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.     

Fidelity of ocean bottom seismic observations

The often poor quality of ocean bottom seismic data, particularly that observed on horizontal seismometers, is shown to be the result of instruments responding to motions in ways not intended. Instruments designed to obtain the particle motion of the ocean bottom are found to also respond to motions of the water. The shear discontinuity across the ocean floor boundary results in torques that cause package rotation, rather than rectilinear motion, in response to horizontal ground or water motion. The problems are exacerbated by bottom currents and soft sediments. The theory and data presented in this paper suggest that the only reliable way of obtaining high fidelity particle motion data from the ocean floor is to bury the sensors below the bottom in a package with density close to that of the sediment. Long period signals couple well to ocean bottom seismometers, but torques generated by bottom currents can cause noise at both long and short periods. The predicted effects are illustrated using parameters appropriate for the operational OBS developed for the U. S. Office of Naval Research. Examples of data from ocean bottom and buried sensors are also presented.     

Quantitative evaluation of a passively leveled ocean bottom seismometer

A problem in the use of ocean bottom seismometers is the difficulty in leveling the sensors while ensuring good coupling to the seafloor. We have investigated the coupling characteristics of the seismic sensors in the new ONR ocean bottom seismometer. In the deployable sensor package for that instrument, a three-component seismometer set is suspended on a 2-axis passive leveling gimbal and is immersed in a viscous fluid. We report tests, conducted in a seismic vault, comparing the output of a gimbaled seismometer set to that of a set rigidly coupled to the ground. Our results show that the degree to which the gimbaled set is coupled to ground motion is a function of the viscosity of the coupling fluid. The coherence between the two sensor sets is poor (<0.4) at some frequencies within the band of interest (0.15 to 20 Hz) and on some components when the viscosity of the coupling fluid is comparatively low (14 Pa-s or 0.16 kSt kinematic viscosity). In addition, the outputs of some components over portions of this frequency band are attenuated and are phase-shifted relative to the outputs of the set rigidly coupled to the ground. Coherence and phase response similarity improve as the viscosity of the coupling fluid is increased. With a coupling fluid viscosity of 980 Pa-s (10 kSt), coherence and phase agreement between the two sensor sets is good (>0.9) across nearly the entire band of interest on all three components. A simple analytical model of the gimbaled seismometer set as a damped, driven, compound-pendulum provides a basis for understanding the test results.     

Optimum design of Ocean bottom seismometers

Ocean bottom seismometers (OBS) have been widely used during the past decade to collect seismic data for determination of the structure of the oceanic lithosphere, stress patterns in regions of earthquake activity, and geoacoustic parameters of the ocean floor. Data quality from these experiments has often been disappointing because of poor signal quality and high noise levels. Many of these problems result from motion of the OBS package that is decoupled from motion of the ocean floor. These coupling problems are more serious in the ocean than on land because of the low shear strengths of most ocean sediments. In this paper we continue to develop the theory of coupling of OBSs to soft sediments and arrive at results suggesting that OBS packages should be designed with: (1) the minimum mass possible, (2) radius of area in contact with the sediment proportional to the cube root of the mass, and the maximum radius less than 1/4 of the shear wavelength, (3) density of the OBS approximately that of the sediment, (4) a low profile and a small vertical cross section with water, and (5) low density gradients, and maximum symmetry about the vertical axis. Agreement of the theory with test data is good; most deviations are reasonable, given limitations of the theory and experiments. The theory also suggests that the coupling frequency, the frequency above which the OBS does not follow the motion of the sediment, is directly proportional to the sediment shear velocity.     

