Shallow Water CSEM Using a Surface-towed Source

We address challenges for using controlled-source electromagnetic (CSEM) surveys at small water depths. A novel deployment setup where electrodes of a conventional CSEM source are suspended from two GPS positioned buoys and towed 10 m below the sea surface is proposed. This setup allows better control of the source position and orientation along with improved speed and manoeuvrability as demonstrated by a test survey in the North Sea. The finite-difference 3D modelling code used in data interpretation has been improved by a careful representation of source and receivers allowing an accurate modelling in shallow waters. Modelling results assuming a 2.2 km deep resistor demonstrate that a surface-towed source has essentially the same efficiency in detecting the target as the traditional deep-towed source if the water depth is within a few hundred meters. Additional attenuation of EM fields travelling through the water layer for surface-towing may be compensated by a better knowledge of these fields due to precise control of source position and orientation.