Isolated sensor Ocean Bottom Seismometer

In most of the Ocean Bottom Seisometers (OBS) used today, the sensors, electronics, recorders, flotation, and ballast are contained in one rigid package. Usually this configuration requires a large mass, a large vertical cross section in the water, and relatively small bearing surface area in contact with the bottom, resulting in poor seismic characteristics and increased noise sensitivity. An OBS recently developed at the Hawaii Institute of Geophysics (HIG) minimizes these problems by physically separating the sensor from the main OBS package. Direct comparison between signals recorded by a standard HIG configuration Pop-up Ocean Bottom Seismometer (POBS) and signals from the new Isolated Sensor Ocean Bottom Seismometer (ISOBS), deployed near each other in deep water, clearly demonstrates the advantages of the isolated sensor configuration. Although the ISOBS is superior to older OBS designs, recent testing suggests that further improvements can be made.     

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.     

南海东部次海盆地震背景噪声分析

背景噪声的强弱是影响地震台站观测的一个重要因素.获取背景噪声的分布特征对评估海底地震仪记录数据质量及对数据的降噪处理均具有重要的指示意义.利用概率密度函数方法获取台站数据的功率谱密度的概率分布特征并与全球背景噪声高值模型和低值模型进行对比是研究台站周围环境背景噪声水平的有利手段.本研究基于南海大规模的被动源海底地震仪台...     

New pop-up type Ocean Bottom Seismometer

We designed a new pop-up type Ocean Bottom Seismometer (OBS) in order to study micro-earthquakes in off-shore areas. With a 57 cm O.D. sphere of high tension aluminium alloy, the OBS system, including one vertical and one horizontal geophone, can safely operate on ocean floors of up to 6000 m depth for seismic observations. The amplified seismic data and the time code are directly recorded on the four-channel cassette deck for periods of up to one month. The frequency response curve throughout the recording and play-back system is flat for the range, 1–15 Hz (–3 dB). The anchor release and the geophone clamp are operated by an acoustic command signal.So far, we have deployed our OBS's 42 times in the ocean. All of the OBS's deployed have been recovered safely. Seismic data has provided seismological evidence for a number of processes associated with tectonism along subduction zones and spreading ridges (e.g., Eguchi et al., 1986).     

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.     

Operating experience with a BM-04 wave-tide gauge

A BM-04 microprocessor wave-tide gauge developed at the Shirshov Institute of Oceanology is used for the acquisition of data on the parameters of surface waves (the mean height \(\bar h\), the mean period \(\bar \tau \), the frequency spectrum S(ω), and so on) and the mean sea level ζ. As a wave gauge, the instrument can be deployed on the sea floor, on piles, and so on at depths down to 10 m in the near-shore zone and can be mounted on subsurface buoys offshore. As a tide gauge, the instrument can be used at depths down to 6000 m. The instruments were successfully tested and intercalibrated in the Black and Baltic seas and the Sea of Okhotsk.     

A note on the seafloor coupling characteristics of the new ONR ocean-bottom seismometer

A series of transient tests were conducted to determine the seafloor coupling characteristics of a new ocean-bottom seismometer (OBS) developed for the United States Office of Naval Research (ONR). The OBS comprises a large recording package and a separate sensor package that is deployed from the recording package. In addition to the coupling characteristics of both the sensor and the recording packages, the seismic energy radiated from the main recording package as a result of motion of the recording package was measured. The observed vertical coupling resonances of both the recording package and the sensor package are in good agreement with those predicted by a simple model of soil-structure interaction. The most important result of this study is that significant energy is radiated from the recording package in response to horizontal motions of the recording package. When the sensor package is 1 m from the recording package, the amplitude of the recorded signal is similar to that recorded in the recording package. In the field, this effect will result in distortion of seismic signals and increased background noise recorded by the sensor package if the recording package is disturbed by seafloor currents or biological activity. The amplitude of this signal attenuates by approximately a factor of two as sensor/recorder separation is increased from 1 to 6 m, suggesting that an improved response can be achieved by increasing the separation between the recording package and the sensors. This effect is much less severe for vertical disturbances of the recording package.     

