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  • 1
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    PANGAEA
    In:  Supplement to: Muller, M R; Robinson, Carlos J; Minshull, Tim A; White, R S; Bickle, Michael J (1997): Thin crust beneath ocean drilling program borehole 735B at the Southwest Indian Ridge? Earth and Planetary Science Letters, 148(1-2), 93-107, https://doi.org/10.1016/S0012-821X(97)00030-7
    Publication Date: 2023-05-12
    Description: A wide-angle seismic experiment at the Atlantis II Fracture Zone, Southwest Indian Ridge, together with geochemical analyses of dredged basalt glass samples from a site conjugate to Ocean Drilling Program hole 735B has allowed determination of the thickness and the most likely lithological composition of the crust beneath hole 735B. The measured Na, composition of 3.3 +/- 0.1 corresponds to a melt thickness of 3 +/- 1 km, a result consistent with rare earth element inversions which indicate a melt thickness of between 1.5 and 4.5 km. The seismic crustal thickness to the north and south of the Atlantis Platform (on which hole 735B is located) is 4 +/- 1 km, and probably consists largely of magmatic material since the seismic and inferred melt thicknesses agree within experimental uncertainty. Beneath hole 735B itself. the Moho is at a depth of 5 +/- 1 km beneath the seafloor. The seismic model suggests that, on average. about 1 km of upper crust has been unroofed on the Atlantis Platform. However, allowing for the inferred local unroofing of 2 km of upper crust at 735B, the base of the magmatic crust beneath this location is probably about 2 km beneath the seafloor, and is underlain by a 2-3 km thick layer of serpentinised mantle peridotite. The P-wave velocity of 6.9 km/s for the serpentinised peridotite layer corresponds to a 35 +/- 10 vol% serpentine content. The Moho beneath hole 735B probably represents a serpentinisation front.
    Keywords: Aluminium oxide; Aluminium oxide, standard deviation; Calcium oxide; Calcium oxide, standard deviation; Cerium; Cerium, standard deviation; Chromium(III) oxide; Chromium(III) oxide, standard deviation; DEPTH, sediment/rock; Dysprosium; Dysprosium, standard deviation; Electron microprobe (EMP); Erbium; Erbium, standard deviation; Europium; Europium, standard deviation; Gadolinium; Gadolinium, standard deviation; Holmium; Holmium, standard deviation; Iron oxide, Fe2O3; Iron oxide, FeO; Iron oxide, FeO, standard deviation; Lanthanum; Lanthanum, standard deviation; LATITUDE; LONGITUDE; Lutetium; Lutetium, standard deviation; Magnesium number; Magnesium oxide; Magnesium oxide, standard deviation; Manganese oxide; Manganese oxide, standard deviation; Neodymium; Neodymium, standard deviation; Phosphorus pentoxide; Phosphorus pentoxide, standard deviation; Potassium oxide; Potassium oxide, standard deviation; Praseodymium; Praseodymium, standard deviation; Samarium; Samarium, standard deviation; Sample code/label; Silicon dioxide; Silicon dioxide, standard deviation; Sodium oxide; Sodium oxide, standard deviation; Terbium; Terbium, standard deviation; Thulium; Thulium, standard deviation; Titanium dioxide; Titanium dioxide, standard deviation; Ytterbium; Ytterbium, standard deviation
    Type: Dataset
    Format: text/tab-separated-values, 310 data points
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  • 2
    Publication Date: 2017-04-04
    Description: We are concerned with the detection and location of small seismic events, such as can be encountered in monitoring hydro-fracturing with surface sensors. Ambient seismic noise is the main problem in detection of weak seismic phases from these events, particularly as the sites of interest are often within or near producing fields. Band-pass filtering and stacking are the most widely used techniques for enhancing the signal-to-noise ratio (SNR) in passive seismic experiments, but they are of limited value when noise and signal share the same frequency band. Seismic arrays can be used to reduce the unwanted noise (e.g., traffic noise, pumping noise, scattering ground roll) by delay-and-sum techniques (also called beamforming) or by frequencywavenumber filtering. Beamforming maximizes the array response for the assumed direction and slowness of the signal. Whereas in some situations it can be highly effective, and the azimuth and slowness of the signal can be determined by a grid search approach, it is vulnerable to contamination by side-lobe energy, particularly for broadband signals and noise (Rost and Thomas, 2002). Frequencywavenumber filtering can be very effective but requires regularly