Modelling Study of Airwave Contribution to the CSEM Signal

The Controlled Source Electromagnetic (CSEM) signal received in a three layer model (air, sea and seabed) has been split into contributions from the direct wave, the signal travelling through the seabed and the direct and indirect airwave by means of three modified models run in addition to the full three layer model. The components have been split for various cases of sea depth, seabed resistivity and frequency. The results show that the contribution from the indirect airwave is of the same order of magnitude as the contribution from the direct airwave and may be even larger. It is also shown that the relationship between the airwave and the signal that has travelled through the seabed is very different depending on whether the seabed resistivity is low or high. This may influence the choice of frequency for a CSEM survey.