南冲绳海槽海域微震活动的初步探讨

通过解析在南冲绳海槽海域投放的海底地震仪（ＯＢＳ）记录到的天然地震资料，得到近垂直海槽走向的微震震中分布．根据震中分布与海底地形、地质构造的对比，提出：南冲绳海槽海域的微震活动频繁且多数集中于海槽轴部的地堑和琉球岛弧中部．     

利用海底地震仪数据分析台风对海底环境噪音的影响

在海底布设的海底地震仪（OBS）能比较清晰地记录到海底的环境噪音,而台风可以直接或间接的产生在海底传播的弹性波,从而影响海底的环境噪音,并在较大程度上影响OBS的数据记录。本文通过分析台风对工作区的整个影响过程中OBS记录数据的振幅变化,再选择合适的滤波方式,首次发现台风产生的风浪及涌浪在短周期海底地震仪的记录数据上有良好的表现特征,指出了台风对海底环境噪音的另一种可能的影响方式,并由此得出：1）台风产生的风浪和涌浪对海底环境噪音的影响模式不同;2）风浪和涌浪所加强的海底环境噪音的范围和程度不同;3）短周期OBS可以比较清晰的记录涌浪信息,其周期主要是6—8 s,且能量稳定（简称“8秒现象”）。这三点结论为后期的海洋地震研究和海洋学其他研究提供经验与借鉴。     

Particulate organic carbon budget in the open Algero-Balearic Basin (Western Mediterranean): Assessment from a one-year sediment trap experiment

With the aim of improving the knowledge of the open ocean carbon cycle, we present a budget of particulate organic carbon (POC) fluxes carried out in the deep central part of the Algero-Balearic Basin (ABB) at 2850 m water depth based on a single mooring equipped with five automated sediment traps deployed from April 2001 to May 2002 at depths of 250, 845, 1440, 2145 and 2820 m. Suspended particulate matter (SPM) and superficial sediments were also used as indicators of hydrodynamics and carbon burial, respectively. The data reveal that the fraction of primary production buried in the sediment, which finally leads to the sequestration of carbon dioxide from the atmosphere, is 0.16%, lower than the values found in the nearby continental margin regions such as the Alboran Sea (0.48–0.89%) but of the same order as recorded at other Mediterranean sites at similar depths, such as the Ionian Sea (0.11%). As they sink through the water column, the particles exhibit decreases in flux that are similar to those observed elsewhere, but also show variations that appear to correlate with hydrological features of the water masses present in the basin, as revealed by SPM concentrations and compositions. The input of the tyrrhenian deep water (TDW) into the ABB at 800–1500 m of water depth exhibits low suspended POC concentrations and low sinking POC fluxes were also observed in this depth range. Gulf of Lions water mass formation appears to also contribute to elevated suspended POC concentrations and perhaps POC accumulation in the traps and sediments by spreading of dense cold water along the whole ABB that supplied POC at depths higher than 2000 m.     

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.     

Detection of Intermediate Mediterranean Waters in the Atlantic Ocean by ARGO Floats Data

Peculiarities of the spatial distribution of intermediate Mediterranean waters (MW), which are the main source to maintain the heat and salt budgets at depths of 600–1500 m in the Atlantic Ocean, have been studied using the ARGO floats measurements database. About 75000 temperature and salinity profiles recorded by 900 ARGO floats in 2005–2014 in the Atlantic Ocean for latitudes from 20° to 50° N were used. To process these data, we used the ARGO-Based Model for Investigation of the Global Ocean (AMIGO). This technique allowed us for the first time to obtain a complete set of oceanographic characteristics up to a depth of 2000 m for different time averaging intervals (month, season, years). Joint analysis of the temperature, salinity, and velocity distributions at 700–1000 m depths made it possible to revise the distribution of MW and their penetration into the western part of the ocean across the Mid-Atlantic Ridge (MAR). It is shown that at depths of 700 and 1000 m, the Mid-Atlantic Ridge is a barrier to advective propagation of salty waters (>35.5 PSU) to the west and is transparent to fragments of destroyed intrathermocline lenses (ITL) with lower salinity (<35.4 PSU). In the Atlantic region, from 20° to 35° N and from 30° to 70° W, individual lens profiles with an anomalous salinity distribution were sought using ARGO measurements to detect ITL and its separate fragments. About 24 000 measurements from 370 ARGO floats were analyzed, and only about 3% of them showed weak salinity anomalies at 800–1200 m depths. No ITL were found from these observations. Analysis of long-term drifting of individual floats recording temperature and salinity profiles with anomalous layers made it possible to study the nature of MW transport through the MAR.     