An overview and general results of the Lopez island OBS experiment

The purpose of the experiment was to determine the effects of coupling and bottom currents on ocean bottom seismometers. Twelve operational OBSs, three specially designed three-component systems, and a hydrophone were compared with each other. Unlike seismometers placed on hard rock at land stations, ocean bottom seismometers can be affected by soft sediments (which act as lossy mechanical springs) and by buoyancy. Coupling through soft sediments can modify the response to ground motion much as a low pass filter does, and high buoyancy tends to counteract this effect. These effects are observed in the Lopez data, which consist of signals from mechanical transient tests, cap shots, airgun pulses, and general background noise. The modification of response is pronounced for some instruments and barely noticeable in others. Instruments that stand high in the water relative to their base width tend to be susceptible to rocking motion that shows up as a mechanical cross coupling between horizontal and vertical motion. Correlation of Lopez results with coupling theory suggests that it is possible to design ocean bottom seismometers that will couple well to any sediment. Current levels at the Lopez site (<5 cm s^-1) were too small to produce noticeable effect on any of the instruments; however, the same design criteria that will minimize coupling problems will also lessen problems caused by ocean currents.Hawaii Institute of Geophysics Contribution No. 1171.     

Current-generated noise recorded on ocean bottom seismometers

High-amplitude, anrrow band noise that correlates with periods of high ocean bottom currents and the tidal cycle is occasionally observed on ocean bottom seismometers (OBS). The geophones on OBSs of different configurations are not equally sensitive to this noise and hydrophones are almost unaffected. With a suitable design, it should be possible to eliminate this noise problem.Hawaii Institute of Geophysics Contribution No. 1173     

海底地震仪及其国内外发展现状

以改进型得克萨斯海底地震仪为主线,详细描述了海底地震仪的工作原理、主要仪器参数、观测方式和回收方法,同时,对美国、日本等几个国家和地区海底地震仪研制和实际观测工作方面的发展状况作了简要介绍,并对海底地震仪的发展趋势和应用前景作了展望。     

SEDIS IV型短周期自浮式海底地震仪数据校正方法

利用15台SEDISIV型短周期自浮式海底地震仪在南海中、北部地壳深部结构调查中所获得的资料,探讨了海底地震仪数据校正的方法和校正后的效果,结果表明:使用该地震仪所获得的原始资料经过放炮时间、炮点坐标数据局部化、海底地震仪位置误差以及记录时间漂移4方面的校正后,数据更趋合理,误差显著降低。放炮时间的校正消除了时钟漂移和时间延迟的误差;炮点坐标数据局部化处理消除了炮点位置整体趋势性偏移的现象;试错法进行位置误差和记录时间的精细校正时,时间漂移的校正量值约为几个到十几个毫秒,位置校正的量值仅在几米到数百米之间,实测数据所绘曲线的形态和位置都与理论曲线十分吻合,可见校正后误差显著降低。     

台湾海峡OBS地震记录中二次Ps震相的特征及应用

在海底地震仪(ocean bottom seismometer, OBS)广角地震记录剖面上, 经常可以见到震相清晰且连续的多次波信号, 多次波和初至波是由相同的震源信号产生的, 也是地壳真实结构的反映。但是在通常的OBS数据处理过程中, 经常将多次波作为无效信号剔除掉, 对其属性及应用的研究比较少。文章通过对台湾海峡南部OBS探测测线HXN01数据的处理, 对多个台站记录到的二次震相进行了识别与拾取, 并以OBS0106台站为例, 对识别出的二次Ps震相进行了系统的研究分析, 发现二次Ps震相的波形特征和质点运动轨迹与初至震相相似, 但波形最大振幅值明显大于初至震相。通过Rayinvr射线追踪方法模拟, 确定了二次Ps震相的主要反射层, 并发现加入二次震相后, 台站下方浅部沉积层射线覆盖密度有显著提升, 射线覆盖的区域也明显增加, 为沉积层精细结构的反演提供了更为丰富的数据基础。另外, 对理论模型的地壳结构进行加入二次Ps震相前后的反演测试, 结果显示加入二次Ps震相数据后, 沉积层的界面深度误差得到明显的改善。     

