An attenuation-based sediment classification technique using Chirp sub-bottom profiler data and laboratory acoustic analysis

A remote sediment classification technique based on attenuation measurements from Chirp sub-bottom profiler data is described. This differs from previously published work in that attenuation measurements are obtained for each stratigraphic unit within a complex, thinly interbedded sedimentary sequence. Compressional wave attenuation measurements are obtained for a wide variety of lithologies, including muds, silts, sands, clayey sands, silty clays and gravel lags, with grain sizes ranging from 8 Phi to -4 Phi. In addition, attenuation measurements from sub-bottom profiler data were calibrated against laboratory acoustic measurements of vibracores and seabed samples from corresponding geographic locations, under simulated in-situ conditions using a Pulse Tube method. We adapt an instantaneous frequency matching method using a causal attenuation filter to model the decay of the Chirp transmitted waveform. From this modelling, a relationship between t* (a causal attenuation operator) and change in instantaneous frequency is established. The Hilbert transform is used to extract instantaneous frequency information from Chirp seismic, which is used to derive attenuation information for selected individual stratigraphic layers imaged by the sub-bottom profiler. This paper draws attention to the limitations in comparing attenuation measurements derived from Chirp sub-bottom profiler data against previously published literature on experimental attenuation measurements, which are limited by the wide variance of these data, and the difficulty in finding a meaningful best fit to these data. This demonstrates the importance of calibrating remote sediment classification observations using complimentary acoustic analysis of seabed samples to generate a site-specific geoacoustic database. A positive correlation between laboratory and sub-bottom profiler attenuation measurements was obtained, with a correlation coefficient of 0.885. Poorly sorted gravels with a mixed lithic and biogenic pebble component are characterised by very high attenuation with values of Q from 4 to 19. These sediments are considerably coarser-grained than those typically described in previously published experimental studies.     

参量阵浅地层剖面的处理方法及在识别浅层天然气水合物中的应用

近年来,参量阵浅地层剖面以其简便的野外采集方式、高分辨率的浅部地层成像能力,迅速发展为海底浅层天然气水合物探测的有效方法。为了更好地获取东海浅层天然气水合物赋存区的地质信息,针对参量阵浅地层剖面数据开展了精细化处理。首先将异常振幅压制和空间振幅均衡等方法有机结合,解决了数据中的各种噪音和能量不均衡等问题,然后利用Hilbert变换提高地层分辨率,最后利用信号增强技术进一步提高同相轴连续性,获得了波组特征更清晰的地震剖面。处理后的参量阵浅地层剖面具有信噪比较高、连续性好、地层结构清晰等特点,可以更好地揭示空白带、气烟囱、亮点和火焰状异常等地震反射特征,为识别浅层天然气水合物赋存区地质信息奠定基础。对精细处理后的数据进一步开展曲率属性、瞬时振幅属性、相干属性等地震属性分析,结果显示,与浅层天然气水合物渗漏相关的声学异常能够被清晰地识别出来。此项研究一方面验证了参量阵浅地层剖面数据处理方法的可行性,另一方面也探索了属性分析技术在海底浅层天然气水合物识别方面的应用。     

Types and characters of sub-bottom acoustic reflection in eastern part of Bohai Sea

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Fluid mud dynamics in the Weser estuary turbidity zone tracked by high-resolution side-scan sonar and parametric sub-bottom profiler

