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1.
R.C. Tyce 《Ocean Engineering》1977,4(3):113-140
This paper describes the development and application of a system for quantitative near-bottom seismic profiling at 4 kHz. Developed as part of the Deep Tow Instrumentation System of the Marine Physical Laboratory, this system represents a capability which is relatively unique in terms of lateral resolution. This capability was developed in order to observe small-scale horizontal variations in acoustic reflectivity of the sea floor as well as similar properties for shallow, layered structures in bottom sediments. The system provides real time computer processing and quantitative displays of processed data in several forms. These displays include a high resolution grey scale display of equivalent plane wave pressure (compensating for Fish altitude), a three-dimensional contourogram display of equivalent intensity, and a plot of integrated equivalent energy for travel-time sections (typically corresponding to 0–5 and 5–55 m into the bottom). These quantitative displays provide a real-time capability for mapping reflectivity of the sea floor and sub-bottom reflectors within a survey area and for estimating acoustic attenuation in marine sediments from observations of layer reflectivity as a function of depth of burial. Such determinations of attenuation can be made directly from the composite computer display which is produced on a facsimile recorder. The data from this system suggest that small scale variations are common to many areas of the sea floor, with poorly explained variations of up to 10 dB over lateral distances of less than 50 m. Values of effective attenuation determined by this system show good agreement with values measured directly in the San Diego Trough, and suggest unusually low values for highly calcareous sediments on the Carnegie Ridge. 相似文献
2.
Dae Choul Kim Gwang Hoon Lee Young Kyo Seo Gil Young Kim Seok Yun Kim Jeong Chang Kim Soo Chul Park Roy Wilkens 《Marine Georesources & Geotechnology》2004,22(1):21-31
Shallow gas in the Korea Strait shelf mud (KSSM) off SE Korea, revealed by high-resolution subbottom profiles, is associated with acoustic blanking, acoustic turbidity, seepages with plumes in the water column, and seafloor depressions. The acoustic blanking, characterized by strong, consistent top reflection and wipeout below, is most dominant. The seaward edge of the acoustic blanking zone generally coincides with the 100-m water-depth contour, suggesting that the water depth (the pressure) may control the distribution of shallow gas. The acoustic turbidity, characterized by diffuse top reflection, is a dark smear, partially blanking the data below. The seepages with plumes, characterized by vertical smearing and disturbed seafloor, are seen only along the shallowest, landward edge of the acoustic blanking zone. This may suggest that the decreased gas solubility at shallow water depths, caused by the lowered pressure, increases the volume of free gas in the sediments, facilitating the gas escape. The seafloor depressions, interpreted as pockmarks, are accompanied by cone-shaped acoustic masking, which is probably the reflection from a narrow vent of gas. The gas-related acoustic anomalies appear to occur mostly in the upper, recent mud of the KSSM. Neither permeable beds nor faults, which can act as vertical migration pathways for deep thermogenic gas, are evident in the recent mud. We interpret that the bacterial degradation of organic matter in situ is the main source for the gas in the KSSM. The upwelling off SE Korea may be an important source for the increased organic matter in the area. 相似文献
3.
DAE CHOUL KIM GWANG HOON LEE YOUNG KYO SEO GIL YOUNG KIM SEOK YUN KIM JEONG CHANG KIM 《Marine Georesources & Geotechnology》2013,31(1-2):21-31
Shallow gas in the Korea Strait shelf mud (KSSM) off SE Korea, revealed by high-resolution subbottom profiles, is associated with acoustic blanking, acoustic turbidity, seepages with plumes in the water column, and seafloor depressions. The acoustic blanking, characterized by strong, consistent top reflection and wipeout below, is most dominant. The seaward edge of the acoustic blanking zone generally coincides with the 100-m water-depth contour, suggesting that the water depth (the pressure) may control the distribution of shallow gas. The acoustic turbidity, characterized by diffuse top reflection, is a dark smear, partially blanking the data below. The seepages with plumes, characterized by vertical smearing and disturbed seafloor, are seen only along the shallowest, landward edge of the acoustic blanking zone. This may suggest that the decreased gas solubility at shallow water depths, caused by the lowered pressure, increases the volume of free gas in the sediments, facilitating the gas escape. The seafloor depressions, interpreted as pockmarks, are accompanied by cone-shaped acoustic masking, which is probably the reflection from a narrow vent of gas. The gas-related acoustic anomalies appear to occur mostly in the upper, recent mud of the KSSM. Neither permeable beds nor faults, which can act as vertical migration pathways for deep thermogenic gas, are evident in the recent mud. We interpret that the bacterial degradation of organic matter in situ is the main source for the gas in the KSSM. The upwelling off SE Korea may be an important source for the increased organic matter in the area. 相似文献
4.
