--> --> Abstract: Marine CSEM in Complex Environments: A Case Study from West Africa, by Lucy MacGregor and Anthony Greer; #90082 (2008)
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Marine CSEM in Complex Environments: A Case Study from West Africa

Lucy MacGregor and Anthony Greer
OHM Ltd, Aberdeen, United Kingdom

Controlled source electromagnetic sounding (CSEM) is now becoming a mainstream exploration tool in a range of geological environments. Both Previous HitdataNext Hit acquisition and interpretation approaches must be tailored for the environment in which they are to be applied to ensure optimum results. We apply a staged approach to the analysis of CSEM Previous HitdataNext Hit. The approach begins with a relatively simple 1-dimensional modelling step, and the generation of Previous HitcommonNext Hit Previous HitoffsetNext Hit normalised anomalies. Although direct interpretation of these can prove misleading, especially in areas of complex structure, they provide an overall check on the internal consistency of the Previous HitdataNext Hit, and highlight any large scale lateral changes in the resistivity of the seafloor. Following this, we use an inversion approach to provide a geo-electric section through the seafloor which can be co-rendered with seismic Previous HitdataNext Hit.

We illustrate this using CSEM Previous HitdataNext Hit acquired by OHM on the Baobab field, offshore Cote d’Ivoire. The Baobab reservoir, operated by CNR, consists of Late and Middle Albian turbidite sands, trapped within a 3 way dip closed tilted fault block at 2100m below seafloor. There were a number of complications in this survey. Firstly the water depth across the survey area varies between 200m and 2500m. There is therefore a dramatic change in the interaction of the transmitted EM signals with the air along the line, which if not accounted for correctly can significantly degrade results. Secondly the area is cut by a number of deep, North-South trending canyons. Rapid variations in topography can cause local distortions in the measured EM field. Finally seafloor infrastructure must be avoided, resulting in rapid changes in source height and hence signal coupling to the seafloor. When these complexities are taken into account we obtain a well resolved image of the resistvity structure of the Baobab field.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery