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  • 1
    Online Resource
    Online Resource
    Abschlussbericht zum Projekt 02WT0248 Integrated electromagnetic geophysical technology applied to the detection of deep fresh/saline groundwater interfaces in northern and central Israel (2005)
    Köln
    Details Availability
    Keywords: Forschungsbericht
    Type of Medium: Online Resource
    Pages: Online-Ressource (258 S., 9,48 MB) , Ill., graph. Darst., Kt.
    URL: https://edocs.tib.eu/files/e01fb06/520864263.pdf
    URL: https://doi.org/10.2314/GBV:520864263
    URL: https://edocs.tib.eu/files/e01fb06/520864263l.pdf
    DOI: 10.2314/GBV:520864263
    Language: English
    Note: Literaturverz , Unterschiede zwischen dem gedruckten Dokument und der elektronischen Ressource können nicht ausgeschlossen werden , Auch als gedr. Ausg. vorhanden , Systemvoraussetzungen: Acrobat reader.
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  • 2
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    Marine-controlled source electromagnetic study of methane seeps and gas hydrates at Opouawe Bank, Hikurangi Margin, New Zealand (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Solid Earth, 122 (5). pp. 3334-3350.
    Details
    Publication Date: 2020-02-06
    Description: Marine controlled source electromagnetic (CSEM) data have been collected to investigate methane seep sites and associated gas hydrate deposits at Opouawe Bank on the southern tip of the Hikurangi Margin, New Zealand. The bank is located in about 1000 m water depth within the gas hydrate stability field. The seep sites are characterized by active venting and typical methane seep fauna accompanied with patchy carbonate outcrops at the seafloor. Below the seeps, gas migration pathways reach from below the bottom-simulating reflector (at around 380 m sediment depth) toward the seafloor, indicating free gas transport into the shallow hydrate stability field. The CSEM data have been acquired with a seafloor-towed, electric multi-dipole system measuring the inline component of the electric field. CSEM data from three profiles have been analyzed by using 1-D and 2-D inversion techniques. High-resolution 2-D and 3-D multichannel seismic data have been collected in the same area. The electrical resistivity models show several zones of highly anomalous resistivities (〉50 Ωm) which correlate with high amplitude reflections located on top of narrow vertical gas conduits, indicating the coexistence of free gas and gas hydrates within the hydrate stability zone. Away from the seeps the CSEM models show normal background resistivities between ~1 and 2 Ωm. Archie's law has been applied to estimate gas/gas hydrate saturations below the seeps. At intermediate depths between 50 and 200 m below seafloor, saturations are between 40 and 80% and gas hydrate may be the dominating pore filling constituent. At shallow depths from 10 m to the seafloor, free gas dominates as seismic data and gas plumes suggest.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2016JB013702
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Marine downhole to seafloor dipole-dipole electromagnetic methods and the resolution of resistive targets (2007)
    SEG
    Details
    Publication Date: 2024-03-15
    Description: Most active marine electromagnetic surveys use an array of transmitting and receiving electric dipoles located on the sea-floor. There are many reports of successful detections of buried resistors with these arrays. Nevertheless, the concept has a significant drawback: Large offsets between the transmitter and receiver with respect to the target depth are needed. A logical alternative is the use of a vertical electric-dipole transmitter in a borehole deeper than the target, combined with receivers located at the seafloor. Because current must pass through the target, even data recorded at short offsets with respect to target depth should be sensitive to the target. We derived the equations for the electric and magnetic field components produced by the vertical electric dipole in a 1D layered medium. The equations are used to model the fields for resistive layers. An eigenparameter analysisis applied to the different field components, and the resolution is compared with the standard seafloor inline array. The analyses show that data for the downhole transmitter are more sensitive to the resistance between the source and the receiver than the sea-floor array. However, model geometry such as depth to the target is resolved less easily. The resolution of the model geometry is improved significantly when a long bipole transmitter is used, extended from below the target layer to a level above it. We carried out 3D modeling for the downhole transmitter through the use of a finite-difference algorithm. Results indicate that these data are very sensitive to lateral changes within the target layer. Differences between electric fields simulated at the seafloor for 3D models and a reference model show a correlation to lateral resistivity differences. This finding suggests that the array is suitable for mapping and monitoring resistivity anomalies outward from the borehole.
    Type: Article , PeerReviewed
    Format: text
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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