This paper discusses the practical considerations associated with analyzing analog data signals to very high resolution in frequency. In the problem considered here, the data set is recorded on analog tape, digitized, and processed on a digital computer. A primary concern is that recorder speed variations, or flutter, can produce artifacts which might become evident when high-resolution spectral analysis is performed. This study concludes that fractional millihertz analysis resolution can be achieved with existing analog tape recorder technology. Another problem which must be considered is the efficient storage of the large data sequences resulting from processing long-time signal records. The use of two-stage frequency analysis is described as an approach to addressing this problem. Finally, the implementation issues associated with the periodogram and AR (autoregressive) estimation algorithms are outlined. It is anticipated that the discussion of practical considerations given here will be useful to researchers who must address problems that are similar to those cited in this paper. 相似文献
5.
William A. Prothero Jr. 《Marine Geophysical Researches》1977,3(1):119-141
The ocean bottom seismometer capsule contains a 1 Hz. vertical seismometer and triggerable or programmable digital recording system. The output of the seismometer is continuously digitized at a preselected rate of 64, 128, or 256 samples/sec. The digital data words are mixed with a time code and synchronization characters, serialized and passed through a 1536 sample shift register which acts as a delay line. The serial output bits are then encoded and recorded on a SONY TC800B tape recorder which is turned on when a seismic event occurs. The event trigger occurs when the seismic signal jumps to 8 times the time averaged input signal. A memory may be programmed to run the recorder on a schedule so that small amplitude signals from refraction shots are sure to be recorded. Data are recovered using the same recorder for playback and a decoder which provides an analog output for field data interpretation or a digital output for computer analysis. An acoustic transponder allows precise ranges between the capsule and ship to be determined. In addition, commands for the capsule to release or to transmit diagnostic data may be given from the surface ship. The capsule falls freely to the ocean bottom. After a predetermined time or when a release command is received, it is released from a 68 kg steel tripod and floats to the surface. A dual timer and explosive bolt system is used to increase recovery reliability.The first capsules were designed and constructed between October 1972 and October 1973. Good results were obtained from 38 out of 43 launchings made on six expeditions in 1974, 1975, and 1976. Four capsules have been lost. 相似文献
6.
声散射是重要的声学现象,海洋水体产生的高频声散射信号既可用于开展多种目的的声学海洋学研究,也可能对水下声学设备产生干扰,而海洋水体背景声散射具有显著的时空变异特征,因此针对特定海区开展声散射时变观测具有重要意义。本文利用在南海北部布放的锚系系统所搭载的声学多普勒流速剖面仪,获取了覆盖4个季节的累计约80 d的声散射数据,数据包括75 kHz和300 kHz两个频段,观测水深几乎覆盖了从海面到约600 m水深的整个水体。结果表明,水体在垂向上分布着上散射层和深散射层2个主要散射层。上散射层分布深度在冬夏较浅,位于约100 m以浅,在春秋较深,位于约200 m以浅;深散射层分布深度同样为冬季最浅,位于约300 m以深,但夏季则最深,位于约400 m以深。因此,两散射层的距离在夏季最远,在春秋最近。2个散射层的声散射强度（Sv）同样具有明显的季节变化,上散射层散射强度夏秋较强而春冬较弱,深散射层则正好相反。 相似文献
7.
An instrumented vehicle is towed at the end of a 5 km long cable, gathering data about the deep sea floor and near-bottom water column. Although bottom-moored acoustic transponders are used to determine the vehicle and ship positions precisely and in real time, predicting the ship manoeuvers required to bring the vehicle over an area of interest on the sea floor is far from trivial for the ship driver. Computer software has been developed which recommends courses for the ship to steer so that the vehicle will pass near a desired target. In trials at sea, the computer steered the vehicle 80, 40 and 85 m from pre-selected targets. Analysis of the causes of the misses suggests future developments which may reduce the miss distance, provide information on current structure of the water column, and reduce the level of skill and attention required of the vehicle pilot. 相似文献
8.
