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Controlled-source Electromagnetic Modelling Studies – Utility of Auxiliary Potentials for Low-frequency Stabilization
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 72nd EAGE Conference and Exhibition incorporating SPE EUROPEC 2010, Jun 2010, cp-161-00065
- ISBN: 978-90-73781-86-3
Abstract
When simulating controlled-source electromagnetic data at frequencies significantly lower than 1 Hz, standard formulations based, e.g., on the vector Helmholtz equation for the electric field, may fail to produce faithful results, because the term in the Helmholtz equation involving conductivity decays linearly with frequency. To stabilize low-frequency simulations, we introduce an auxiliary potential, constrained to be zero by boundary conditions, and explicitly enforce a divergence condition. We present an implementation of our stabilization technique within a finite-difference frequency-domain algorithm. The utility of the stabilized approach is demonstrated by showing synthetic data for frequencies as low as 0.001 Hz for a 3D model roughly mimicking the geometry of the CO2 sequestration pilot site in Ketzin, Germany. In these synthetic data, we observe that instability occurring primarily in the air for non-stabilized simulations disappears for stabilized computations. As expected, the auxiliary potential assumes near-zero values, but its amplitude increases with decreasing frequency due to numerical limitations. The amplitude characteristics of the auxiliary potential, and good accuracy of our synthetic data in comparison with 1D simulation results, suggest that the stabilization is usable down to frequencies at which electromagnetic fields can effectively be considered static.