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), but these approaches cannot easily describe electromagnetic field propagation although they are great for computational applications. 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). These articles equate electromagnetic field diffusion as 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 while Figure 2 provides another representation. While this comparison makes it easier to describe MCSEM methods to seismic practitioners it may cause confusion amongst MCSEM practitioners as these two representations appear fundamentally in opposition while both can be similarly true.
Figure 1 : An example of representing the MCSEM with seismic refraction, taken from Hesthammer et al. (2010). 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 state that “the energy propagation is shown as raypaths in the figure, although the energy at the low frequencies used mainly propagates through diffusion”.
Another example of electromagnetic field propogation showing the electric field lines generated by the electric bipole transmitter. Taken from http://www.acceleware.com/
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).
References
Carstens, H. (2009). Technology: Changing exploration - using non-seismic technology. GEO ExPRO 6 (1).
Constable, S. (2010). Ten years of marine CSEM for hydrocarbon exploration. Geophysics 75 (5), 75A67–75A81.
Fischer, P. (2005). New em technology offerings are growing quickly. World Oil 226 (6), 9.
Pound, G. (2007). Multi-transient em technology at pgs. Tech Link 7 (4), 8.
Thirud, P. (2002). Waves of information. Scandinavian Oil and Gas Magazine 3 (4), 2.
Zhdanov, M. (2009). Geophysical Electromagnetic Theory and Methods. Elsevier.
For More Information View : Understanding Electromagnetic Field Behaviour
Understanding Marine Controlled Source Electromagnetic Field Propogation
Streamlines
In reality, electromagnetic field propagation is extremely complex. Everything in the earth influences the recorded electromagnetic response, from the highly resistive air to the most seemingly insignificant conductive brine filled sandstone unit. Everybody has a preferred method to understanding electromagnetic field behaviour. It could be mathematically, rules of thumb or with static field lines. I like the idea of streamlines as they are able to visualise simulated electric, magnetic and Poynting vector field lines in time. Each to their own.
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