Simultaneous acquisition of towed EM and 2D seismic – a successful field test

Summary

A towed marine EM acquisition system has been under development for seven years, and two field tests were recently completed in the North Sea. Traditional CSEM technology is based on sparsely spaced receiver stations placed on the seafloor and the source dipole is towed close to the seafloor. The source signal is a square-wave, or a modified square-wave, that is emitted continuously.

In the towed EM system, the source dipole is towed at 10 m below the sea-surface and the receiver cable is towed at a nominal depth of 100 m. The prototype system described here is sufficiently powerful to work in water depths up to 400 m, with a nominal depth penetration of 2,000 m below the seafloor. The signal is a transient signal that can be a modified square-wave, or a PRBS. All aspects of the data acquisition are monitored real-time and pre-processed on-board facilitating quality assurance and optimization of all acquisition parameters.

Two successful tests were conducted over the Peon shallow gas field and the Troll oil and gas field. One of the sail-lines over the Troll field was simultaneously acquiring EM and 2-D seismic data. By keeping the EM source and the seismic streamer separated, the level of induced electrical noise on the streamer was never reaching levels where it would become an issue.

In total 615 line km were acquired over 138 hrs and the data has been successfully processed and inverted to delineate all targets.

Key words::

• CSEM
• Marine EM
• Transient EM
• Towed EM