spaced arrays and implicitly assumes plane-wave propagation. Both methods perform poorly when the waveform changes significantly between stations of the array, as might be caused, for example, by differences in site response. In this article, we present a multichannel Wiener filtering technique, which allows the removal of coherent noise from three-component 2D arrays without making a priori assumptions about the mode of propagation (e.g., no planewave assumption is required for the noise field). We test the effectiveness of this filter with two case studies. In the first case, we add synthetic signals of varying strengths to actual noise data recorded with a hexagonal array during hydrofracturing within a producing oil field in Wyoming, USA. Using this test, we are able to provide estimates of the smallest event detectable with the filtered data, and compare the results with conventional techniques, such as stacking. The second test case is a dense, small-aperture 2D seismic array of 95 stations placed within an area of 130 m 56 m on a landslide deposit in the Northern Apennines, Italy. Numerous microearthquakes have been recorded with this array, whose faint P-phases serve as an ideal data set for testing filtering techniques. Using the two case studies, we discuss the effectiveness of the multichannel Wiener filter on SNR improvement, and show that including horizontal components into the analysis increases the SNR improvement more than using only vertical components.
    Description: Acknowledgments: We thank Schlumberger Cambridge Research for providing funding for this project and for the hydrofracture surface monitoring experiment. However, the views expressed here are those of the authors, who are solely responsible for any errors. For the hydrofracture surface data, we thank the Schlumberger office in Rock Springs, Wyoming, for help with logistics and deployment, BP for permission to deploy seismometers on one of their fields, Anna Horleston and Sharif Aboelnaga for assistance in the field, and SEIS-UK for the loan of the seismometers. The Cavola data were acquired by the Istituto Nazionale di Geofisica e Vulcanologia, Italy. We thank the Cavola Experiment Team, particularly John Haines (University of Cambridge), Giuliano Milana, Giuseppe Di Giulio and Fabrizio Cara (Istituto Nazionale di Geofisica e Vulcanologia). Ed Kragh and Everhard Muyzert provided helpful advice. University of Cambridge, Department of Earth Sciences contribution No. ES9031.
    Description: Published
    Description: 232-238
    Description: 1.1. TTC - Monitoraggio sismico del territorio nazionale
    Description: N/A or not JCR
    Description: partially_open
    Keywords: seismic noise ; Wiener filters ; 04. Solid Earth::04.06. Seismology::04.06.10. Instruments and techniques
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 3
    Publication Date: 2017-04-04
    Description: Hydraulic-fracture induced microseismic events are usually small, and noise levels are high at the surface due to the activities associated with a producing oil field. Similarly, local arrays for the detection of local earthquakes will also benefit from reduced noise levels and detect smaller events. We present a frequency-dependent multi-channel Wiener filtering technique with linear constraints, which employs an adaptive least-squares technique to remove coherent noise in seismic array data. The noise records on a number of reference channels is used to predict the noise on a primary channel, which can then be subtracted. We implement and test first an unconstrained version of this filter, where maximal noise suppression can lead to signal distortion. Two methods of imposing constraints are then introduced to achieve signal preservation. We test this technique with two case studies. First, synthetic signals are added to actual noise from a pilot deployment of a hexagonal array (9 three-component seismometers, approximate size 150 m × 150 m) in an oil field; noise levels are suppressed by up to 11 dB (at 1 - 6 Hz). Secondly we use natural seismicity recorded at a dense array (∼10 m spacing) in Italy where the application of the filter reduces the signal-to-noise ratio by more than 20 dB (at 8 - 15 Hz), using 35 stations. In both cases, the performance of the multi-channel Wiener filters is significantly better than stacking, especially at lower frequency where stacking does not help to suppress the coherent noise. The unconstrained version of the filter yielded the best improvement in the signal-to-noise ratio, but the constrained filter is useful when waveform distortion is not acceptable.
