Mitobs: A seismometer system for ocean-bottom earthquake studies

The MIT ocean-bottom seismometer is a free-fall, pop-up instrument capable of recording three components of seismic data on the sea floor for periods of at least one month. Data are recorded in digital format on a specially designed magnetic tape recorder. An event recording scheme and semiconductor memories assure both efficient data storage and preservation of first motion information. Sensors and recording electronics are housed in a cylindrical pressure vessel, which sits vertically atop an expendable base plate on the ocean bottom. Attached to the pressure case are three glass spheres for buoyancy. After a pre-set time interval, a motor-driven mechanical latch release frees the instrument to float to the ocean surface for recovery.     

The evolution of deep ocean pressure measurements in the UK

This paper is a brief review of the work carried out by the Proudman Oceanographic Laboratory (POL) in measuring tides and sub-surface pressure variations in the deep water of the ocean basins using Bottom Pressure Recorders (BPRs). It deals particularly with the work of David Cartwright which he began in the early 1970s and carried out until he left POL in 1986 but is continued by his co-workers of that time.The paper mentions the early work in the north Atlantic when instrument deployments were limited to one month duration and describes what was achieved from the measurements. It concentrates mainly on measurements that were made since the early 1980s when it became possible, because of developments in instrument technology, to achieve individual measurements lasting for one year or more. The results obtained from these measurements are described and some interesting features of the spectrum of the pelagic sub-surface pressure are discussed.As instrument technology further improved, it became possible to study low frequency variations in sub-surface pressure. The contributions made by POL to studies of the Atlantic and Indian Oceans and the contribution to ocean circulation studies during the World Ocean Circulation Experiment are discussed. This led to the development of an instrument capable of operating continuously at ocean depths for five years. The preliminary results from a four year deployment of this instrument which was completed in November 1996 are included.     

A deep ocean release mechanism

A simple release system for a deep ocean instrument package is described. An evacuated chamber holds the instrument to the anchor and an explosive device floods the chamber, allowing the instrument to float to the surface after a present time.     

水下光谱辐射测量技术

海水中的叶绿素、泥沙、黄色物质等及其含量直接影响海水的光谱特性。此外，海洋水色遥感算法的建立及验证也离不开现场相关光学参数的测量，准确的海水现场光谱辐射测量，是提高海洋水色定量化遥感精度的重要环节。文章通过开展水下光谱辐射测量方法的讨论、仪器总体方案的设计及相应的试验的结果分析，提出了有关水下光谱辐射测量仪器构成的一些想法。     

A free-fall direct-recording ocean bottom seismograph

A compact and relatively low-cost ocean bottom seismograph has been constructed for use in marine seismic research. The instrument is deployed as a free-fall package and can record for 12.5 days. Signals from a vertical geophone, a hydrophone, and a clock are direct recorded on four channels of 6.35×10^-3 m magnetic tape. A timed-release system returns the instrument to the surface.     

Long-period acoustic and seismic measurements and ocean floorcurrents

Pressure fluctuations caused by a strong ocean floor current are evident during most of an eighty-day-long record of very-low-frequency acoustic ambient noise measured by an instrument on the seafloor in the western Atlantic in the framework of the HEBBLE (High Energy Benthic Boundary Layer Experiment). The differential pressure gauges on the instrument produce useful measurements over a wide frequency band extending from 0.0005 to 16 Hz. The spectrum of current-induced pressure fluctuations is red with a power-law dependence on frequency with an exponent of -1.5. Turbulence in the ocean floor boundary layer is the source of these pressure fluctuations rather than the effects of flow around the transducers. This record of boundary-layer pressure fluctuations is used to predict the effect of seafloor currents on long-period seismograph measurements from the seafloor and from under the seafloor in boreholes     

