Impacts of Complex Geological Structures on Marine Controlled-Source Electromagnetic Responses

Folds and faults are the most common types of basic structures in geological formations. Marine controlled-source electromagnetic (CSEM) data are strongly affected by folds, faults and other complex irregular geological structures in subsea geo-environments because of their electrical anisotropy and the bathymetry generated by these types of structures. In this work, we investigate the effects of two typical geological structures, folds and faults, on marine CSEM responses by using an adaptive finite-element modeling approach. The adoption of unstructured tetrahedral grids in our code easily allows the inclusion of complex model geometries so that natural environments can be mapped into the model realistically. For the fold models, we investigate the impacts of anticlines, synclines and mixed anticline-syncline-type structures; numerical results indicate that both anticlines and synclines can produce considerable negative electromagnetic anomalies, which may be neutralized to a certain extent by the positive electromagnetic anomalies of a highly resistive target reservoir. The response curve of a syncline displays a “W” shape whose central part consisting of rising positive anomalies may be mistaken as the response of a highly resistive reservoir when interpreting CSEM data. Furthermore, the negative anomalies caused by the anticlines/synclines increase with the increasing inclination of their limbs. For the fault models, we investigate the effects of horsts and grabens on marine CSEM responses and results show that the responses of horsts/grabens can be large enough to mask the target reservoir's response. We conclude that folds and faults clearly have to be taken into account to avoid the misinterpretation of marine CSEM data.