Overview of Marine Controlled Source Electromagnetic Methods Electromagnetic (EM) methods can be divided into active and passive categories. There has been a transition from passive source EM such as the magneto-telluric (MT) method towards active source EM such as the controlled source electromagnetic (CSEM) method. The MCSEM method goes by many other names. Some of these are CSEM imaging (CSEMi) and sea bed logging (SBL). MCSEM detects electrical resistivity contrasts (Kong, 2002). Marine CSEM (i.e., MCSEM or mCSEM) often targets thin electrically resistive hydrocarbon reservoirs in conductive surroundings (e.g., ocean and sub-ocean sediments). MCSEM was originally developed for deep water studies of oceanic lithosphere in the 20th century (Cox, 1981). The method expanded into hydrocarbon exploration, beginning with the first survey carried out on an oilfield in offshore Angola (Ellingsrud et al., 2002). The MCSEM method has been advanced by the efforts of university and industry researchers such as Scripps Institution of Oceanography, The University of Toronto, Cambridge and Southampton University over the past two decades. MCSEM and MT have been linked in their development (Constable, 2010). Early MCSEM research combined both natural and active EM sources to image resistivity variations beneath the sea floor (Constable, 2010), while MT identifies resistivity variations on large scale structures at basin level. Most modern MCSEM are performed to ascertain nearsurface shape resistivity variation, in particular the detection of electrically thin resistive bodies, although they are still useful for deeper oceanographic studies (Peace, 2005).
Seismic methods are an industry standard hydrocarbon exploration technique. Combinations of seismic and non-seismic methods have been utiltized to reduce exploration risk in recent times. Methods such as gravity, magnetism, well logging and more recently MCSEM have been incorporated into hydrocarbon exploration (i.e., Coraggio et al., 2012; Yang et al., 2011 and Jain et al., 2008). MCSEM is useful as a supplementary non-seismic method as it detects resistivity rather than acoustic properties. MacGregor (2006) demonstrates that a strong relationship between the MCSEM electromagnetic response and fluid saturation percentage exists. Phillips (2007) states hydrocarbon reservoir saturation can be characterised ahead of drilling. Hydrocarbon saturation can be characterised because the electromagnetic field amplitude increases proportionally with hydrocarbon saturation. MacGregor (2006) highlights that amplitude and velocity analysis can only accurately detect hydrocarbon saturation between 0 and 10% (see Figure 1). MCSEM otherwise has the ability to detect hydrocarbon saturation due its resistivity variation. Typical MCSEM targets include petroleum, natural gas and gas hydrates.

Figure 1 - The relationship between hydrocarbon saturation on seismic and MCSEM electric field
observations. The MCSEM electric field response is characterised by increasing normalised electric
field response with increasing hydrocarbon saturation. The seismic method fails to distinguish between
a hydrocarbon saturation of 10 and 100 percent %. The ability to detect hydrocarbon saturation is
the main benefit of the MCSEM method (Figure modified from Phillips, 2007).
Targets which are thin, or underneath thick tabular salt/basalt targets, or deep and with low resistivity comparable to the surrounding geology (see Table 1) will limit MCSEM’s effectiveness. MCSEM is useful in deep water turbidites and deltas, over stacked reservoir
sequences or where there are large resistivity contrasts such as in shallow gas hydrates (MacGregor, 2006). Geological environments such as reservoirs at the edges of salt diapirs and in carbonates can be explored by the MCSEM method.

The practicality of the MCSEM method in different geological environments. MCSEM
works in a variety of geological environments however it may not work in some geological settings.
MCSEM surveys have become an integral part of deep ocean petroleum exploration and appraisal (Gribenko and Zhdanov, 2007) and future use for monitoring purposes may be possible as investigated by Noel et al. (2010), Noel et al. (2011) and Liang et al. (2012).
References
Constable, S. (2010). Ten years of marine CSEM for hydrocarbon exploration. Geophysics 75 (5), 75A67–75A81.
Cox, C. S. (1981). On the electrical conductivity of the oceanic lithosphere. Physics of the Earth and Planetary Interiors 25 (3), 196–201.
Coraggio, F., P. Bernardelli, and G. Gabbriellini (2012). Structural reconstruction using potential field data in hydrocarbon exploration, pp. 1–6.
Ellingsrud, S., T. Eidesmo, S. Johansen, M. Sinha, L. MacGregor, and S. Constable (2002). Remote sensing of hydrocarbon layers by seabed logging (sbl): results from a cruise offshore angola. Leading Edge 21 (10), 972–982.
Jain, M., S. N. Mohanty, and S. V. Yalamanchili (2008). Gravity, magnetic and seismic data integration for structural configuration and its hydrocarbon evaluation in the San Juan-Tumaco Basins, offshore Colombia, pp. 774–778.
Kong, E.N.; Westerdahl, H. E. S. E. T. J. S. (2002). Seabed logging: A possible direct hydrocarbon indicator for deepsea prospects using em energy. Oil and Gas journal 100, 6.
Gribenko, A. and M. Zhdanov (2007). Rigorous 3d inversion of marine csem data based on the integral equation method. Geophysics 72 (2), WA73–WA84.
MacGregor, L. M. (2006). Ohm short course.
Peace, D. (2005, 2 May). How to plan for a successful c.s.e.m. survey.
Phillips (2007). Feasibility of the Marine Controlled Source Electromagnetic Method for Hydrocarbon Exploration. B.sc. Curtin University.
Yang, H., Y. Zhang, B. Wen, S. Yu, X. Qi, D. Ma, and Z. Xu (2011). Exploring shallow biogenic gas with high-precision gravity data, pp. 892–896.
Recent Comments