• Login
    About WHOAS
    View Item 
    •   WHOAS Home
    • Woods Hole Oceanographic Institution
    • Applied Ocean Physics and Engineering (AOP&E)
    • View Item
    •   WHOAS Home
    • Woods Hole Oceanographic Institution
    • Applied Ocean Physics and Engineering (AOP&E)
    • View Item
    JavaScript is disabled for your browser. Some features of this site may not work without it.

    Browse

    All of WHOASCommunities & CollectionsBy Issue DateAuthorsTitlesKeywordsThis CollectionBy Issue DateAuthorsTitlesKeywords

    My Account

    LoginRegister

    Statistics

    View Usage Statistics

    On the physics of frequency-domain controlled source electromagnetics in shallow water. 1: isotropic conductivity

    Thumbnail
    View/Open
    ggw435.pdf (1.804Mb)
    Date
    2017-11-17
    Author
    Chave, Alan D.  Concept link
    Everett, Mark E.  Concept link
    Mattsson, Johan  Concept link
    Boon, James  Concept link
    Midgley, Jonathan  Concept link
    Metadata
    Show full item record
    Citable URI
    https://hdl.handle.net/1912/8896
    As published
    https://doi.org/10.1093/gji/ggw435
    DOI
    10.1093/gji/ggw435
    Keyword
     Electrical properties; Marine electromagnetics 
    Abstract
    In recent years, marine controlled source electromagnetics (CSEM) has found increasing use in hydrocarbon exploration due to its ability to detect thin resistive zones beneath the seafloor. It is the purpose of this paper to evaluate the physics of CSEM for an ocean whose electrical thickness is comparable to or much thinner than that of the overburden using the in-line configuration through examination of the elliptically polarized seafloor electric field, the time-averaged energy flow depicted by the real part of the complex Poynting vector, energy dissipation through Joule heating and the Fréchet derivatives of the seafloor field with respect to the subseafloor conductivity that is assumed to be isotropic. The deep water (ocean layer electrically much thicker than the overburden) seafloor EM response for a model containing a resistive reservoir layer has a greater amplitude and reduced phase as a function of offset compared to that for a half-space, or a stronger and faster response. For an ocean whose electrical thickness is comparable to or much smaller than that of the overburden, the electric field displays a greater amplitude and reduced phase at small offsets, shifting to a stronger amplitude and increased phase at intermediate offsets and a weaker amplitude and enhanced phase at long offsets, or a stronger and faster response that first changes to stronger and slower, and then transitions to weaker and slower. These transitions can be understood by visualizing the energy flow throughout the structure caused by the competing influences of the dipole source and guided energy flow in the reservoir layer, and the air interaction caused by coupling of the entire subseafloor resistivity structure with the sea surface. A stronger and faster response occurs when guided energy flow is dominant, while a weaker and slower response occurs when the air interaction is dominant. However, at intermediate offsets for some models, the air interaction can partially or fully reverse the direction of energy flux in the reservoir layer toward rather than away from the source, resulting in a stronger and slower response. The Fréchet derivatives are dominated by preferential sensitivity to the reservoir layer conductivity for all water depths except at high frequencies, but also display a shift with offset from the galvanic to the inductive mode in the underburden and overburden due to the interplay of guided energy flow and the air interaction. This means that the sensitivity to the horizontal conductivity is almost as strong as to the vertical component in the shallow parts of the subsurface, and in fact is stronger than the vertical sensitivity deeper down. However, the sensitivity to horizontal conductivity is still weak compared to the vertical component within thin resistive regions. The horizontal sensitivity is gradually decreased when the water becomes deep. These observations in part explain the success of shallow towed CSEM using only measurements of the in-line component of the electric field.
    Description
    Author Posting. © The Authors, 2016. This article is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 208 (2017): 1026-1042, doi:10.1093/gji/ggw435.
    Collections
    • Applied Ocean Physics and Engineering (AOP&E)
    Suggested Citation
    Geophysical Journal International 208 (2017): 1026-1042
     

    Related items

    Showing items related by title, author, creator and subject.

    • Thumbnail

      Electromagnetic flow sensors 

      Lawson, Kenneth; Kanwisher, John W. (Woods Hole Oceanographic Institution, 1974-03)
      Flow sensors based on the principle of electromagnetic induction were investigated as alternatives to commonly used mechanical devices utilizing rotors and propellers. Prototype sensors were constructed showing ...
    • Thumbnail

      Mantle dynamics beneath the East Pacific Rise at 17°S : insights from the Mantle Electromagnetic and Tomography (MELT) experiment 

      Baba, Kiyoshi; Chave, Alan D.; Evans, Rob L.; Hirth, Greg; Mackie, Randall L. (American Geophysical Union, 2006-02-17)
      The electromagnetic data from the Mantle Electromagnetic and Tomography (MELT) experiment are inverted for a two-dimensional transversely anisotropic conductivity structure that incorporates a correction for three-dimensional ...
    • Thumbnail

      An electromagnetic method for measuring the velocities of ocean currents from a ship under way 

      Von Arx, William S. (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1950-03)
      During the past four years a deliberate effort has been made at the Woods Hole Oceanographic Institution to devise methods of kinematic observation generally suited to the needs of oceanographers. One result of this work, ...
    All Items in WHOAS are protected by original copyright, with all rights reserved, unless otherwise indicated. WHOAS also supports the use of the Creative Commons licenses for original content.
    A service of the MBLWHOI Library | About WHOAS
    Contact Us | Send Feedback | Privacy Policy
    Core Trust Logo