Precise Positioning of Ocean Bottom Seismometer by Using Acoustic Transponder and CTD

We have obtained precise estimates of the position of Ocean Bottom Seismometers (OBS) on the sea bottom. Such estimates are usually uncertain due to their free falling deployment. This uncertainty is small enough, or is correctable, with OBS spacing of more than 10 km usually employed in crustal studies. But, for example, if the spacing is only 200 m for OBS reflection studies, estimates of the position with an accuracy of the order of 10 m or more is required.The determination was carried out with the slant range data, ship position data and a 1D acoustic velocity structure calculated from Conductivity–Temperature–Depth (CTD) data, if they are available. The slant range data were obtained by an acoustic transponder system designed for the sinker releasing of the OBS or travel time data of direct water wave arrivals by airgun shooting. The ship position data was obtained by a single GPS or DGPS. The method of calculation was similar to those used for earthquake hypocenter determination.The results indicate that the accuracy of determined OBS positions is enough for present OBS experiments, which becomes order of 1 m by using the DGPS and of less than 10 m by using the single GPS, if we measure the distance from several positions at the sea surface by using a transponder system which is not designed for the precise ranging. The geometry of calling positions is most important to determine the OBS position, even if we use the data with larger error, such as the direct water wave arrival data. The 1D acoustic velocity structure should be required for the correct depth of the OBS. Although it is rare that we use a CTD, even an empirical velocity structure works well.     

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.     

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.     

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.     

基于LZW的海底声学探测数据压缩方法研究

针对海底声学探测仪器采集数据量大而存储容量有限、数据传输带宽不足的实际问题,基于Lempel-Ziv-Welch(LZW)无损压缩算法,研究海底声学探测数据的实时压缩方法,提高数据压缩效果、节省传输带宽。并在LZW无损压缩算法的基础上结合数据存储的特点对压缩结果进行内存重新分配,极大提高压缩比(压缩数据大小/原始数据大小)。利用海底地震仪(OBS)采集的原始声学探测数据进行测试验证,结果表明该方法对于OBS声学探测数据有很好的压缩比,可用于对OBS采集的声学探测数据进行压缩处理,对于海底探测仪器的研发有很好的指导意义。     

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.     

Coupling parameters of the MIT OBS at two nearshore sites

A model representing the coupling of an ocean-bottom seismometer (OBS) to the seafloor as a mass-spring-dashpot system satisfactorily explains the results of transient tests performed on different instruments during the Lopez Island intercomparison test. In this paper, we compare the results obtained for the MIT OBS at Lopez Island to results from similar tests at a dockside site at Woods Hole, Massachusetts. The vertical instrument response at the Lopez Island site shows a highly damped resonance at a frequency of 22 Hz, whereas the response at the Woods Hole site shows a marked resonance at 13 Hz. The difference between the responses at the two sites can be qualitatively attributed to the difference between the surficial sediments.     

Evidence that biological activity affects Ocean Bottom Seismograph recordings

Brief and impulsive signals of uncertain origin appear regularly on records from Ocean Bottom Seismographs (OBS) of several institutions. These signals have been recorded on nearly all deployments of the Texas OBS, including sites at depths greater than 7000 m. At some sites, they account for over 90% of the events recorded. They are of short duration (usually 0.5–4.0 s) and have a characteristic frequency (usually in the range of 4–18 Hz) that differs from site to site. When networks of OBS instruments are deployed, the signals are not recorded simultaneously by different instruments. Neither the frequency content nor the distribution of durations of these signals is similar to what is observed for known earthquake events.We present evidence suggesting that the signals are of biological origin, perhaps caused by animals touching the OBS units. (1) The distribution of these signals on instruments deployed at depths shallower than 1000 m shows a 24 h periodicity, while there is a 24 h periodic pattern on instruments deployed at sites deeper than 1000 m (where there is no visible light). (2) The frequency of occurrence of signals is similar to the vertical distribution of biomass in the oceans, i.e., they appear most frequently on OBS instruments deployed at very shallow depths. (3) Biological material has been found attached to several OBS units upon recovery.University of Texas Institute for Geophysics contribution number 468.     

