Desired Signal Enhancing And Airwave Mitigating Methods In Marine CSEM

In the quest to find hydrocarbon reservoirs,Controlled Source Electromagnetic method(CSEM)could add the key information to determine the pore-fluid of the potential reservoir.The crucial problem related to interpretation of the CSEM data is that the signal transmitted by source does not only interact with subsurface on its path from source to reservoir.A large part of the signal diffuses from the source to the receiver directly through the water and water-air interface.This signal does not contain any information from the subsurface,on the contrary,it could mask the subsurface response making the interpretation of the CSEM data difficult.That's so called airwave effect.In this thesis,three processing approaches,synthetic aperture source,field decomposition and interferometry by multidimensional deconvolution,for marine CSEM data to overcome these problems,are introduced.The capabilities and limitations of these three methods are demonstrated at the hand of numerically modeled marine CSEM data.The goal of applying synthetic aperture source is to increase the detectability of the hydrocarbon reservoir and the focus lies on how to search the optimal weights for individual source.A unified equation to separate electromagnetic field into upward and downward components is derived in frequency and wavenumber domain.In particular,the 1D special case is applied to marine electromagnetic.After field decomposition just below the seabed,the upward component of electric field does not contain any primarily downward-traveling energy and most of airwave effect has been removed.Finally,the thesis is rounded off with interferometry by MDD,since this method replaces the medium above receiver level by a homogeneous half space,the effect related to water volume has been removed.Synthetic data with different transmitter location demonstrate that interferometry by MDD is also effective for weakness signal.