| dc.description.abstract | Seismic surveys are well established in hydrocarbon prospecting, and technology for processing seismic data has been developed through decades. The first electromagnetic survey for hydrocarbon prospecting however, was performed in 2000, and as a consequence of the short time span the technology is not as well developed as in seismics. For instance, the need for efficient and robust forward modelling software and inversion schemes for collected data is urgent. In this thesis forward modelling using integral equations and the Contrast Source Inversion (CSI) method is investigated for forward and inverse 3D electromagnetic scattering experiments in hydrocarbon prospecting, respectively. The mathematical model is developed in an arbitrary isotropic, conductive medium, with contrast in electric permittivity and electric conductivity between the scattering object and the background, while in the numerical examples the background model is restricted to a horizontally layered medium with variations in the $z$-direction only and contrast in electric conductivity. The difference in electric conductivity is considered the backbone of electromagnetic hydrocarbon prospecting. The main result concerning forward modelling in this thesis is the establishment of a, to my knowledge, previously unpublished method for solving the electric Lippmann-Schwinger equation in a conductive medium by fixed point iteration. In the inversion part of this thesis the previously scalar CSI method is extended to a full vector valued method. A new CSI method for inversion with respect to all the electromagnetic parameters (the electric permittivity, electric conductivity and magnetic permeability) is also presented, which I have yet to find treated elsewhere. Only the former method is tested numerically, using synthetic data, due to the computational complexity of the latter. The numerical results from the forward modelling show the numerical validity of integral equation modelling and the fixed point iteration, whereas the results from the inversion show some promise. With several source and receiver lines present the lateral position of the scattering object is reconstructed well, whereas the vertical position causes problems. When textit{a priori} information about the position of the scattering object is introduced to further regularise the problem, the approximate position of it is successfully inverted, which illustrates the essential part additional regularisation plays in this inverse scattering problem. Thus the CSI method could be useful in petroleum geophysics, and should be developed further for the purpose of locating hydrocarbons in the subsurface. Several possibilities for further work is noted. This work was performed for StatoilHydro ASA. | nb_NO |