海底地震仪的性能对比及在渤海试验中的应用

2010年3月,国家海洋局第一海洋研究所在渤海海域开展了深部地震探测试验,试验过程中使用了3种海底地震仪(OBS),包括德国的SEDISⅥ型OBS、法国的Micr OBS和中国的I-7C型OBS共计51台。文章详细描述并比较了以上3种OBS的性能指标,通过对比发现:I-7C型OBS的耦合技术比其他两种OBS更加完善,功耗和数据存储容量方面优势明显;I-7C型OBS和Micr OBS在设置参数和充电时,操作方便,提升了海上工作效率;Micr OBS的水听器性能略有优势,获取的数据清晰度高,连续性好;SEDIS VI型OBS的垂直分量数据在三者之间最优。本文提出了OBS的发展趋势:(1)小型化、便携式;(2)低功耗、大容量;(3)长周期、宽频带;(4)低成本,易回收。最后简要介绍了本次海上试验过程、数据处理方法以及探测试验取得的成果。本次试验成功取得了渤海海域深部地球物理数据,对开展该区域内的深部地质研究具有重要的意义。     

The estimation of ocean bottom seismographs' coupling characteristics by means of microshock recordings

A transient technique was used for estimating the bottom-instrument response function in experiments with bottom seismographs (OBS) in deep ocean. The sharp mechanical impacts on bottom seismograph usually induced by bottom displacements under the instrument (microshocks) are suggested as rough analogues of the impulses for the bottom-instrument system transient calibration. It has been found that microshocks has usually sufficiently small duration to be used for coupling characteristics estimates. Test measurements have shown that in many cases this method makes it possible to distinguish spectral features characterising earthquakes and seismic noise wavetrains from those which are caused by coupling resonances of the OBS-sediment inter-face.     

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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Ocean-bottom-seismograph of the Institut für Geophysik,Hamburg

The ocean bottom seismograph (OBS) of the Institut für Geophysik, Hamburg (IfG) is designed for refraction seismic experiments and for recording microseismic noise. Hydrophone signals are recorded directly on a casette tape recorder with a band width of 3–60 Hz. Signals from three component 1 Hz seismometers are recorded on a 2nd casette tape recorder in FM for a frequency range of 0.1–1 Hz. A telemetering buoy at the surface is connected with the OBS by a polypropylene rope.     

Infrasonic seismic and acoustic measurements in the deep ocean

The authors compare the signal-to-noise ratios obtained on bottomed seismometers, bottomed hydrophones, and buried seismometers from near-surface explosions in the Ngendei Expedition. The data were recorded in 5.5-km-deep water in the south central Pacific Ocean with a triaxial borehole seismograph and four triaxial ocean-bottom seismographs having externally mounted hydrophones. At ranges less than 35 km, the data indicate that the ocean bottom seismometer is a superior signal detector than the ocean-bottom hydrophone, and that the subbottom seismometer is superior in performance to the ocean-bottom seismometer. Above 4 Hz, the seismometer appears to have a 10-dB signal-to-noise advantage over the hydrophone for surface explosions at ranges less than 30 km     

Instrumental waveform distortion on ocean bottom seismometers

Data from the 1978 Lopez Island OBS Intercomparison Experiment and deep sea data from University of Washington OBSs show that there is a considerable amount of waveform distortion resulting from the conversion of horizontal motion into vertical motion, here called cross-coupling distortion. This distortion, which substancially reduces the significance of waveform matching with synthetic seismograms, appears to result from rotation imparted to the OBS package by near-vertically traveling shear energy. The degree of this rotation seems to depend on the instrument surface area above the seafloor and the geometry and surface area of the feet connecting the package to the seafloor. The sensitivity and response of the seismometers within the package to this rotation depends on the precise location of the seismometers with respect to the axis of rotation. The results suggest how to modify OBS designs to minimize these effects.University of Washington Contribution No. 1225.     

A digital event-recording ocean bottom seismometer capsule

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.     

