Next in our program was the deployment of 14 Ocean Bottom Electric (OBE) field receivers from the Ocean Bottom Instrument Consortium (OBIC). A recent development is the vertical arm so that 6 stations recorded not only the horizontal but also the vertical electric field. Deployment of the OBEs extended over two sessions with an intermediate record-time deployment of 19 Geomar Ocean Bottom seismometers (OBS).
Photos by Romina Gehrmann
We were then ready to deploy the Deep-towed active source instrument (DASI, Sinha 1990) and two towed electric field receivers (Vulcan, Constable 2016) in the early hours of the morning, and finished just in time for breakfast.
Photos by Romina Gehrmann
Controlled Source Electro-Magnetic, or CSEM, is a geophysical technology deployed offshore for the detection of changes in electrical properties of the subsurface sediments.
Sketch of the University of Southampton towed CSEM system: DASI [Sinha, 1989] towing the dipole transmitting antenna array (white lines represent current streamlines generated by the antenna), the three-axis electric field Vulcan receiver [Constable et al., 2016] and three-axis ocean bottom electric field receivers (OBIC). Figure adapted from Weitemeyer (pers. communication)
To achieve this, a powerful electrical dipole (the source) is towed above the seafloor emitting a low frequency (1 Hz) electromagnetic signal into the subsurface. Receivers can be placed on the seabed on a regular grid or towed behind the source, to record the signal that has propagated through the subsurface and returns. In the subsurface, the absence of saline formation water will render rock more resistive than when this is present. Thus, CSEM resistivity results can be used to indicate the possible presence or absence of fluids like hydrocarbons or fresh water, as well as differentiating between permeable and impermeable rock layers.
Hence CSEM data is a powerful tool, when integrated with seismic and rock sample analysis, to identify potential permeable structures in the North Sea basin from where gases in the subsurface, methane or carbon dioxide, could migrate through from the layers where they are stored and migrate to the surface.
After 2 days of survey all the EM instruments were recovered in record time before tea with a beautiful data set. Thanks for the great work done by the watch keepers, engineers and the ship’s crew! The cruise moved on to deploy the seismic streamer array and air guns.
Your Scientists on board of RV Maria S. Merian
Martin C Sinha, PD Patel, MJ Unsworth, TRE Owen, and MRG MacCormack. An active source electromagnetic sounding system for marine use. Marine Geophysical Researches, 12(1-2):59-68, 1990.
Steven Constable, PK Kannberg, and K Weitemeyer. Vulcan: A deep-towed CSEM receiver. Geochemistry, Geophysics, Geosystems, 17(3):1042-1064, 2016.