need to be integrated instead of two. To illustrate the capability of the method, we present numerical reconstructions which are based on synthetic reflection data for a geoacoustic model that represents the acoustic properties of the surficial sediments for a site in the Hatteras Abyssal Plain.     

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。     

Tests of models for high-frequency seafloor backscatter

The interaction of high-frequency sound with the seafloor is inherently a stochastic process. Inversion techniques must, therefore employ good stochastic models for bottom acoustic scattering. An assortment of physical models for bottom backscattering strength is tested by comparison with scattering strength data obtained at 40 kHz at three shallow water sites spanning a range of sediment types from fine silt to coarse sand. These acoustic data are accompanied by sediment physical property data obtained by core sample analysis and in situ probes. In addition, stereo photography was used to measure the power spectrum of bottom relief on centimeter scales. These physical data provided the inputs needed to test the backscatter models, which treat scattering from both the rough sediment-water interface and the sediment volume. For the three sites considered here, the perturbation model for scattering from a slightly rough fluid seafloor performs well. Volume scattering is predicted to be weak except at a site having a layer of methane bubbles     

海底地形变化对地震波场以及偏移成像的影响

本文针对海洋地球物理勘探数据处理当中偏移成像剖面中深层层位模糊,像场能量较弱的问题,从起伏海底在反射地震数据采集当中某个时刻发生变化着手,分析讨论了当海底发生微小变化以及海底发生稍大变化时模型差异对波场记录以及偏移成像剖面的影响,通过分析对比原始速度模型以及变化后速度模型同一点位上采集的波场记录以及波场记录在变化前后模型上的偏移成像剖面可以得出：不管数据采集是沿着测线方向进行还是逆着测线方向进行,对于起伏海底发生微小变化的速度模型,海底的微小变化对波场记录以及偏移剖面的影响很小;对于起伏海底发生较大变化的速度模型,海底的变化对波场记录以及偏移剖面的影响非常大。     

Backscattering from bioturbated sediments at very high frequency

Recent backscattering measurements made in the Gulf of La Spezia, Italy, using a sonar operating at 140 kHz combined with thorough characterization of seabed interface and volume properties illustrate the importance of seabed volume scattering. Three-dimensional fluctuation statistics of density variability and vertical density gradients, both of which are attributed to the level of bioturbation (e.g., sea shell fragments, burrows, pockets of water) have been quantified using X-Ray computed tomography. Two-dimensional interface roughness spectra have also been determined using a digital stereo photogrammetry system. The combined ground truth has allowed a backscattering model to be fully constrained. Measured backscattering strength versus angle is compared to a model that includes the effects of varying density and sound speed. Data-model comparisons show that scattering from the volume of strongly inhomogeneous sediments can often be a primary contributor to seafloor scattering away from normal incidence.     

Processing of high-frequency multibeam echo sounder data for seafloor characterization

Processing simultaneous bathymetry and backscatter data, multibeam echosounders (MBESs) show promising abilities for remote seafloor characterization. High-frequency MBESs provide a good horizontal resolution, making it possible to distinguish fine details at the water-seafloor interface. However, in order to accurately measure the seafloor influence on the backscattered energy, the recorded sonar data must first be processed and cleared of various artifacts generated by the sonar system itself. Such a preprocessing correction procedure along with the assessment of its validity limits is presented and applied to a 95-kHz MBES (Simrad EM 1000) data set. Beam pattern effects, uneven array sensitivities, and inaccurate normalization of the ensonified area are removed to make possible further quantitative analysis of the corrected backscatter images. Unlike low-frequency data where the average backscattered energy proves to be the only relevant feature for discriminating the nature of the seafloor, high-frequency MBES backscatter images exhibit visible texture patterns. This additional information involves different statistical distributions of the backscattered amplitudes obtained from various seafloor types. Non-Rayleigh statistics such as K-distributions are shown to fit correctly the skewed distributions of experimental high-frequency data. Apart from the effect of the seafloor micro-roughness, a statistical model makes clear a correlation between the amplitude statistical distributions and the signal incidence angle made available by MBES bathymetric abilities. Moreover, the model enhances the effect of the first derivative of the seafloor backscattering strength upon statistical distributions near the nadir and at high incidence angles. The whole correction and analysis process is finally applied to a Simrad EM 1000 data set.     

Effects of using inclined parametric echosounding on sub-bottom acoustic imaging and advances in buried object detection

This study reports an adaptation of a parametric echosounder system using 15 kHz as secondary frequency to investigate the angular response of sub-bottom backscatter strength of layered mud, providing a new method for enhanced acoustic detection of buried targets. Adaptions to achieve both vertical (0°) and non-vertical inclination (1–15°, 30°, 45° and 60°) comprise mechanical tilting of the acoustic transducer and electronic beam steering. Data were acquired at 18 m water depth at a study site characterized by a flat, muddy seafloor where a 0.1 m diameter power cable lies 1–2 m below the seafloor. Surveying the cable with vertical incidence revealed that the buried cable can hardly be discriminated against the backscatter strength of the layered mud. However, the backscatter strength of layered mud decreases strongly at >3±0.5° incidence and the layered mud echo pattern vanishes beyond 5°. As a consequence, the backscatter pattern of the buried cable is very pronounced in acoustic images gathered at 15°, 30°, 45° and 60° incidence. The size of the cable echo pattern increases linearly with incidence. These effects are attributed to reflection loss from layered mud at larger incidence and to the scattering of the 0.1 m diameter buried cable. Data analyses support the visual impression of superior detection of the cable with an up to 2.6-fold increase of the signal-to-noise ratio at 40° incidence compared to the vertical incidence case.     

