This paper discusses an application of electromagnetic (EM) migration for sea-bottom EM imaging. We apply to marine EM data the method of frequency domain EM migration developed by Zhdanov et al. (1996) for fast imaging of land EM data. We study a synthetic survey with electric receivers measuring the natural telluric electric field at the sea bottom over a 3-D geoelectrical model of a sea-bottom petroleum reservoir. We also demonstrate that the EM migration can be applied for fast imaging of marine controlled-source electromagnetic data. The results of this modeling show that the migration method can be effectively used for fast sea-bottom imaging of resistive reservoir structures.
The application of EM methods in petroleum exploration requires development of appropriate imaging techniques which provide the means for fast but accurate evaluation of the observed data. In seismic exploration, which is a leading method for oil and gas prospecting, the most widely used imaging technique is seismic migration (Tarantola, 1984; Claerbout, 1985). In a series of publications, Zhdanov and his co-authors have extended the method of seismic migration to the case of a low frequency diffusive EM field (Zhdanov, 1981; Zhdanov and Frenkel, 1983; Zhdanov et al., 1996; Zhdanov, 1999; and Zhdanov, 2002). The US patent for this technology was issued in 2001 (Zhdanov, 2001). The EM migration has important features in common with seismic migration, but differs in that, for geoelectrical problems, EM migration is done on the basis of Maxwell’s equations, while in the seismic case it is based on the wave equation. EM migration, similar to seismic migration, is based on a special form of downward continuation of the observed field or one of its components. This downward continuation is obtained as the solution of the boundary value problem in the lower half-space for the adjoint Maxwell’s equations, in which the boundary values of the migration field on the earth’s surface are determined by the observed EM data. It was shown in the original paper by Zhdanov and Traynin (1997) that EM migration can be treated as an approximate solution of the corresponding EM inverse problem. It was also demonstrated in subsequent publications (see, for example, Zhdanov, 2002) that EM migration can be applied iteratively, which results in a rigorous inverse EM problem solution. Tompkins (2004) has reported an application of this migration technique for fast imaging of Sea Bed Logging (SBL) EM data. In the present paper, we investigate a 3-D geoelectrical model of a sea-bottom petroleum reservoir. We extend the method and numerical algorithm of frequency domain (FD) EM migration, developed by Zhdanov et al. (1996) for processing land EM data, to be effectively used for seabottom imaging of resistive reservoir structures.
We will analyze a model of a sea-bottom petroleum reservoir to illustrate seabed imaging with frequency domain EM migration. Figure 1 shows a 3-D view of a model with the corresponding system of Cartesian coordinates.
