Seismics and EM complement each other
Seismic surveys can be used to locate reservoirs but cannot differentiate between oil and water in reservoirs. Today oil companies use EM as well as seismic to achieve a more certain estimate of the contents of subsurface structures.
Petroleum exploration with seismic surveying works by sending powerful acoustic waves into the subsurface. The returning echo is recorded and used to create an image of subsurface reflectors. The method can be compared to an ultrasound examination at the doctor, where high frequency sound waves are sent out from a transducer. The reflected sound waves are processed to give a visual interpretation of internal organs.
Surveying with EM-technology operates in an entirely different way. The source is towed by a cable that creates a localized EM field. The induced current lines aredeflected by the different geological structures they encounter. Sand and gravel affect current flow lines differently than granite or gneiss and water affects current flow lines differently than oil and gas.
The induced currents modify bi\oth the electric and magnetic fields, which are then recorded at the seafloor receivers. EM surveys give no direct image as with seismic surveys, rather a huge data flow which must be analysed with specially developed computer programs.
Analysis of EM records gives rise to an unclear picture of the resistive structures below. High electrical resistivity can indicate the presence of hydrocarbons but can also be due to bedrock topography, basalt layers or carbonates. When EM data is analysed together with seismic imaging and detailed geological models; results can give a more precise analysis of the geology to be found down there.
NGI combines EM and seismic
The risk of drilling a dry well is considerably reduced with EM, but even the combination of seismic and EM cannot guarantee 100 % success. Great demands are imposed on processing EM signals which are influenced by complex 3D geological structures, shallow water, reservoirs close to salt deposits or basalt structures, etc. NGI continues, therefore, to research and eliminate the limitations of EM.
To effectively produce seismic and EM data in a joint project; NGI developed a seabed cable system for Multifield Geophysics, a growing entrepreneur in the EM market. A cable-based system has the great advantage of higher resolution data than traditional node-based surveying. All EM software solutions developed at NGI rely upon close integration with seismic information to achieve the best possible interpretation of ambiguous EM data.
Today's most advanced oil exploration at sea involves laying a long cable with both seismic equipment and EM receivers on the seabed. The cable remains fixed while the subsurface is surveyed and must then be retrieved and relocated to a new area of exploration. Illustration: Multifield Geophysics.