基于水下滑翔机的湍流观测平台研究

海洋湍流混合过程对大洋热盐环流具有重要调控作用,因此海洋湍流混合特征量如湍动能耗散率等物理量的定量刻画,主要通过现场湍流观测来实现。为满足长期连续海洋湍流混合观测资料的迫切需求及打破国外的技术封锁,自行研制了基于水下滑翔机的湍流观测仪。为了测试滑翔机在小尺度湍流观测方面的可行性,将一个携带有剪切探头的湍流仪安装在滑翔机本体上,并在千岛湖进行了一系列的实验。滑翔机平台的主要噪声来源于浮力调节机构中的油泵,油泵会在接近剖面的底端时开启。三轴加速度谱在30 Hz左右有明显的谱峰,说明滑翔机油泵的振动频率应该是在30 Hz左右,在其他频率点上振动较小。由剖面下潜过程(开泵)的波数谱可知,开泵时虽然对湍流观测有影响,但是振动频率在波数空间中处于无效观测谱一侧,由此说明平台的振动对剪切信号影响很小。剪切探头测得的耗散率在10-7 W/kg量级上,且都与Nasymth理论谱吻合良好,验证了滑翔机在小尺度湍流观测方面的可行性。     

Instrument calibration of ocean bottom seismographs

To increase the accuracy of measuring sea floor motion with ocean bottom seismometers, we calibrate the seismometer system on the ocean floor. Data from the sea floor calibration, augmented with electronic and land calibration data, enables us to find the OBS transfer function to an accuracy of 0.5% in the frequency range of 0.1 to 32 Hz. We are able to distinguish between temperature, instrument and OBS ground coupling effects, all of which alter the transfer function. This paper reviews our method of calibration and discusses the effects of temperature and some of the instrument design features on the vertical seismometer transfer function.     

The vertical response of an ocean bottom seismometer: Analysis of the Lopez Island vertical transient tests

A lumped-parameter model was developed to predict the response of an ocean bottom seismometer, resting on relatively non-stiff sediments, to vertical ground-motion. The model predictions were compared with the response of an instrument on a foundation of foam rubber to a sinusoidal input. Comparison of the model data to the measured Lopez Island vertical transient test data showed that bearing pressure of the instrument in a nonuniform vertical soil profile causes certain instruments to experience a shear modulus higher than the mean.     

GPS浮标数据反演海浪谱的理论仿真与试验验证

GPS浮标作为一种新型的海洋测量设备,近年来在海面高度现场测量和星载高度计定标方面取得了重要应用。通过仿真试验对反演海浪谱的方法和流程进行研究,旨在探索从GPS浮标测量的海面高度序列中提取海浪谱的方法。首先,使用Longuest模型生成了海浪波面位移时间序列,并通过Pierson-Moscowitz风浪谱对波面位移的统计特性进行约束,其随机性由相位引入。结合典型潮汐和GPS浮标仪器噪声的仿真时间序列,合成了仿真时间长度1h的1Hz(每秒1次)随机海面高度序列。然后,利用自相关函数法,进行高通滤波和数据压缩,得到了仿真的海浪谱。该仿真结果和理论海浪谱非常接近,可满足海浪谱反演的需求。最后,通过山东石岛外海的GPS浮标现场试验,验证了本文提出的反演方法的适用性。本文的研究解决了GPS浮标反演海浪谱的关键问题,丰富了海浪谱反演的手段,拓展了GPS浮标的应用领域,有望为未来我国的星载波谱仪定标服务。     

Altimeter Signal-to-Noise for Deep Ocean Processes in Operational Systems

The ocean signal for this study is the sea surface height due to the slowly varying (greater than 5-day) ocean processes, which are predominantly the deep ocean mesoscale. These processes are the focus of present assimilation systems for monitoring and predicting ocean circulation due to ocean fronts and eddies and the associated environmental changes that impact real time activities in areas with depths greater than about 200 m. By this definition, signal-to-noise may be estimated directly from altimeter data sets through a crossover point analysis. The RMS variability in crossover differences is due to instrument noise, errors in environmental corrections to the satellite observation, and short time period oceanic variations. The signal-to-noise ratio indicates that shallow areas are typically not well observed due to the high frequency fluctuations. Many deep ocean areas also contain significant high frequency variability such as the subpolar latitudes, which have large atmospheric pressure systems moving through, and these in turn generate large errors in the inverse barometer correction. Understanding the spatial variations of signal to noise is a necessary prerequisite for correct assimilation of the data into operational systems.     

