Fast imaging techniques of marine controlled source electromagnetic (CSEM) data

Obtaining information regarding the resistivity structure of the subsurfacefrom marine CSEM data involves complex processes. 1D and 2D forward and inversemodelling are currently the standard approaches used to produce geoelectrical models,with 3D inversion fast becoming a realizable method. However, these methods aretime consuming, require expert knowledge to produce reliable results, and suffer fromthe non-uniqueness of the EM problem. There is therefore considerable scope fordeveloping imaging techniques for marine CSEM data that do not require lengthy,time consuming computations, but make use of entire datasets. These could provide a“first look” for possible structural information conveyed by the data, and may providestarting points or other constraints for inversion. In this thesis, a number of differentimaging techniques for marine CSEM data are assessed, with particular reference toapplications in hydrocarbon exploration.T-X and F-K imaging are widely used seismic reflection processingtechniques that can be applied to CSEM data. Features produced in the T-X and F-Kdomains by 1D subsurface resistivity structures are investigated. The dip of an arrivalcorresponding to a subsurface resistive feature is found to depend on its resistivity,with reduction in resistivity producing steeper dipping events. The separation ofarrivals according to their dips in the T-X domain is used as a basis for the attemptedseparation of the airwave, by filtering in the F-K domain. However, this does notprove to be useful.Secondly, in a adaptation of the F-K migration method used in seismicprocessing, EM migration is investigated, following the approach by (Tompkins,2004b). The results of the migration method are compared and contrasted to a 1Dsmooth inversion algorithm. It is found that the migration is mostly dependent on theconductivity contrast across a geoelectrical boundary, whereas the inversion recoversthe resistivity thickness product (transverse resistance). Hence, EM migration is aviable alternative to inversion and usefully complements it in regions of largeconductivity contrasts.Normalized ElectroMagnetic Imaging (NEMI) extends the standard approachof normalizing the recorded electric field data by a 1D background model, to identifylarge lateral resistivity variations over a survey area. This is achieved by firstly sortingthe data based on sensitivity to the target layer, and then distributing the normalizedanomaly in the horizontal plane between the source and receiver using a simple quasitomographicalapproach. In some scenarios this provides a reasonable estimation ofthe lateral extent of a 3D resistive body buried in a conductive background.Lastly, Apparent Resistivity Imaging (ARI) is adapted for the use with themarine CSEM method. This generates pseudo-sections in which offsets are mappedinto apparent depths. This study shows that whilst vertical resolution of resistivebodies is poor, lateral resolution is high and provides a good estimate of the trueextent of a target body. Apparent resistivity pseudo-sections therefore provide a veryeffective means of “first look” imaging and assessment of marine CSEM data.