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P231 Extraction of the Ground Wave from Marine CSEM Data B.A. Farrelly* (Hydro Oil & Energy) & K.N. Madsen (Hydro Oil & Energy) SUMMARY The desired signal in marine Controlled Source Electromagnetics is the ground wave. However this may be swamped by the airwave and not least by multiples. A ray theory taking account of multiples is described. However the results are not much better than so-called up-down separation below the sea-floor which is an approximation of the ray theory. In this method the ground wave is calculated as a linear combination of the inline electric and crossline magnetic fields