Altimeter Signal-to-Noise for Deep Ocean Processes in Operational Systems

The ocean signal for this study is the sea surface height due to the slowly varying (greater than 5-day) ocean processes, which are predominantly the deep ocean mesoscale. These processes are the focus of present assimilation systems for monitoring and predicting ocean circulation due to ocean fronts and eddies and the associated environmental changes that impact real time activities in areas with depths greater than about 200 m. By this definition, signal-to-noise may be estimated directly from altimeter data sets through a crossover point analysis. The RMS variability in crossover differences is due to instrument noise, errors in environmental corrections to the satellite observation, and short time period oceanic variations. The signal-to-noise ratio indicates that shallow areas are typically not well observed due to the high frequency fluctuations. Many deep ocean areas also contain significant high frequency variability such as the subpolar latitudes, which have large atmospheric pressure systems moving through, and these in turn generate large errors in the inverse barometer correction. Understanding the spatial variations of signal to noise is a necessary prerequisite for correct assimilation of the data into operational systems.     

An ocean bottom,microprocessor based seismometer

We describe the design and construction of an ocean bottom seismometer configured as a computer, based on an Intersil IM6100 microprocessor plus appropriate peripheral devices. The sensors consist of triaxial 1 Hz seismometers and a hydrophone, each sensor channel being filtered prior to digitizing so that typical noise spectra are whitened. Digital data are recorded serially on magnetic tape. The instrument is placed on the ocean bottom by allowing it to fall freely from just below the surface. An acoustic system allows precise determination of instrument position, acoustic recall, and transmission of operational information to the surface. Release from an expendable anchor is accomplished by redundant pyrotechnic bolts which can be fired by acoustic command or by precision timers.The operational flexibility provided by the micro-computer, which executes the DEC PDP8/E instruction set, enables optimum use of the 6-hr recording capacity (at 128 samples/second/channel) in the context of the particular experiment being performed.

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Precise optical path length measurement through an optical fiber: Application to seafloor strain monitoring

An optical fiber strainmeter intended for measuring tectonic strains on the seafloor is under development. In this instrument, an optical fiber is stretched between two points fixed to the ocean bottom; relative displacement of these points causes a change in the elongation of the fiber. This associated change in optical path length is monitored by an electronic distance meter. The dominant sources of noise in determining the optical path length of the fiber stem from the dependence of the fiber's index of refraction on both wavelength and temperature. In a 50 day long experiment performed in the shallow ocean, a test fiber was installed along a 210 m long baseline on the bottom. The RMS variation in length was 5 mm except for two displacements of order 10 cm caused by known effects.     

On the use of an externally deployed geophone package on an Ocean Bottom Seismometer

We describe a recent modification to the MIT Ocean Bottom Seismometer by which the geophones are housed in a separate package that is deployed on the sea floor about 1 m distance from the main unit several hours after the OBS reaches the ocean bottom. Records from deep-sea experiments and shallow-water tests show two improvements over records from geophones housed in the main instrument package. Signals recorded by the external geophones have a much better signal-to-noise ratio because tape recorder noise and instrument vibrations generated by water currents are effectively eliminated. As a result, the overall frequency response of the sensors to ground motion has a demonstrably smoother spectral shape. The second improvement is that the cross coupling of horizontal instrument motion to vertical ground motion is apparently greatly reduced because of the simpler design for the sensor package.     

