Modeling of seafloor exploration using electric-source frequency-domain CSEM and the analysis of water depth effect
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For the final goal of solving the problem about the feasibility and techniques of resistive target (e. g. hydrocarbon) exploration in the environment of China sea using CSEM, this paper studies the frequency-domain electromagnetic responses of a marine geological model with finite depth of seawater excited by a horizontal electric source, establishing theoretical base for 1D and 3D electromagnetic simulation. In the derivation of electric field formulas and magnetic field formulas, the Lorentz-gauged potential in each layer is solved first, and then the Coulomb-gauged potential in each layer is derived from the solution of Lorentz-gauged potential in the corresponding layer by the relationship between these two kinds of potentials. Although the electric fields and magnetic fields in all layers can be computed from the Lorentz-gauged potentials or the Coulomb-gauged potentials, the Coulomb-gauged potentials are advantageous in finiteelement computation. The electromagnetic fields and potentials for an electric source with finite length can be obtained by integrating those for an electric dipole along the length of source. The electromagnetic field formulas in seawater both for a horizontal electric dipole and for an electric source with finite length are represented in this paper and they are used to simulate the distribution of electric fields and magnetic fields over seafloor in different water-depth environments. The effect of water depth on the electromagnetic anomalies of hydrocarbon buried in seafloor is discussed in the end. The results show that the intensity and shape of the electromagnetic anomalies markedly change with the decrease of water depth and only the Ex and E-components reveal perceptible anomalies when seawater is very shallow, for example, 50 m. At last, the electric-field anomalies for two well known oil fields in different water depth are calculated, which shows the good future of electric-source frequency-domain CSEM in seafloor exploration, even in shallow sea. The problems that need further study in the practical application of this method are also discussed in the last part of this paper.
