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

Abstract

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.

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