New insights of airwave in controlled-source electromagnetic offshore data

The marine controlled-source electromagnetic (CSEM) method has been used for offshore natural resource exploration for a few decades, and it has the potential to be used for offshore CO₂ storage monitoring. Airwaves, previously treated as distortions, often dominate marine CSEM data when the offshore seawater is shallower than a couple of kilometers. Therefore, different methods have been proposed to distinguish and then correct the airwaves in marine CSEM data. In this study, we analyzed the airwave features by different model parameter perturbations with two-dimensional (2D) modeling. After a thorough study of differentiated EM fields and Poynting vectors for each single model component, we summarize the airwave propagation features and sensitivities regarding different modeling parameters. Particularly, the scenario with and without an oil reservoir or CO₂ storage is carefully studied. It turns out that the airwave can provide useful information at certain transmitter and receiver offsets. Nevertheless, we propose to model the airwave as what it is in the marine CSEM data rather than correct it before feeding the offshore CSEM data to inversion. This idea is demonstrated with a field example from offshore of Svalbard, Norway.