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.
Streamlines represents ﬂow and ﬂow paths, so for ease of interpretation
- the electric ﬁeld can be considered to visualize the direction of the ﬂow of charged particles
- the magnetic ﬁeld the direction of the force on moving charged particles
- the Poynting vector the ﬂow of energy ﬂux.
A static interpretation guide is shown below. The electric ﬁeld streamlines show the airwave as an interaction between two vortices; the earth vortex and air vortex. The centre of the earth vortex corresponds with the electric x phase inﬂection point a significantly small amplitude. The air wave front (per indicated by the dashed red line) represents the point where the contribution of “earth” and “air” energy ﬂow is equal. That is above the line is predominantly air wave energy and below the line predominantly earth energy. I have seen early CSEM papers in the past where they show an apparent 'direct wave'. While these papers made it easier for seismic specialists to understand marine CSEM, I don't really see apparent direct or reﬂected energy, however it appears to have “guided” energy ﬂow (see Weidelt, 2007) within the hydrocarbon and along the air-ocean boundary.
The videos presented below show the three planar perspectives of the electromagnetic field generated from an electric dipole transmitter. These three cases videos below have been taken from,
A.M. Pethick, B.D. Harris, Interpreting marine controlled source electromagnetic field behaviour with streamlines, Computers & Geosciences, Volume 60, October 2013, Pages 1-10, ISSN 0098-3004, http://dx.doi.org/10.1016/j.cageo.2013.04.017.
The final case shows the electromagnetic field generated by a MCSEM survey within a conductive 1D layered earth containing a 3D resistive hydrocarbon body. The video show how the streamlines tend to “avoid” more electrically resistive areas; either bending around resistive bodies or “jumping” across the resistive area along the shortest possible path. In particular note how the streamlines travel perpendicular across the high resistivity reservoir (i.e. the resistive slab). Conversely the electric ﬁeld streamlines tend to concentrated in the more conductive geo-electric features.
Let's also not forget the size of the fields we are dealing with. The EM Fields are REGIONAL SCALE!
Generally the detectable electric ﬁeld streamlines generated by the lowest MCSEM frequency (0.1Hz), This is generated using an earth with an ocean depth of 1.5km and halfspace resistivity of 1.5Ohm m are overlaid above the city of Perth, Western Australia. These fields generated are massive! The engulfing electromagnetic ﬁeld is large (50km×50km×4km!!!) and would easily span the sprawling metropolitan city of 2 million people. Although many MCSEM surveys transmit using a higher base frequency and as a result will reducing the volume the detectable EM field will inﬂuence.
Pethick, Andrew M., (2013), Multidimensional computation and visualisation for marine controlled source electromagnetic methods. PhD Thesis. Curtin University, Australia.
Pethick, A., and B. Harris. "Poynting Vector Streamlines and the Marine Controlled Source Electromagnetic Airwave." 75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013. 2013.
Weidelt, P. (2007). Guided waves in marine csem. Geophysical Journal International 171(1), 153–176