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。     

Earthquake activity in the Southern Vanuatu arc recorded by the Texas digital OBS

We have reconfigured the Texas digital ocean bottom seismograph (OBS) to operate in a triggered mode and record regional earthquake signals. This paper reports the results of a deployment program designed to test these digital OBS, by moni toring earthquake activity in and near the trench in southern Vanuatu (formerly, the New Hebrides). We successfully recorded hundreds of earthquakes, including 133 located regional earth quakes recorded by three or more stations. We also report J-B residuals for 21 earthquakes reported and located by the ISC. Fourier analysis of seismograms from regional earthquakes suggest that the frequencies of spectral peaks at any station were nearly the same for large, small, nearby, and distant events. However, we obtained very disparate frequencies when we analyzed seismograms for the same earthquake recorded at different stations. The most plausible interpretation is that spectral peaks do not depend on the characteristics of the earthquake source, but instead on site characteristics, or, more specifically, on the coupling of the instrument to the seafloor. To record reliable spectral data, we need to overcome this problem.     

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.     

2.5-D seismic tomographic modelling of the crustal structure of north-western Spitsbergen based on deep seismic soundings

Deep seismic sounding measurements were performed in the continent-ocean transition zone of the northern Svalbard continental margin in 1985 and 1999. Data from seismic profile AWI-99200 and from additional crossing profiles were used to model the seismic crustal structure of the study area. Seismic energy (airgun and TNT shots) was recorded by land (onshore) seismic stations, ocean bottom seismometers (OBS), and hydrophone systems (OBH). 3-D tomographic inversion methods were applied to test the previous 2-D modelling results. The results are similar to the earlier 2-D modelling, supplemented by new off-line information. The continental crust thins to the west and north. A minimum depth of about 6 km to the Moho discontinuity was found east of the Molloy Deep. The continent-ocean transition zone to the east is characterized by a complex seismic velocity structure according to the 2-D model and consists of several different crustal blocks. The zone is covered by deep sedimentary basins. Sediment thicknesses reach a maximum of 5 km. The Moho interface deepens to 28 km depth beneath the continental crust of Svalbard.     

Marine seismics with a pulsed combustion source and Pseudo Noise codes

There has been a long-standing debate concerning how dangerous seismic surveys are with respect to marine life. Marine seismic work today is dominated by airgun technology, where high energy is generated by a release of compressed air into the water. The objective of the “Time coded impulse seismic technique” project is to examine whether a new low energy acoustic source can be used for seismic purposes. If the method turns out to be successful, the low output energy and continuous operation will make the source suitable in environmental sensitive areas. The Low level Acoustic Combustion Source (LACS) is a petrol driven pulsed underwater acoustic source. It operates at a few meters depth, and each shot can be digitally controlled from the surface by a computer located in the mother vessel. A presentation of the recorded LACS signal characteristics, the modulation, the Pseudo Noise coding/decoding principles and field test results, is given. The importance of using an optimized code with fine resolution and of using the near field recording as a correlator sequence is demonstrated. Clear correlation peaks could then be seen from the bottom and sub bottom reflectors.     

Coupling of ocean bottom seismometers to soft bottom

Unlike response of seismometers resting on hard rock where the seismometer case moves with the rock to high frequencies, the response of ocean bottom seismometers (OBS) can be strongly affected by the low mechanical strength of ocean sediments. The motion as measured by the seismometer will not follow the expected relationships between pressure and particle motion for different wave types. Cross coupling between horizontal and vertical motions can occur, especially when there is differential motion between water and sediment. Resonant amplification and attenuation of higher frequencies also occur. Secondary seismic arrivals are especially subject to distortion. Overall response is strongly dependent upon the mass and configuration of the OBS and the rigidity and density of the bottom material. Tests at Lopez Island, Puget Sound using both directly applied mechanical transients and seismic signals with various instrument configurations demonstrate the above effects and provide some guidance for improved designs.Hawaii Institute of Geophysics Contribution No. 1172.     