OBS海洋环境信号分析与应用

为研究海洋环境信号在OBS(Ocean Bottom Seismograph)原始数据中的规律及应用,根据OBS原始数据的波形及频谱特征,将研究区划分为5个时间段,依次为旧涌浪阶段、风浪渐强阶段、风浪全盛阶段、风浪消退阶段和新涌浪阶段。结合海洋天气预报,认为上述现象是由偏南风风浪对海流的影响造成的。参考野外地震数据采集记录班报,得到各阶段的时长和距离,计算风浪渐强、全盛和消退阶段OBS附近海流的平均速度。结果表明:OBS原始资料中浅海海洋环境噪音增强的主要因素是风浪,且风浪引起的噪音信号的波形变化特征是渐进式的;OBS可用于接收某种特殊阶段(如台风、海啸等)的噪音信号,并根据噪音信号的波形特征、频谱变化规律和持续时间估算该阶段的海流速度变化。     

Effect of the water-saturated sediment layer on recording seismic signals with a bottom seismometer

Recording seismic signals on the bottom is accompanied by specific distortions caused by resonance phenomena. In the literature, such distortions are explained by the natural vibration of the heavy housing of a seismometer on a soft elastic sediment layer. Meanwhile, there are experimental results that contradict this model. In the present paper, we consider the rheological properties of the bottom sediments, which in fact were not taken into account previously. The model of a viscoplastic medium was used (the Bingham model), and the parameters of the model were experimentally determined. The estimates show that, in the frequency range from 0.003 to 30 Hz used in broadband bottom seismology, the effect of the mass of the seismometer on the results of recording on a soft bottom is negligible. Large errors can be introduced only when a seismometer is placed on rubberlike media such as peat soil, algae aggregations, etc. Resonance phenomena in recording signals on the bottom can occur when seismic waves propagate through a layer of water-saturated sediments. These phenomena are more pronounced for shear waves, whereas the distortions of the longitudinal waves propagating through the water-saturated layer are relatively weak.     

Ocean bottom seismograph development at Hawaii Institute of Geophysics

Three distinct ocean bottom seismograph (OBS) systems have been developed at the Hawaii Institute of Geophysics to satisfy the different requirements for short-range refraction and anisotropy experiments, long-range refraction experiments, and short-term and semi-permanent monitoring for earthquakes. One system, originally designed for semi-permanent use in conjunction with a monster buoy of the IDOE North Pacific Experiment has been modified for emplacement off Oahu. It contains 3-component 1 Hz seismometers and a hydrophone and obtains power and transmits data via tow conductor cable. Two additional systems were designed for short-term use: a 2 Hz telemetering system (TOBS); and 4.5 Hz free-fall pop-up system (POBS). The TOBS contains 3-component seismometers and a hydrophone and transmits data to the ship via light-weight single-conductor electromechanical cable and an HF-VHF radio link from a surface buoy. The bottom package also includes a backup tape recorder. This system exhibits the advantages of real-time data acquisition (e.g. precise timing, rapid appraisal of data quality, optimum use of explosives, and common recording with other data) and the complexities and difficulties associated with a deep-sea mooring. However, use of cable with near neutral bouyancy permits the design of a deep-water system with low weights and stress levels. The POBS is a self-contained package containing a vertical and single horizontal seismometer, hydrophone, cassette tape recorder, and pre-set timed release. This system is relatively simple and inexpensive. Total weight of 150 kg in air (before launch) permits emplacement and retrieval from a ship with no special equipment by two (strong) persons. Experience to data suggests that the optimum deployment scheme for many studies is a combination of TOBS's and POBS's.Hawaii Institute of Geophysics Contribution 835.     

On the nature of microshocks recorded by ocean bottom seismographs

The results of the study of short impulsive signals (microshocks) which constitute a specific type of noise on the records of ocean bottom seismographs are given. Various possible causes of their origin have been analysed. It is shown that the great majority of microshocks are produced by external causes: bottom displacements under an instrument at the deployment site and the mechanical action of marine organisms on OBSs. To cope with this kind of noise the use of parallel recording at two seismometers some distance apart is suggested.