Fluid mud in estuarine turbidity maximum zones (TMZ) can pose considerable navigation risks due to potentially substantial reductions in nautical depth, coupled with an inherent difficulty of detection by conventional echo-sounders. Despite intensive research efforts, however, our knowledge about the spatial and temporal dynamics of fluid mud is still not sufficient. In this study, the combined use of a side-scan sonar (Sportscan®, Imagenex) and a parametric sub-bottom profiler (SES-2000®, Innomar Technology GmbH) has proved successful for high-resolution fluid mud detection and volumetric quantification in an estuarine environment. In 2004 and 2005, repeated surveys were conducted in the navigation channel of the upper meso- to lower macrotidal Weser estuary TMZ (German North Sea coast) at different tidal stages and river discharges. Current velocity data were simultaneously collected by 1,200-kHz broadband ADCP (RDInstruments) measurements. Ground-truthing was carried out by means of grab sampling and gravity coring, adapted to fluid mud conditions. It was found that fluid mud occurrence in the Weser estuary is highly variable on time scales of a few hours and spatial scales of several metres. The riverbed is characterised by sand and mud deposits, and a complex morphology including subaqueous dunes and smooth bed deposits intermittently overlain by fluid mud. Thus, a continuous, coherent fluid mud body covering the entire TMZ riverbed was not observed. Rather, spatial distribution was patchy and highly dependent on suspended particulate matter (SPM) concentrations in the water column, as a result of which local fluid mud deposits varied in thickness from centimetres to metres. The formation of fluid mud was largely restricted to slack water, although slack-water conditions were not necessarily associated with large-scale fluid mud appearance. Advective SPM transport of resuspended fluid mud seems to be the most plausible explanation for the high spatial variability observed, even between two successive tides. The amount of fluid mud deposited and resuspended in the course of a tidal cycle can reach several 10s of tons even in small riverbed depressions.     

Autonomous underwater vehicle-based concurrent detection and classification of buried targets using higher order spectral analysis

This paper presents a processing concept for autonomous underwater vehicle (AUV)-based concurrent detection and classification (CDAC) of mine-like objects. In the detection phase, the AUV seeks objects of interest using a simple energy detector combined with a peak tracking mechanism. Upon detection, the processing mechanism changes to a higher order spectral (HOS) classification process. The system is demonstrated through theory, simulation and at-sea experiments to have promise in reducing the false alarm rate of mine detections. The HOS classification mechanism is also shown to have some benefit over classical spectral estimation in all cases. Components of the system concept were also demonstrated live onboard the AUV during the Generic Oceanographic Array Technology Sonar (GOATS 2002) experiment off the coast of Italy, while others are demonstrated using a comprehensive AUV sonar simulation framework.     

Object detection and tracking method of AUV based on acoustic vision

This paper describes a new framework for object detection and tracking of AUV including underwater acoustic data interpolation, underwater acoustic images segmentation and underwater objects tracking. This framework is applied to the design of vision-based method for AUV based on the forward looking sonar sensor. First, the real-time data flow (underwater acoustic images) is pre-processed to form the whole underwater acoustic image, and the relevant position information of objects is extracted and determined. An improved method of double threshold segmentation is proposed to resolve the problem that the threshold cannot be adjusted adaptively in the traditional method. Second, a representation of region information is created in light of the Gaussian particle filter. The weighted integration strategy combining the area and invariant moment is proposed to perfect the weight of particles and to enhance the tracking robustness. Results obtained on the real acoustic vision platform of AUV during sea trials are displayed and discussed. They show that the proposed method can detect and track the moving objects underwater online, and it is effective and robust.     

Aerial and acoustic marine mammal detection and localization on navy ranges

Acoustic monitoring and aerial visual surveys of marine mammal activity were conducted simultaneously at the Navy's Pacific Missile Range Facility near Kauai, HI, during times of both high- and low-whale density from February 2002 to March 2003. Specifically, recordings from the range's 24 broadband hydrophones were made during 11 of 16 "in-season" and during six of ten "off-season" aerial surveys. Basic acoustic detections consisted primarily of humpback whale calls and sperm-whale clicks, and those two species were also reported in the visual surveys. The relative number of acoustic detections roughly corresponded with the visual survey results throughout the year. The same acoustic data were also provided to a passive-acoustic-localization algorithm based on acoustic propagation models which generated estimates of sperm-whale movement through the range. The acoustic localizations are in close proximity in space and time to the visual observations of sperm whales. Verification of the model-based localization algorithm's accuracy was demonstrated in a controlled-source experiment at the Navy's Atlantic Undersea Test and Evaluation Center (AUTEC) range in the Bahamas where the recordings of sperm-whale clicks were broadcast and successfully tracked. The localization accuracy of the model-based technique and traditional hyperbolic techniques is compared. These results raise the possibility of using existing Navy assets to detect and track marine mammals, particularly during times when visual sighting conditions are not favorable, in efforts to minimize their exposure to underwater sound.     

