A theoretical calculation of GIC in a given network (power grid, pipeline etc.) is conveniently divided into two steps:
The geophysical step is generally more difficult, partly because the space and geophysical input parameters are not well known.The complex image method (CIM) (see list of publications) allows for a fast computation.
In the engineering step two different cases may be separated: a discretely-earthed network (like a power system earthed at transformers), and a continuously-earthed network (like a buried pipeline earthed through the coating). Formulas for considering the former case were derived at FMI/GEO in the 1980's. The distributed-source transmission line (DSTL) theory recently investigated and extended in connection with the collaborative project between Gasum Oy and FMI/GEO makes the treatment of a pipeline possible.