The spatial statistics of the acoustic field in shallow water are strongly affected by interfacial roughness and volume fluctuations in the water column or the seabed. These features scatter energy, reducing the coherence of the acoustic field. This paper introduces a consistent, mode-based modeling framework for ocean scattering. First, the rough surface scattering theory of Kuperman and Schmidt is reformulated in terms of normal modes, resulting in computation times which are reduced by several orders of magnitude. Next, a perturbation theory describing scattering from sound speed and density fluctuations in acoustic media is developed. The scattering theories are combined with KRAKEN, creating a unified normal mode code for wave theory modeling of shallow-water spatial statistics. The scattered field statistics are found to be a complicated function of scattering mechanism, scatterer statistics, and acoustic environment. Bottom properties, including elasticity, strongly influence the scattered field 相似文献
9.
The broad-band acoustic characterization of the Hudson Canyon region off the New Jersey Continental Shelf is studied with an analysis of pressure time series generated by small explosive sources and recorded on a vertical line array (VLA). The average water depth is about 72 m and the average sound-speed profile (SSP) is downward-refracting over the midportions of the water column. The seabed is characterized by sediment layers possessing sand-like characteristics. The sound-speed structure of the water column and the seabed structure create distinguishing modal features in the impulse response in the 250-500-Hz hand. The details of the depth and range dependence of the time series on the VLA are sensitive to small perturbations of the structure of the upper layer of the SSP, the water depth, and the seabed structure. This sensitivity of the acoustic field is investigated using a broad-band range-dependent normal mode model called NAUTILUS. The representation of the spatial and temporal structure of the time series in terms of a modal structure reveals several unique effects of the SSP and the geoacoustic structure of the bottom on the group velocity of the modes over a large bandwidth. Individual modes can be identified in the measured data using direct data-simulation comparisons. Cross-correlation values between data and simulations in a 155-ms time window generally vary from 0.7 to 0.9 for sensors below the thermocline but are much smaller for sensors above the thermocline 相似文献
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11.
A. Peter Slootweg 《Marine Geophysical Researches》1986,8(2):149-174
The multistreamer Side-Looking Seismic system presented in this paper makes a sonograph of uncovered or buried crustal topography, thus revealing the structural fabric of the oceanic basement, even when this is covered with a sedimentary layer. Major elements of the system are an airgun as a sound source, five single-channel parallel streamers and two minicomputers for signal capture and processing.The system is used simultaneously for enhanced single-channel seismic profiling and for side-looking seismics. A vertical section with an improved signal-to-noise ratio and a suppression of side-echoes is produced on a digital seismic recorder. Primary side-looking seismic output in the form of 5 profiles with different angles of incidence is obtained within 10 seconds. This part of the processing can be done in real time.In sediment-covered areas the low frequencies used cause the slanted profiles (the side beams in the primary output) to be side-looking sonar images of buried topography. The projection process yielding final side-looking output corrects for slant range deformation caused by the water column and, if necessary, for deformation caused by refraction within the sedimentary column. The result approaches a conformal map of the structure of the traversed basement. Swath width is mainly determined by water depth and refraction effects in the sediment. In Madeira abyssal plain a swath width of 8000 m was attained in a water depth of 5000 m.Within the swath, oceanic basement structures are recognized in the form of elongate more or less parallel reflectors. They are interpreted as buried spreading topography. The lack of side-echoes within fracture zones combined with typical wall signatures can be used to trace fracture zones. These features are demonstrated for an area in Madeira abyssal plain. 相似文献
12.
R. I. C. C. Francis 《新西兰海洋与淡水研究杂志》2013,47(3):375-389
The design and execution of two acoustic surveys of Hawke Bay in February and September/October 1980 are described. Though the resulting acoustic data were, for technical reasons, of limited value, they illustrate the information which may be drawn from acoustic surveys. Fish distributions may be rapidly mapped, not only by area, but throughout most of the water column and over time. Qualitative observations on fish distribution may be quantified. The estimation of variance in acoustic surveys is discussed in detail with particular emphasis on the problems of systematic surveys. The role of replicate observations is stressed. Coefficients of variation for relative fish density estimates from the two surveys probably lie between 5 and 30%. 相似文献
13.
Optimal localization of a seafloor transponder in shallow water using acoustic ranging and GPS observations 总被引：2，自引：0，他引：2
This study utilized circular and straight-line survey patterns for acoustic ranging to determine the position of a seafloor transponder and mean sound speed of the water column. To reduce the considerable computational burden and eliminate the risk of arriving at a local minimum on least-squares inversion, the position of a seafloor transponder was estimated by utilizing optimization approaches. Based on the implicit function theorem, the Jacobian for this inverse problem was derived to investigate the constraints of employing circular and straight-line survey patterns to estimate the position of a transponder. Both cases, with and without knowledge of the vertical sound speed profile, were considered. A transponder positioning experiment was conducted at sea to collect acoustic and GPS observations. With significant uncertainties inherent in GPS measurements and the use of a commercial acoustic transponder not designed for precise ranging, experimental results indicate that the transponder position can be estimated accurately on the order of decimeters. Moreover, the mean sound speed of the water column estimated by the proposed optimization scheme is in agreement with that derived from conductivity, temperature, and density (CTD) measurements. 相似文献
14.