    Description: Published
    Description: v133-v141
    Description: 1.11. TTC - Osservazioni e monitoraggio macrosismico del territorio nazionale
    Description: JCR Journal
    Description: open
    Keywords: Weiner filters, noise suppression, cavola array ; 01. Atmosphere::01.01. Atmosphere::01.01.08. Instruments and techniques
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 4
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The determination of melt distribution in the crust and the nature of the crust–mantle boundary (the ‘Moho’) is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading ...
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Quantifying the melt distribution and crustal structure across ridge-axis discontinuities is essential for understanding the relationship between magmatic, tectonic and petrologic segmentation of mid-ocean-ridge spreading centres. The geometry and continuity of magma bodies beneath ...
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 125 (1996), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The Southwest Indian Ridge marks an end-member of mid-ocean ridge spreading rates, with a current full rate of only 13–15 mm a−1. Most thermal models of mid-ocean ridges suggest a decrease in crustal thickness at such slow spreading rates, because conductive heat loss from the upwelling asthenospheric mantle decreases the volume of melt generated by decompression. Seismic measurements of the thickness of crust formed at very slow-spreading ridges are sparse. We have reanalysed data from a two-ship, split spread seismic refraction experiment conducted in 1962 on the southern flank of the Southwest Indian Ridge (Francis & Raitt 1967). We used synthetic seismograms to model amplitude variations, which were carefully recorded by the original investigators. 1- and 2-D modelling suggests that the seismic velocity increases smoothly with depth within the igneous crust, with an unusually high velocity of ∼6.0 km s−1 near the top of the crust, increasing to ∼7.0 km s−1 just above the Moho, and a crustal thickness of 5 km. Converted shear waves yield a Poisson's ratio of 0.30±0.01 in the crust, which is intermediate between values for gabbros and serpentinized upper mantle. the crustal thickness is consistent with a passive mantle upwelling model of melt generation at mid-ocean ridges. the unusual velocity structure may indicate the presence of gabbroic rocks near the seabed, unroofed by extension.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 119 (1994), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: As part of an intensive study of a small area of oceanic lithosphere, the British Institutions Reflection Profiling Syndicate (BIRPS) acquired closely spaced deepseismic-reflection profiles over the Early Cretaceous crust of the Cape Verde abyssal plain off West Africa. The survey consisted of profiles spaced at 4 km arranged into strike lines parallel to the old sea-floor spreading axis (‘isochron’ profiles) and orthogonal dip lines oriented in the original direction of spreading (‘flow’ profiles). A large-capacity, well-tuned airgun source and very quiet shooting conditions ensured a high signal-to-noise ratio for deep reflection. Devising a strategy for mitigating contamination from ‘wrap-around’ multiples arriving from previous shots enabled us to use the minimum possible shot-point interval (50 m) allowed for collecting long (18 s) records. Data processing was oriented towards a medium with low root-mean-square velocity, steeply dipping structure, and pervasive low apparent velocity noise from diffraction at the top of the igneous crust. The contrast between the isochron and flow profiles is striking. Isochron profiles are typically highly reflective throughout the igneous crust, consisting of bright, bidirectionally dipping reflection sets that extend in places from the top of the igneous basement down to the interpreted Moho reflection. These reflections do not offset intracrustal or top-basement structure and thus are not interpreted as faults: an igneous intrusive origin seems more likely. Flow profiles are more sparsely reflective but show individual steeply dipping reflections best developed in the upper igneous crust, continuing down in places to the Moho. Dipping reflections on the flow profiles are interpreted as major normal faults since they are clearly associated with offsets of the top of the basement as well as truncation of horizontal reflections within the igneous crust. The dominant dip of these reflections is to the west towards the spreading ridge axis. Reflections from the vicinity of the Moho, while well developed in some places, are not particularly prominent across the survey area. Moho reflections appear to show a different structural relation to crustal features on the isochron and flow profiles: on isochron profiles, dipping reflections occasionally flatten out into, and may merge with, the Moho reflection; on flow profiles, as dipping crustal reflections approach the Moho reflection, they are usually abruptly cut off by it without extending deeper. This survey shows how oceanic crustal structure can vary rapidly over relatively small areas, provides convincing evidence that a structurally complex fabric dominates oceanic igneous crust, and gives a conclusive observation of faults that penetrate the entire oceanic crust.