Three-dimensional electromagnetic responses of disk-shaped hydrocarbon reservoir in marine sediments

摘要

This paper presents a three-dimensional (3D) marine controlled-source electromagnetic modeling algorithm using primary fields for a homogeneous half-space to accurately account for airwave effects. This algorithm is validated with analytic solutions for 1D two- and four-layer models and numerical results from another 3D model. Using this code, we investigate the 3D electromagnetic responses of a 100 m thick, 5 km disk-shaped hydrocarbon reservoir buried at a depth of 1 km below the seafloor. From the numerical results, we can recognize that a 3D effect of the reservoir typically produces a transition zone in comparison with 1D model responses. The transition zone decreases with the airwave effect as the depth of water becomes shallow. As the source frequency increases, the sensitivity to the reservoir increases, whereas the amplitude decreases and falls at higher than 1 Hz below the current system noise floor. Broadside electric fields for a 10-km diameter disk model are only about 5% of in-line electric fields for the 5-km disk model. The Tequivalence is observed at such a low frequency of 1 Hz for the thin resistive tabular target, whose response varies almost linearly with the target thickness and resistivity even in the transition zone.