Effects of steel casing in mCSEM: Analysis on the air-sea interface and a steel infrastructure

Abstract

One of the challenging tasks for applying controlled-source electromagnetic method to various fields is to consider the steel casing which strongly distorts electromagnetic fields. In this study, we developed an algorithm calculating the electric field due to the casing when a background is set as a three-layered earth (air, sea, and sea sediment). Through setting the three-layered earth as the background, we can incorporate the air-sea interface which shows strong conductivity contrast into the algorithm. To investigate the effect of the air-sea interface, we compare the horizontal electric fields caused by the casing assuming different backgrounds. The comparisons showed that if the background is inappropriately chosen, the response by the casing cannot be accurately simulated. In addition, since steel infrastructures near the survey environment can affect electromagnetic fields, the effect of a riser, a representative infrastructure, was analyzed. The analysis showed that the riser has to be considered as it locates closer to the air-sea interface and affects more to the electromagnetic fields than the casing. We concluded that appropriate background should be chosen for calculating the response by the casing reasonably and other steel infrastructures can significantly affect electromagnetic fields.