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

• Main Author: Morris, Edward C.
• Published: University of Southampton 2008
• Subjects: 550; GC Oceanography
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.561458

Obtaining information regarding the resistivity structure of the subsurface from marine CSEM data involves complex processes. 1D and 2D forward and inverse modelling are currently the standard approaches used to produce geoelectrical models, with 3D inversion fast becoming a realizable method. However, these methods are time consuming, require expert knowledge to produce reliable results, and suffer from the non-uniqueness of the EM problem. There is therefore considerable scope for developing 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 provide starting points or other constraints for inversion. In this thesis, a number of different imaging techniques for marine CSEM data are assessed, with particular reference to applications in hydrocarbon exploration. T-X and F-K imaging are widely used seismic reflection processing techniques that can be applied to CSEM data. Features produced in the T-X and F-K domains by 1D subsurface resistivity structures are investigated. The dip of an arrival corresponding to a subsurface resistive feature is found to depend on its resistivity, with reduction in resistivity producing steeper dipping events. The separation of arrivals according to their dips in the T-X domain is used as a basis for the attempted separation of the airwave, by filtering in the F-K domain. However, this does not prove to be useful. Secondly, in a adaptation of the F-K migration method used in seismic processing, EM migration is investigated, following the approach by (Tompkins, 2004b). The results of the migration method are compared and contrasted to a 1D smooth inversion algorithm. It is found that the migration is mostly dependent on the conductivity contrast across a geoelectrical boundary, whereas the inversion recovers the resistivity thickness product (transverse resistance). Hence, EM migration is a viable alternative to inversion and usefully complements it in regions of large conductivity contrasts. Normalized ElectroMagnetic Imaging (NEMI) extends the standard approach of normalizing the recorded electric field data by a 1D background model, to identify large lateral resistivity variations over a survey area. This is achieved by firstly sorting the data based on sensitivity to the target layer, and then distributing the normalized anomaly in the horizontal plane between the source and receiver using a simple quasitomographical approach. In some scenarios this provides a reasonable estimation of the lateral extent of a 3D resistive body buried in a conductive background. Lastly, Apparent Resistivity Imaging (ARI) is adapted for the use with the marine CSEM method. This generates pseudo-sections in which offsets are mapped into apparent depths. This study shows that whilst vertical resolution of resistive bodies is poor, lateral resolution is high and provides a good estimate of the true extent of a target body. Apparent resistivity pseudo-sections therefore provide a very effective means of “first look” imaging and assessment of marine CSEM data.