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 101 (1990), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Diapiric structures showing striking spatial periodicity have been widely observed at shallow levels on single and multichannel seismic reflection profiles within Indus fan sediments. The study area is some 200 to 400 km from the mouth of the Indus at the fan's northern margin in the region of the Murray Ridge. Neighbouring diapirs are generally associated with the same, acoustically transparent layer. They form at depths less than 500 in below the seafloor and have the form of gentle waves on the upper surface of this ‘parent’ layer. The acoustic appearance of the features may be controlled by the presence of methane, which has been widely reported in the area.The diapirs are formed by gravitational instability of overpressured muds within the rapidly deposited sedimentary sequence. Simple 2-D, two-layer gravity instability modelling of diapir wavelengths and growth rates has determined values of approximately 104 to 106 for the ratio of equivalent viscosity of the overburden sediments to that of the diapiric layer; and a maximum equivalent viscosity of 1011 to 1012 Pa s for the diapiric layer itself.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 2482-2486 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The magnetization of a high-beta (plasma energy density/magnetic field energy density≡β≥1) hydrogen-plasma beam injected into a vacuum transverse magnetic field is studied experimentally. Nominal parameters were Ti≈1 eV, Te≈5 eV, n≤3×1013 cm−3, vi≤7×106 cm/sec, tpulse〈70 μsec, Bz≤300 G. Plasma characteristics were measured for a wide beam, a/ρi≤35, and a downstream distance, x≤300ρi, where a is the beam radius, x is the downstream distance, and ρi is the ion gyroradius. A brief initial state of diamagnetic propagation is observed, followed by E×B (magnetized) propagation; E×B propagation is accompanied by beam compression transverse to B with as much as a factor of 4 increase in density and a slight drift of the beam in the ion Lorentz force direction. For Bz=200–300 G the observed magnetization time is much faster than calculated from classical Spitzer conductivity and is more of the order of the magnetization time based on Hall conductivity.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 98 (1989), S. 0 
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In two recent papers, normal incidence and expanding spread profiles (ESPs) shot across the Hatton Bank passive continental margin showed seaward dipping reflectors in the upper crust and elevated velocities in the lower crust. The reflectors were interpreted as subaerial basalt flows and the high velocities as underplated or intruded igneous material in the lower crust. A recent gravity model shows high densities in the lower crust supporting the velocity model. In this study we present the forward modelling of two wide-angle seismic refraction datasets shot perpendicular to strike and through the ESP midpoints. The final velocity model across the margin is consistent with all the seismic datasets but differs in some important respects from the ESP-based model: a high velocity volcanic pile is outlined mid-way up the continental slope and the inferred velocities in the lower crust are slightly lower. The differences between the two models can be explained by the three-dimensional nature of the margin: the ESPs were modelled assuming that the sub-sediment structure was invariant along strike. The final velocity model is used to obtain an estimate of the total thickness of melt accreted across the margin, which is calculated to thicken from zero at Hatton Bank to about 12km mid-way down the continental slope. At the bottom of the continental slope the adjacent igneous section forming the oceanic crust is about 18 km thick and thins rapidly northwestwards to 11 km. As the oceanic crust thins its seismic velocity decreases; this suggests that it is less mafic and that there is a post-rift decrease in the percentage of mantle melting. This decrease may be explained by a decrease in asthenospheric temperature and/or by a reduction in volatile content.
    Type of Medium: Electronic Resource
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