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 exceeding 1000 A (Pethick, 2008). A horizontal electric bipole is a long electric field transmitter towed approximately 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 time varying 3D 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.
The electromagnetic fields generated during a MCSEM survey are described by differing and possibly contradictory methods. These include mathematical, electromagnetic wave propagation and seismic analogies. A number of mathematical approaches to define the electromagnetic field propagation exits (i.e., Zhdanov, 2009). A popular method to describe the electromagnetic field behavior is to compare the MCSEM method with seismic refraction (e.g., Thirud, 2002; Fischer, 2005; Pound, 2007 and Carstens, 2009). Many of these will explain that the "refraction-paths" show the direction of energy flow. While some articles do equate electromagnetic field diffusion with ray-paths. It also has been thought of as a diffusing wave (e.g., Constable, 2010). Figure 1 represents a common practice of comparing EM field propagation as raypaths rather than diffusion. This comparison makes it easier to describe MCSEM methods to seismic practitioners but this may ultimately lead to confusion amongst MCSEM practitioners as these two representations appear fundamentally in opposition while in fact they both can be correct.
For simplicity the MCSEM method transmits an electromagnetic field from an electrical bipole source. The transmitted wave diffuses through the water column and into seabed. The electric fields attenuate less in resistive mediums. The presence of the reservoir increases the electrical field amplitude which can be measured at the seafloor at offsets roughly double the depth of the reservoir below the seabed (Pethick 2008). As the electromagnetic field encounters a conductive region of earth, the field changes in phase. The level of phase change is on the conductivity (i.e., the greater the conductivity, the greater the phase variation).
Figure 2: Is MCSEM like seismic refraction? The answer is a confusing yes and no. Taken from GeoExPro This diagram represents the electromagnetic field as a number of rays, similar to the seismic refraction method. This comparison is made to demonstrate the path of energy. The authors do state that "the energy propagation is shown as raypaths in the figure, although the energy at the low frequencies used mainly propagates through diffusion".
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