Overview of SAX99: environmental considerations

A 1-km² area located 2 km off the Florida Panhandle (30°22.6'N; 86°38.7'W) was selected as the site to conduct high-frequency acoustic seafloor penetration, sediment propagation, and bottom scattering experiments. Side scan, multibeam, and normal incidence chirp acoustic surveys as well as subsequent video surveys, diver observations, and vibra coring, indicate a uniform distribution of surficial and subbottom seafloor characteristics within the area. The site, in 18-19 m of water, is characterized by 1-2-m-thick fine-to-medium clean sand and meets the logistic and scientific requirements specified for the acoustic experiments. This paper provides a preliminary summary of the meteorological, oceanographic, and seafloor conditions found during the experiments and describes the important physical and biological processes that control the spatial distribution and temporal changes in these characteristics     

多波束反向散射强度数据处理研究

在探讨多波束测深系统反向散射强度与海底底质类型的关系基础上,研究影响反向散射强度的各种因素,主要分析了海底地形起伏、中央波束区反射信号对反向散射强度的影响,并给出了消除这些影响的方法;将处理后的“纯”反向散射强度数据镶嵌生成海底声像图,为海底底质类型划分以及地貌解译提供了基础数据和辅助判读依据.     

Inhomogeneous substrate analysis using EM300 backscatter imagery

Backscatter reflectivity from multibeam echo-sounders provides a powerful tool to efficiently characterize seafloor substrates. A comprehensive EM300 bathymetric and backscatter survey has been completed of Cook Strait, in central New Zealand. This paper presents a detailed analysis of the realtime corrections applied to the raw EM300 multibeam data and additional corrections required to compute angular variations of the backscatter strength. The corrections, including the local absorption coefficient, the influence of seafloor topography and sound refraction in the water column, are determined for different Cook Strait seafloor substrates. Modifying MB-System software code, we extracted the backscatter signal parameters in order to quantify the raw backscatter strength and apply additional processing. Profiles of backscatter strength versus incidence angle were computed for a variety of sites characterized by flat seafloor and homogeneous substrates, and for which ground-truth data were available. For each homogeneous site, different but characteristic backscatter profiles are observed that can be interpreted in terms of sediment facies. To analyze heterogeneous substrates, we present a statistical technique, based on a 3-dimensional distribution of (incidence angle, backscatter strength) couples that preserves geological information of the substrate components. This analysis, using backscatter data acquired on a submarine volcano, north of New Zealand, clearly differentiates soft sediments and lava flows within a heterogeneous substrate.     

一种分层海底反向散射模型

In order to predict the bottom backscattering strength more accurately, the stratified structure of the seafloor is considered. The seafloor is viewed as an elastic half-space basement covered by a fluid sediment layer with finite thickness. On the basis of calculating acoustic field in the water, the sediment layer, and the basement, four kinds of scattering mechanisms are taken into account, including roughness scattering from the water-sediment interface, volume scattering from the sediment layer, roughness scattering from the sediment-basement interface,and volume scattering from the basement. Then a backscattering model for a stratified seafloor applying to low frequency(0.1–10 kHz) is established. The simulation results show that the roughness scattering from the sediment-basement interface and the volume scattering from the basement are more prominent at relative low frequency(below 1.0 kHz). While with the increase of the frequency, the contribution of them to total bottom scattering gradually becomes weak. And the results ultimately approach to the predictions of the high-frequency(10–100 kHz) bottom scattering model. When the sound speed and attenuation of the shear wave in the basement gradually decrease, the prediction of the model tends to that of the full fluid model, which validates the backscattering model for the stratified seafloor in another aspect.     

Processing Multibeam Backscatter Data

A new highly precise source of data has recently become available using multibeam sonar systems in hydrography. Multibeam sonar systems can provide hydrographic quality depth data as well as high-resolution seafloor sonar images. We utilize the seafloor backscatter strength data of each beam from multibeam sonar and the automatic classification technology so that we can get the seafloor type identification maps. In this article, analyzing all kinds of error effects in backscatter strength, data are based on the relationship between backscatter strength and seafloor types. We emphasize particularly analyzing the influences of local bottom slope and near nadir reflection in backscatter strength data. We also give the correction algorithms and results of these two influent factors. After processing the raw backscatter strength data and correcting error effects, we can get processed backscatter strength data which reflect the features of seafloor types only. Applying the processed backscatter strength data and mosaicked seafloor sonar images, we engage in seafloor classification and geomorphy interpretation in future research.     