Bistatic synthetic aperture imaging of proud and buried targets from an AUV

The use of autonomous underwater vehicles (AUVs) for the detection of buried mines is an area of current interest to the Mine CounterMeasures (MCM) community. AUVs offer the advantages of lower cost, stealth, reduced operator risk, and potentially improved coverage rates over more traditional mine hunters. However, AUVs also come with their own set of difficulties, including significant error in navigation and low communication rates with the mother platform and each other. In the case of bistatic detection scenarios, AUVs will therefore have difficulty knowing where exactly in space they are and the trigger time of sources on other platforms, be they ships or other AUVs. However, the potential improvement in detection and coverage rates offered by bistatic sonar concepts makes resolution of these issues a high priority. In this paper, the problems of inaccurate navigation and source timing information are addressed for the Generic Oceanographic Array Technology data set. In this experiment, conducted off Marciana Marina during June 1998, a MIT AUV with a SACLANTCEN acoustic array and acquisition system was used together with a TOPAS parametric sonar to explore issues of buried target detection using AUVs. In this paper, solutions to the navigation and timing problems are proposed which enable the effective use of bistatic synthetic aperture sonar (SAS) concepts for the detection of buried objects in the mid-frequency regime of 2-20 kHz.     

海底冷泉气泡羽流声学探测参数研究

对气泡羽流的声学探测是目前海底冷泉调查的主要手段。在声学探测理论基础上, 分析了声学探测过程中CW声波脉冲信号的发射频率、发射功率、声波脉冲宽度(脉冲持续时间)3个参数对海底冷泉气泡羽流探测成像的影响。同时, 结合黄、渤海老铁山水道海域海底冷泉外业调查和人工模拟冷泉实验的数据, 进一步明确了这些参数的选取原则和范围。针对声学水体剖面上出现的异常干扰带, 还提出了改变脉冲发射时延值来消除水体声学剖面图中干扰带的方法, 从而进一步优化海底冷泉气泡羽流的声学成像效果。     

掩埋海底管道探测方法及新技术应用研究

<正>在海洋油气资源开发中,最常用和有效的油气输运方法是使用海底管道进行输送,因此,随着海洋油气资源的不断开发,海域内所铺设的海底管道数量越来越多,密度不断增高,种类越来越复杂。这些海底管线长期在海洋复杂环境下工作,出现悬空、平面位移、管体损伤等情况,与原始设计状态有很大差异,但未得到精确探测与校核,存在很多事故隐患,对环境和生产造成极大的威胁[1]。因此,准确探明海底管道的状态和位置,评价其安全风险,对于     

Effects of Korean littoral environment on acoustic propagation

Environmental acoustics experiments were recently conducted in shallow to intermediate water depths in the Sea of Japan, east of Korea, along the shelf and slope, covering frequencies from 25 to 800 Hz. These were operational experiments carried out in three different seasons. The primary objectives of the data reported here are: (1) to characterize the Korean coastal environment during May 1998, September 1998, and February 1999 and (2) to assess how complexities of the environment might impact acoustic propagation in May and February, as measured by its transmission loss. Propagation data were obtained from broadband explosive SUS sources and sonobuoy receivers. The tests were conducted over varying bottom depths and slopes, both approximately normal and parallel to the bathymetric contours. Two different source depths were included. Environmental and acoustic data are reviewed and discussed. While many aspects of the observed propagation remain ill understood, on the whole a consistent and useful picture has emerged of acoustic propagation in this region. Environmental impacts on propagation are associated mainly with bottom properties, somewhat less so with source depth in relationship to sound speed profiles, and almost not at all with range-dependent profiles of a water mass front     

