Dr. Marion Jegen-Kulczar
Phone: +49 (0)431 600-2560
Fax: +49 (0)431 600-2922
Electromagnetic (EM) methods are used to image the resistivity structure of the subsurface by means of measuring components of the electric and/or magnetic field. Such measurements can not only be performed directly on land, airborne using a helicopter, or in boreholes, but also in marine environments. For the marine case, changes in the electrical resistivity of the subsurface can be detected, where saline (i.e. low resistive) pore water is replaced by resistive materials like hydrocarbons (oil, gas, gas-hydrates) or fresh water. While EM methods generally lack the structural resolution of seismics, the complementary information about the electrical resistivity makes the combination of these methods especially interesting for the detection of hydrocarbons.
Starting in 2006, new marine electromagnetic instruments have been developed at GEOMAR. These OBEM (ocean bottom electromagnetic) systems are used to measure the horizontal components of the electrical field with two orthogonal, 10m long dipole arms as well as the three components of the magnetic field using a high-precision fluxgate magnetometer. Generally, the OBEM systems allow for two types of experiments:
Evans et al., 2005
R.L. Evans, G. Hirth, K. Baba, D. Forsyth, A. Chave & R. Mackie: Geophysical evidence from the MELT area for compositional controls on oceanic plates. Nature, 2005, vol. 437 (7056), pp. 249-252.
Hoversten et al., 2000
G.M. Hoversten, S.C. Constable, & H.F. Morrison: Marine magnetotellurics for base-of-salt mapping: Gulf of Mexico field test at the Gemini structure. Geophysics, 2000, vol. 65 (5), pp. 1476-1488.
Jegen & Edwards, 1998
M. Jegen, and R.N. Edwards: The electrical properties of a 2D conductive zone under the Juan de Fuca ridge. GRL, 1998, vol. 25 (19), pp. 3647-3650.
Worzewski et al., 2011
T. Worzewski, M. Jegen, H. Kopp, H. Brasse, & W. Taylor: Magnetotelluric Image of the Fluid Cycle in the Costa Rican Subduction Zone. Nature Geoscience, 2011, vol. 4 (2), pp. 108-111.