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

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

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Morris, Edward C.