潮汐测量标准装置系统设计

为使仪器检测能力满足海洋观测业务发展对潮位测量仪器性能的要求,结合各类验潮仪的检定规程,设计潮汐测量标准装置,应用先进的电气控制技术进行水位精准控制和潮汐模拟,实现仪器定点值测量和动态响应测试,通过图像识别软件实现水位和标尺动态识别,提高标准值的精确度,并集成远程控制功能,提升仪器检测效率,使装置能对潮位测量仪器开展完善合理且高效的检测,从而进行全面系统的仪器性能评价。     

海水温盐深剖面测量技术综述

温盐深是反应海洋物理学特性的重要参数,是海洋水文观测的基本要素。CTD剖面仪(Conductivity-Temperature-Depth profiler)是进行海水温盐剖面观测的主要仪器,利用CTD剖面仪可精确测得水下不同深度上海水的温度和电导率参数,进而能够推算出海水盐度、密度、声速等相关信息,对于海洋经济开发、海上国防建设、海洋环境保护等都具有非常重要的意义。本文介绍了温盐深剖面测量技术的基本原理与发展现状,对几种典型的温盐深测量设备及各种海洋观测平台中搭载的CTD传感器进行了介绍,论述了CTD传感器的标定和测试技术,并对其发展趋势进行了分析。     

Extending the versatility of the Bullard heat probe

A versatile probe for simultaneous studies of heat flow and near-bottom water parameters has evolved through modifications of the Bullard heat probe frame. Suitable sensor arrays have been used with this instrument to study (1) heat flow through the ocean floor, (2) water column temperature structure, (3) near-bottom current speeds, and (4) the differential cooling of water-column temperature sensors placed in a current speed gradient.Some of the advantages of such a modified Bullard probe are: (1) several parameters, including heat flow, can be measured across the sediment-water interface simultaneously, (2) the instrument frame is rigidly pinned to the ocean floor during measurement, permitting true Eulerian measurement in the water column with no effects of ship movement, swaying moorings or cable oscillation, and (3) the device is inexpensive and simple.     

The international ocean regime at the new millennium: a survey of the contemporary legal order

This study surveys the international regime governing the use of ocean space by examining various agreements that have come to comprise it. While the regime for the oceans is commonly viewed within the context of a general, universal convention for the law of the sea, the contemporary scope of global oceans law extends far beyond that single instrument. Indeed, the law of the sea is broad, far-reaching and reflective of the increasingly complex, interdependent nature of contemporary international relations. In this regard, by end of the 20th century there has emerged a pronounced trend toward worldwide participation in and adherence to the multifaceted legal regime established to manage the oceans. This panorama of legal instruments focuses both on international instruments and regional accords. The study's premise is this: Since 1958 several multilateral agreements have been negotiated and implemented specifically to regulate various uses of ocean space. Viewed as an integrated whole, these agreements comprise at the beginning of the 21st century an intertwined, sophisticated international legal regime for managing ocean activities. The range of agreements discussed here, however, is neither exhaustive nor meant to suggest that sufficient law is available for regulating all activities affecting the oceans now or in the future. Rather, the survey furnishes a starting point for appreciating just how far the international community has come in establishing a legal regime for managing people and their technological activities that impact on ocean space.     

海床基自动监测系统中央控制单元设计与实验

根据海洋监测仪器及系统工作环境的特点,需要从高可靠性、低功耗、长时间工作、小体积、轻重量等几个方面进行深入分析和研究。论文设计的海床基自动监测系统的中央控制单元,采用工业MSP430、带中断输出的时钟芯片DS1305、FLASH RAM和低阈值VMOS功率管等低功耗3.3 V器件,应用中断和休眠软件设计方法,很好地满足了使用要求。通过实验和分析,指出了低功耗海洋监测仪器及系统设计的基本原则,总结了MSP430闲置的管脚处理、上拉电阻阻值合理选择和电池自放电电流等问题的解决方案和应用经验。