Effects of sensor placement on acoustic vector-sensor arrayperformance

We consider the role played by the sensor locations in the optimal performance of an array of acoustic vector sensors, First we derive an expression for the Cramer-Rao bound on the azimuth and elevation of a single far-field source for an arbitrary acoustic vector-sensor array in a homogeneous wholespace and show that it has a block diagonal structure, i.e., the source location parameters are uncoupled from the signal and noise strength parameters. We then derive a set of necessary and sufficient geometrical constraints for the two direction parameters, azimuth and elevation, to be uncoupled from each other. Ensuring that these parameters are uncoupled minimizes the bound and means they are the natural or “canonical” location parameters for the model. We argue that it provides a compelling array design criterion. We also consider a bound on the mean-square angular error and its asymptotic normalization, which are useful measures in three-dimensional bearing estimation problems. We derive an expression for this bound and discuss it in terms of the sensors' locations. We then show that our previously derived geometrical conditions are also sufficient to ensure that this bound is independent of azimuth. Finally, we extend those conditions to obtain a set of geometrical constraints that ensure the optimal performance is isotropic     

Internal wave effects on high-frequency acoustic propagation tohorizontal arrays-experiment and implications to imaging

An experiment was carried out over a nine day period from August 18 to 27, 1996 to examine acoustic wave propagation in random media at frequencies applicable to synthetic aperture sonar. The objective was to test experimentally the hypothesized imaging effects of variations in the sound speed along two different acoustic paths as put forth by F.S. Henyey et al. (1997). The focus of this paper is on describing the experiment and carrying out an initial analysis of the data in the context of the effect of ocean internal waves on imaging resolution. The oceanography is summarized to the extent needed to discuss important aspects relative to the acoustics experiment. In the acoustics experiment transmissions at 6, 20, 75, and 129 kHz between sources and receiver arrays were carried out. Source to receiver separation was about 815 m. All sources and receivers were mounted on bottom-deployed towers and were at least 9 m off the seafloor. The analysis concentrates on the 75-kHz data acquired during one day of the experiment. The time span examined Is sufficient to examine a diurnal tidal cycle of the oceanographic conditions. The results indicate the IW phase perturbations would have a significant effect on imaging for even the most benign conditions of the experiment if no autofocusing scheme is used. Also, though autofocusing should be useful in recovering the focus for these conditions, there are conditions (e.g., for the path that has a turning point at the thermocline and during times when solibores are present), where more sophisticated compensation schemes would be needed     

声跃层结构变化对深海汇聚区声传播的影响

根据射线理论建立了线性声速结构条件下的声跃层强度与深海汇聚区关系模型,用最小位移角讨论了海洋环境变化（如声跃层强度变化、声跃层位置变化及季节性跃层生消等）与汇聚区距离和宽度变化的相关性.结果表明,声跃层的结构变化对汇聚区特征影响很大.声跃层强度增大使汇聚区向远离声源的方向变化,跃层强度每增加0.01 s-1对应的汇聚区位移增大约为3.5～5.0 km.声跃层位置变化对汇聚区的影响小于声跃层强度,与两层结构的声速剖面相比,上行结构使汇聚区向靠近声源的方向变化,声跃层上升200 m对应的汇聚区位移减小约为1.0～1.5 km,声跃层越浅,汇聚区距离越近;下行结构使汇聚区向远离声源的方向变化,混合层加深200 m对应的汇聚区位移增大约为1.0～1.5 km,混合层越深,汇聚区距离越远.季节性跃层的生消使近表层有负梯度、零梯度和正梯度的变化.负梯度结构的变化规律与两层结构条件下的声跃层强度变化类似,但对汇聚区的影响程度相对较小;正梯度结构使汇聚区在近表层出现表面声道,梯度值的增强将使汇聚区向靠近声源的方向变化.     

