--> Modeling of Electromagnetic Response on Rugged Seafloor by Using Finite-Difference Method

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Modeling of Electromagnetic Response on Rugged Seafloor by Using Finite-Difference Method

Abstract

Controlled source electromagnetic (CSEM) method is proved to succeed in detecting the high resistance of thin layer with higher credibility in confirming hydrocarbon bearing reservoir, water or gas compared with the reflection seismic method. When considering the resistive thin layer, 1D simulation results can give a satisfactory explanation. In many oil and gas fields, the reservoir exists under the rugged seafloor, and is more difficult to be identified. In fact, 3D modeling and analysis of CSEM response can be helpful for understanding the qualitative and/or quantitative information in resistive anomaly detection. 3D staggered grid finite-difference (SFD) scheme is used to simulate the response of marine CSEM in frequency-domain, while the linear system is solved by using the generalized productive bi-conjugate gradient (GPBiCG) Krylov subspace iterative operator (Shen, 2003), and a divergence correction method (Smith, 1996) is applied to speed up the convergence. Finally, we discuss the response characteristics of the EM response of gas hydrate model under the rugged seafloor in common offsets profile based on 3D modeling (Sasaki, 2009, 2010). The characteristics of the gas hydrate EM responses simulated in deep and shallow water environment have been analyzed, and also the normalized response and the corrected response have been derived by using the method suggested by Sasaki (2010) to indicate the gas hydrate distribution. We concern a layered reservoir model in shallow water which performs a poorer iterative convergence compared with the deep water case. A model grid that consists of 154×72×55 cells is set up for modeling the CSEM response of the wide offset range. The spacing is variable both in horizontal x, y and vertical z axis, with the thinnest cell in z-axis of 50 m in the anomalous region and nearby the source when the total field scheme is employed. In our work, the minimum cell in x- and y-axis is 100 m, and the maximum cell is 4000 m in the corner of the boundaries. A 200 m HED source of 0.25 Hz is positioned 50 m above the seafloor. We calculate the CSEM response use both total field scheme and secondary field scheme, and the amplitude and phase of the inline electric field Ex are plotted as a function of offset, also the comparison with the 1D semi-analytic solution is shown and significant improvement in iterative convergence when a divergence correction procedure is adopted.