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:
- Magnetotelluric (MT) measurements:
In this type of experiments OBEM stations measure the natural variations of the earth's electromagnetic fields at long periods (0.1 ¨C 10,000s), which allows the detection of resistivity anomalies down to depths of several 10s of kilometers. Marine MT has been successfully employed in various settings and different depth scales to image partial melt zones in the crust and mantle along mid ocean ridges (e.g. Jegen and Edwards, GRL, 1998; Evans et al., 2005) and potentially oilbearing reservoirs covered by basalt and salt layers (Hoversten et al., 2000). The OBEM system of GEOMAR was successfully used in an MT experiment to image the fluid cycles of the Costa Rican subduction zone (s. Worzewski et al., 2011).
- Controlled source electromagnetic (CSEM) measurements:
in this type of experiment a transmitter is used to generate artificial EM fields. The OBEM receivers are used to measure the transmitted electric field at high frequencies (10kHz). This type of acquisition has a penetration depth down to a few hundreds of meters.
A first CSEM experiment was carried out during a mud volcano study in the West Nile Delta in November 2008. By means of a ROV-mounted small scale, time domain transmitter (also developed at GEOMAR) a 3D style, tomographic study of the structure was performed. Results of this CSEM experiment were presented at several conferences and will be published in 2012.
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.