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  • 1
    Electronic Resource
    Electronic Resource
    Quantitative evaluation of a passively leveled ocean bottom seismometer (1994)
    Springer
    Marine geophysical researches 16 (1994), S. 347-363 
    Details
    ISSN: 1573-0581
    Keywords: Ocean bottom seismometer ; seismic coupling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01203972
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  • 2
    Electronic Resource
    Electronic Resource
    A simple shaker table for seismometer calibration (1984)
    Springer
    Marine geophysical researches 6 (1984), S. 311-328 
    Details
    ISSN: 1573-0581
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A unique and simple shaker table (shake table or shaking table), designed, constructed, and installed at the Hawaii Institute of Geophysics, has proven to be a valuable aid in testing and calibrating short period seismometers, as well as ocean bottom and ocean sub-bottom seismometer/tilt meter packages. It consists of a platform suspended in a stairwell by a single elastic cord (10 m extended length) driven by GeoSpace HS-10 geophones. Platform motion is monitored by orthogonal reference geophones and tilt meters. The relatively low natural periods of the platform, about 1.9 sec vertical and 6.5 sec horizontal, provide sufficient isolation from local vibrations that calibration can be made near operational amplitudes. Vertical or horizontal driver geophones can be driven by a commercial signal generator or white noise generator, or from magnetic tape output. The table can also be tilted with respect to the drivers to determine tilt tolerances and to calibrate tilt meters. A Hewlett-Packard 3582-A spectrum analyzer, used to analyze both reference and output signals, provides near real-time system cabibration and is an efficient means for investigating parasitic system resonances. The analyzer can also provide a white noise signal source to the driver geophones.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00286532
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  • 3
    Electronic Resource
    Electronic Resource
    Isolated sensor Ocean Bottom Seismometer (1983)
    Springer
    Marine geophysical researches 5 (1983), S. 437-449 
    Details
    ISSN: 1573-0581
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00305393
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  • 4
    Electronic Resource
    Electronic Resource
    Optimum design of Ocean bottom seismometers (1987)
    Springer
    Marine geophysical researches 9 (1987), S. 47-65 
    Details
    ISSN: 1573-0581
    Keywords: ocean bottom seismometer optimization
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00338250
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  • 5
    Electronic Resource
    Electronic Resource
    A note on the seafloor coupling characteristics of the new ONR ocean-bottom seismometer (1994)
    Springer
    Marine geophysical researches 16 (1994), S. 91-103 
    Details
    ISSN: 1573-0581
    Keywords: Ocean-bottom seismographs ; coupling seismographs to sea floor
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01224755
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  • 6
    Electronic Resource
    Electronic Resource
    Fidelity of ocean bottom seismic observations (1995)
    Springer
    Marine geophysical researches 17 (1995), S. 535-555 
    Details
    ISSN: 1573-0581
    Keywords: Ocean bottom seismometer ; coupling to the ocean floor ; seismic noise
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01204343
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  • 7
    Electronic Resource
    Electronic Resource
    Ocean bottom seismograph development at Hawaii Institute of Geophysics (1977)
    Springer
    Marine geophysical researches 3 (1977), S. 153-177 
    Details
    ISSN: 1573-0581
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00310108
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  • 8
    Electronic Resource
    Electronic Resource
    Current-generated noise recorded on ocean bottom seismometers (1981)
    Springer
    Marine geophysical researches 5 (1981), S. 109-115 
    Details
    ISSN: 1573-0581
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00310316
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  • 9
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    A note on the seafloor coupling characteristics of the new ONR ocean-bottom seismometer (1994)
    Springer
    Details
    Publication Date: 1994-04-01
    Print ISSN: 0025-3235
    Electronic ISSN: 1573-0581
    Topics: Geosciences , Physics
    Published by Springer
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  • 10
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    Fidelity of ocean bottom seismic observations (1995)
    Springer
    Details
    Publication Date: 1995-12-01
    Print ISSN: 0025-3235
    Electronic ISSN: 1573-0581
    Topics: Geosciences , Physics
    Published by Springer
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