Traditionally, matched-field processing (MFP) has been used to localize low-frequency sources (e.g., <300 Hz) from their acoustic signals received on long vertical arrays. However, some sources emit acoustic signals of much higher frequency. Applying MFP to signals in the mid-frequency range (e.g., 1-4 kHz) is a very challenging problem because MFP's sensitivity to environmental parameter mismatch becomes more severe with increasing frequency. Robust MFP techniques are required to process signals in the mid-frequency range. As a practical issue, short vertical arrays are more convenient to work with than are long vertical arrays; they are easier to deploy and are less prone to large amounts of deformation. However, short vertical arrays undersample the water column, which can result in severely degraded MFP performance. In this paper, we present experimental data results for this nonconventional paradigm. Using the environmentally robust broad-band L/sub /spl infin//-norm estimator, MFP results are given using shallow-water experimental data. This data consisted of broad-band signals in the 3-4-kHz band collected on an eight-element 2.13-m vertical array. These results serve to demonstrate that good localization performance can be attained for this difficult problem. Guidelines on the appropriate use of ray and normal-mode propagation models are also presented. 相似文献
15.
Hiroaki Ochiai 《Journal of Oceanography》1973,29(1):8-15
This paper presents a system of the line scanner with InSb detector of filter band 2.0–5.5 microns, for obtaining infrared imagery from an aero-plane. Since its scanning sector is as large as 80 degrees, wide thermal imagery can easily obtained from comparatively low altitude. Displaying techniques of thermal pattern of sea surface are also discussed.All data were stocked in the four-channel data recorder during measurements and analysed at the laboratory after landing.Colored imagery and contoured imagery were made from the original thermal imagery by using computer. 相似文献
16.
A pop-up bottom seismic recorder designed for seismic refraction experiments was built by the Institute of Oceanographic Sciences in 1968. The device is housed within a 71 cm diameter sphere weighing 270 kg when launched. signals picked up by a hydrophone are recorded in analogue form on magnetic tape in the band 2–100 Hz. The total continuous recording period is 12 hr but the lifetime of the system can be effectively extended by cycling the tape-recorders to allow shooting to go on for up to 3 days. Ballast release is by acoustic command or by pre-set clock. The instruments have been used in water depths from 150 to 4820 m making a total of 63 deployments with a 95% recovery rate. A new version with three-component geophones is being built. 相似文献
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海底底质声学参数测量系统设计 总被引：4，自引：1，他引：3
海底底质声学参数测量系统用于测量海底沉积物的声速和声衰减参数,它由水上主控子系统和水下测量子系统构成.水上主控子系统由工控机、数据传输接口和GPS定位模块组成,实现实时控制、数据处理与记录功能;水下测量子系统由声系、数据采集和传输模块组成,完成水下声波信号数字化及数据上传.水上设备与水下装置间通过电缆建立低速的下行命令信道和高速的上行数据信道,数据通讯采用曼彻斯特编码调制方式.水上计算机通过下传命令实时改变数据采集模块的控制参数,能够在各种环境条件下获得最佳测量效果.实验证明,仪器测量达到预期效果. 相似文献
20.
南黄海西部地区浅层气地震特征 总被引：11，自引：2，他引：9
通过近年来南黄海西部地区的浅层地震调查,发现该地区浅层气地震特征丰富而多变,特别是多处海底面、海水层中的浅层气地震记录,揭示本区存在较多浅层气,且部分浅层气由海底逸出,释放到海水层中。该区的浅层气地震特征按空间位置分为3大类:(1)地层中的特征:声学空白、声学幕、声学扰动、不规则强反射顶界面、两侧相位下拉;(2)海底面的特征:海底麻坑、大型塌陷坑;(3)海水层中的特征:声学羽流、云状扰动、点划线反射。选择相应特征的典型浅层气地震记录,进行了声学成因解释,讨论了大型塌陷坑的浅层气成因、海水层中声学反射与浅层气体的成因关系以及浅层气地震特征的气体浓度指示作用。根据南黄海西部地区浅层气地震特征,绘制了该区的浅层气分布图。 相似文献