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.     

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.     

Very low frequency Ocean Bottom ambient seismic noise and coupling on the shallow continental shelf

Sources of very low frequency (0.01 to 1.0 Hz) ambient seismic noise in the shallow (<100 m) water continental margin sediments are investigated using Ocean Bottom Seismometers (OBS). The predominant seismic motions are found to be due to surface gravity (water) waves and water-sediment interface waves. Actual experimental measurements of seabed acceleration and hydrodynamic pressure are given, including side by side comparisons between buried and plate-mounted OBS units. OBS-sediment resonant effects are found to be negligible at the low frequencies under investigation. Wherever there exists relative motion between the seabed and the water, however, an exposed OBS is subject to added mass forces that cause it to move with the water rather than the sediments. Calculations based on measured seabed motions show that a neutral density, buried seismometer has superior sediment coupling charactersitics to any exposed OBS design.     

Vertical distribution of mesozooplankton in the interaction zone of the Canary and the Northern Branch of the Intertrades currents

The analysis of the vertical distribution of the zooplankton and of the peculiarities of the specific, biotopic, trophic, and spatial structures in relation to the hydrological features was performed on the basis of the data on the vertical zooplankton sampling fulfilled at a transect along the coast of Morocco and Mauritania over depths of 1000 m in the summer of 2004. The surveyed area is inhabited by five zooplankton complexes: three surface and two interzonal ones associated with the water masses of different origin. The vertical boundary between the southern and northern complexes corresponds to the southern boundary of the Senegal-Mauritanian frontal zone, while the horizontal boundary between the surface and the interzonal complexes is mainly at the depths of about 200 m, and its location is basically determined by the water’s vertical dynamics. The surface and interzonal complexes of northern origin are involved in the formation of a highly productive zone in the waters of Cape Blanc. The formation of this zone is not related to upwelling.     

Bottom seismometer observation of airgun signals at Lopez Island

First arrival compressional wave signals from an airgun source, as detected by a variety of seismometers in a shallow bay, are remarkably uniform. However, minor variations in wavelet appearance imply some combination of the instrument response and coupling to the bottom. Signal spectra show typically a spectral peak at 12 Hz and an envelope very similar to that expected from an airgun source. Those instruments with a decoupled geophone package have spectra most like the theoretical spectrum but spectra for the other instruments are not significantly different. Little variation exists in spectra between tripod-mounted and inverted-pendulum OBS configurations for the low amplitude P-waves observed here. The signal source is the principal influence on the resulting spectra rather than OBS configuration or bottom coupling.     

Nature of “transparency windows” in the broadband spectrum of seafloor microseisms

The peculiarities of the microseisms spectra shape when recording on the ocean floor in the frequency band of 0.003–20 Hz are examined. The origin of the stable minima in the microseisms spectrum (“transparency windows”) at frequencies of about 0.02–0.1 Hz and 5–15 Hz is analyzed. In these frequency bands, weak earthquake signals are recorded by bottom instruments. The origin of the low-frequency “transparency windows” can be explained by the conditions of the microseisms propagation in the oceanic waveguide (between the bottom and the water’s surface) in the abyssal plain zones. The results of the full-waveform numerical simulation of the seismoacoustic waves propagation in the oceanic environment and on the ocean-continent border are presented, and the experimental data as well. The peculiarities of the microseisms spectra in the band of high-frequency “transparency windows” can be caused by the constructive resonance in the water-saturated layer of bottom sediments. The theoretical foundation and experimental results are given.     

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.     

Submerged terraces in the Laccadive Islands,India

The Laccadive atolls have recently been surveyed in detail (1:5,000 and 1:10,000). These surveys indicated the occurrence of a number of terraces on the seaward reefs of these islands at 7–12, 15, 21–36 (prominent) and 43–47 m depths. The first break in the profile of the seaward reefs occurs at depths of 3–5 m and on the windward side this extends down to about 12 m, the break at this shallow depth represents a wave-cut platform. The deeper terraces (>12 m) appear to have been formed during periods of lowered sea level. The most prominent terrace at 21–36 m is correlable with similar terraces recorded on other atolls. The terraces at Bangaram Island are covered by sediment while those at Kadmat do not have any sediment. In some of the atolls, surfaces at depths comparable to the seaward terraces are found in the lagoons also indicating extensive effects of lowered sea levels.