Experimental detection and reception performance for uplink underwater acoustic communication using a remote, in-air, acousto-optic sensor

Covert communications between underwater and aerial platforms would increase the flexibility of surface and air vehicles engaged in undersea warfare by providing a new netcentric warfare communications capability and could have a variety of commercial and oceanographic applications. Research into an acousto-optic sensor shows promise as a means for detecting acoustic data projected toward the water surface from a submerged platform. The laser-based sensor probes the water surface to detect perturbations caused by an impinging acoustic pressure field. Experimental studies were conducted to demonstrate acousto-optic sensor feasibility for obtaining accurate phase preserved recordings of communication signals across the air-water interface. The recorded surface velocity signals were transferred to an acoustic communications receiver that used conventional acoustic telemetry algorithms such as adaptive equalization to decode the signal. The detected, equalized, and decoded bit error rate performance is presented for hydrostatic and more realistic, hydrodynamic water surface conditions.     

High-frequency acoustic backscattering characteristics for acoustic detection of the red tide species Akashiwo sanguinea and Alexandrium affine

Harmful algal blooms (HABs), caused by the overgrowth of certain phytoplankton species, have negative effects on marine environments and coastal fisheries. In addition to cell-counting methods using phytoplankton nets, a hydroacoustic technique based on acoustic backscattering has been proposed for the detection of phytoplankton blooms. However, little is known of the acoustic properties of HAB species. In this study, as essential data to support this technique, we measured the acoustic properties of two HAB species, Akashiwo sanguinea and Alexandrium affine, which occur in the South Sea off the coast of Korea. Due to the small size of the target, we used ultrasound for the measurements. Experiments were conducted under laboratory and field conditions. In the laboratory experiment, the acoustic signal received from each species was directly proportional to the cell abundance. We derived a relationship between the cell abundance and acoustic signal received for each species. The measured signals were compared to predictions of a fluid sphere scattering model. When A. sanguinea blooms appeared at an abundance greater than 3 500 cells/mL, the acoustic signals varied with cell abundance, showing a good correlation. These results confirm that acoustic measurements can be used to detect HAB species.     

Effects of East China Sea shallow-water environment on acoustic propagation

Operational environmental acoustics experiments were conducted over the frequency range of 25 to 800 Hz in September 1997 in the East China Sea, where the water depth was about 100 m. Objectives of the data analysis reported here are to characterize this environment and to assess its complexities as they may impact acoustic propagation as measured by its transmission loss (TL). Conductivity-temperature-depths and expendable bathy-thermographs sampled the ocean, such that its spatial and temporal variability could be approximately separated. The sound-speed profiles are downward refracting, involve two water masses associated with the Kuroshio Current and Taiwan Warm Current, and have thermocline variations caused by internal tides. The bottom geoacoustic characteristics, presumed to be approximately horizontally isotropic, were based on data atlases and were estimated from the measured TL, for some interpretations. The TL data were obtained in octave bands from explosive signal underwater sound sources and sonobuoy receivers, both deployed at a depth of about 18 m. Tests were conducted in directions approximately normal and parallel to the bathymetric contours and the measured TL was, to zero order, independent of the direction of propagation. To higher order, directional differences in the TL were observed and ascribed to anisotropies in bottom properties. A state-of-the-art TL model was adopted, based on environmental idealizations typical of operational forecasting and compared with the measured TL. The comparison yields a probability density function that quantifies the uncertainty of such a TL model, caused by the stochastic variability of the environment, typically unknown a priori. For the model used, the pdf has a standard deviation of about 2 dB from 50 to 800 Hz and larger below 50 Hz.