Processing Multibeam Backscatter Data

A new highly precise source of data has recently become available using multibeam sonar systems in hydrography. Multibeam sonar systems can provide hydrographic quality depth data as well as high-resolution seafloor sonar images. We utilize the seafloor backscatter strength data of each beam from multibeam sonar and the automatic classification technology so that we can get the seafloor type identification maps. In this article, analyzing all kinds of error effects in backscatter strength, data are based on the relationship between backscatter strength and seafloor types. We emphasize particularly analyzing the influences of local bottom slope and near nadir reflection in backscatter strength data. We also give the correction algorithms and results of these two influent factors. After processing the raw backscatter strength data and correcting error effects, we can get processed backscatter strength data which reflect the features of seafloor types only. Applying the processed backscatter strength data and mosaicked seafloor sonar images, we engage in seafloor classification and geomorphy interpretation in future research.     

Recent transfer of coastal sediments to the Laurentian Channel,Lower St. Lawrence Estuary (Eastern Canada), through submarine canyon and fan systems

Multibeam sonar data, acoustic sub-bottom profiles and box cores were used to study the activity of submarine canyons and fans near the city of Les Escoumins, on the North Shore of the Lower St. Lawrence Estuary (Eastern Canada). The multibeam data were used to generate a high-resolution digital terrain model that reveals the presence of a large number of canyons and fans along the northern slopes of the Laurentian Channel. This paper focuses on two of the larger canyons, and their associated submarine fans. The sub-bottom profiles on the fans reveal high-amplitude reflections at the sediment/water interface and near the seafloor surface, indicating the occurrence of layers of coarse material. A turbidite was observed in a box core sampled in one of the fans, confirming the nature of the coarse layer. Geophysical and sedimentological data indicate that the canyons and fans play an important role in transferring coastal sandy sediments to the deeper marine environments by longshore drift-initiated turbidity flows, and thereby contribute to the negative sediment budget along the coast. The morphology of the canyons indicates that they were produced by a combination of erosive turbidity flows and retrogressive failures. The two box cores sampled on the fans reveal a recent (~last 60 years) quasi-exponential increase in sand content near the surface of the cores, possibly reflecting recent deforestation and/or increased coastal erosion.     

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.     

海洋测深中垂直分置海底混响信号的仿真分析

海洋测深中,海底混响信号是测深仪回波信号检测的主要内容。测深仪通常采用信号的相关处理方法对其进行检测,因此在设计测深仪的回波处理单元时,系统地分析海底混响信号的相关特性就显得尤为重要,对混响信号仿真是分析其特性的有效手段。基于单元散射理论,依据海底散射系数的空间相关半径划分散射单元,给出垂直分置海底混响信号的仿真方法。研究结果表明,该模型物理意义明确,计算简单。仿真得到的海底混响信号具有非常好的空间相关性和时间自相关性,与实测的海底混响信号相符,可用于对混响场特性的分析,改善测深仪的设计,从而有效提高测深仪的测量精度。     

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。     

An overview of SAX99: acoustic measurements

A high-frequency acoustic experiment was performed at a site 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18-19 m depth. The goal of the experiment was, for high-frequency acoustic fields (mostly In the 10-300-kHz range), to quantify backscattering from the seafloor sediment, penetration into the sediment, and propagation within the sediment. In addition, spheres and other objects were used to gather data on acoustic detection of buried objects. The high-frequency acoustic interaction with the medium sand sediment was investigated at grazing angles both above and below the critical angle of about 30°. Detailed characterizations of the upper seafloor physical properties were made to aid in quantifying the acoustic interaction with the seafloor. Biological processes within the seabed and the water column were also investigated with the goal of understanding their impact on acoustic properties. This paper summarizes the topics that motivated the experiment, outlines the scope of the measurements done, and presents preliminary acoustics results     

Study of the sedimentation and self-weight consolidation behavior of seafloor sediments using a radioisotope densitometer

ABSTRACT

In this study, settling tests were conducted to investigate the sedimentation and self-weight consolidation behavior of seafloor sediments from Isahaya Bay, Ariake Sea, Japan. During the tests, the density variations with depth and time were measured by a gamma-ray transmission radioisotope densitometer. The test results show that the settling process of the seafloor sediments can be classified into the flocculation stage, settling stage, and consolidation stage. The settling rate of the seafloor sediments in the settling stage is dependent on the temperature and initial water content, while the settling rate in the consolidation stage is independent of the temperature and initial water content. The density profile changes from a constant density profile to a linear density profile when the sedimentation process transitions to the self-weight consolidation process. The relations between the void ratio (e) and effective vertical stress (p’) at very low pressures can be calculated from the measured density values, and this can be used for the analysis of the self-weight consolidation of seafloor sediments. For the seafloor sediments tested in this study, the undrained shear strength (s_u) values are almost the same when the density values are less than 1.14?g/cm³, and the s_u values increase linearly with an increase in density when the density values are in the range of 1.14–1.2?g/cm^3.