Controlled-source electromagnetic (CSEM) surveys are a common geophysical investigation tool in the search for, amongst other, groundwater, hydrocarbons, and minerals. The numerical modelling of CSEM data requires the solution of the Maxwell equations. These can be simplified in the particular case of CSEM, as the frequencies used in surveys are usually sufficiently low to ignore any displacement currents. A diffusive problem remains, which has the resulting system of equations given in the frequency domain by