An active source electromagnetic sounding system for marine use

Instrumentation has been developed for carrying out active source electromagnetic sounding experiments in the deep oceans. Experiments of this type are directly and uniquely sensitive to the presence of molten or partially molten material, to temperature structure and to the porosity of upper crustal rocks such as those that accommodate hydrothermal circulation systems. Electromagnetic sounding experiments therefore represent an extremely desirable addition to the existing range of geophysical techniques for studying geological processes in thermally, hydrothermally or magmatically active regions—for example, at oceanic spreading centres.The instruments can be operated in regions of rugged, unsedimented sea bottom terrain, and are designed for investigating the distribution of electrical conductivity within the oceanic crust and uppermost mantle. The instrumentation consists of a deep towed, horizontal electric dipole transmitter and a set of free-fall, sea bottom, horizontal electric field recording devices.The transmitter is a deep-towed instrument, which is provided with power from the towing ship through a conducting cable. The transmitter package is fitted with an integral echo sounder, which allows it to be towed safely a short distance above the seabed. Electromagnetic signals are transmitted from a neutrally-buoyant antenna array, which is streamed behind the deep tow.The sea bottom receiving instruments each consist of a recoverable package which contains the instrumentation and digital recording system, an acoustic release unit, four low-noise, porous electrodes arranged in two orthogonal, horizontal dipoles, and a disposable bottom weight.The instruments have been used at sea on three occasions. On their most recent use, active source signals were successfully recorded during an experiment to investigate crustal magmatism and hydrothermal circulation beneath the axis of the East Pacific Rise.     

海底地震仪在天然气水合物勘探中的应用综述

海底地震仪(OBS)直接布设在海底,在地球动力学研究、油气勘探以及地震学研究等众多领域中应用广泛。随着采集技术的提高,海底地震仪也越来越广泛地应用到水合物的勘探中。文中从OBS数据采集、数据处理以及速度反演三个方面对OBS在水合物勘探中的应用现状进行综述,重点在于综述目前的应用现状和OBS使用的难点。数据采集方面主要综述了在OBS水合物勘探中几种常见的观测系统;OBS数据处理方面主要综述OBS数据预处理和OBS数据镜像成像;OBS速度反演方面主要综述常用的反演方法及其优缺点;最后,在以下几个方面对OBS在水合物勘探中的应用进行了展望:(1)优化观测系统的设计;(2)发展更先进的波场分离算法;(3)基于全波形反演以及基于面波的速度反演。     

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。     

OBS记录的时间和定位误差校正

根据地震波传播的时距关系,利用海底地震仪子母钟对时记录、测深数据和近炮点数据,使用数据处理软件OBSTOOL,对南海北部采集的海底地震记录进行高精度时钟校正处理。通过反演确定放炮延迟和海底地震仪的漂移参数,为准确反演速度奠定了基础。分析结果显示,南海北部OBS数据放炮延迟为1.278s,最大时钟校正量为2.394 s,OBS偏离测线最大距离为652m,这些将会导致浅层正、反演结果不准确,影响BSR的正确识别。     

Biological interference of optical backscatterance sensors in Tampa Bay, Florida

Optical backscatterance (OBS, D&A Instruments, Inc.¹) sensors for measuring suspended-solids concentrations have been deployed in Tampa Bay to monitor resuspension of bottom sediments. This paper describes biological factors that affected the OBS sensors deployed in Tampa Bay and discusses deployment strategies that minimize biological interference. Phytoplankton may interfere with the OBS sensors when the suspended-solids concentration is near or below the sensor response threshold. Fish swimming in front of the OBS sensors caused spikes in the OBS sensor output, so the median average was more appropriate than the mean average. An algal slime on the OBS sensors caused excessive backscatterance that dominated the backscatterance from suspended material. Because of the fouling problem, deployments were limited to less than a week, and OBS sensors were cleaned daily, if possible. Calibration of OBS sensors with water samples collected from Tampa Bay was satisfactory when biological interference was not significant. When properly deployed, the OBS sensors can successfully monitor sediment resuspension in Tampa Bay and similar subtropical estuaries.     

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.