The geoelectric coast effect: a stepped-depth sea model

It is known that geoelectric fields, induced at ground level, during geomagnetic storms may represent a threat for infrastructures such as power lines, pipelines, and railway lines. Particularly, when one of these structures is located inside a region characterised by large lateral ground conductivity contrast (typically a coastal region where the seawater conductivity is significantly higher than that of the land), the commonly named geoelectric coast effect manifests and consists in an increased level of the induced geoelectric field near the coastline with penetration inside the land region for several kilometres. A convenient calculation method, capable of assessing the geoelectric coast effect, is the generalised thin sheet model. In literature, this method is generally applied by considering the idealised case of a constant-depth sea. On the contrary, in this paper, we extend its use to the more realistic case of a stepped-depth sea model. In such a way, we show that, by considering a shelf region, characterised by different sea depth levels and located between the coastline and the maximum sea depth region, the application of such model results in significant differences in the predicted geoelectric field with respect to the simpler constant-depth sea model. Moreover, the influence of certain parameters characterising the model (shelf region length and slope, and maximum sea depth) is put into evidence. These are the main novelties introduced by this paper.