Typical marine CSEM surveys work by using a horizontal electric bipole to transmit a low frequency 0.1 to 5 Hz square wave under high power up to 1000 A (Pethick, 2008). A horizontal electric bipole is a 100 to 1000 m long electric field transmitter towed 50 m above of the ocean bottom (MacGregor, 2006). Receivers can be placed to record the perturbations in the electric and magnetic fields for all Cartesian directions. There is typically a resistivity contrast between the conductive host rock and the resistive hydrocarbon. Saturated mudstone rocks, sandstones and shales with low resistivity dominate deep water environments.

Video 1 shows that traditional MCSEM surveys consist of a large moving high power transmitter and stationary magnetic and electric field receivers. Seafloor electromagnetic receivers are deployed to the ocean floor through the use of heavy concrete pads. Three
component electrical bipole (Ex, Ey and Ez) and two component magnetic (Hx and Hy) fields are typically recorded in modern surveys. These ocean bottom receivers start recording the small changes in the electromagnetic field once deployed. The MCSEM transmitter is dragged as close to the ocean floor as is practical (e.g., approximately 30 to 50 m from the ocean floor). The transmitter is typically towed inline with the receiver line. The transmitter sends a large amplitude transient current into a long electrical bipole source (e.g., 1000 A at 1 Hz into a 300 m long wire cable) (Harris and Pethick, 2008; MacGregor, 2006). The changing current generates 4D coupled electric and magnetic fields. The magnetic and electric fields circulate around each other with a strict geometry. The mathematical expression of the interactions between electric and magnetic fields is captured in Maxwell’s equations. A hydrocarbon reservoir can be 10 to 100 times greater in resistivity (Eidesmo and Ellingsrud, 2002). Sediments containing oil or gas have typically higher electrical resistivity compared to the ocean and brine-saturated host sediments. The electric and magnetic field patterns and amplitudes become altered or distorted if an electrically resistive hydrocarbon reservoir is present.

Video 1 : A video showing the schematic of a typical MCSEM survey. The video shows the electric dipole transmitter; transmits electric and magnetic fields while the seafloor receivers record subtle variations in the electric and magnetic fields. The end of the video shows the two modes of propagation which are commonly used to describe EM field behaviour. The black line represents the “direct wave”, the green arrow represents the “guided” wave and the blue lines represent the electric field.

 

References

Eidesmo, T. and S. Ellingsrud (2002). How electromagnetic sounding technique could be coming to hydrocarbon e and p. First Break 20 (3), 11.

Harris, B. and A. Pethick (2008). Marine controlled source electromagnetic methods for hydrocarbon exploration. Preview 137, 4.

MacGregor, L. M. (2006). Ohm short course.

Pethick, A. (2008). Planning and 4D Visualisation of the Marine Controlled Source Electromagnetic Method. Bsc thesis, Curtin University.