Norm Allegar and Kurt M. Strack
KMS Technologies, Houston, TX
Most service providers of CSEM technology transmit a frequency-targeted source into the earth, often this is referred to as frequency domain CSEM or fCSEM. This source is almost always a continuous square-wave and both the active source and the subsurface response are recorded at nodes distributed along the seafloor. High resistivity lithologies and pore fluids are the resistors that then alter this electric field.
In time domain CSEM (tCSEM), the duration for the “on” and “off” times of the source is optimized for the particular problem. Transient responses to this artificial electric field are then measured by sensors that record both the electric and magnetic components. As tCSEM is collected in a style similar to seismic, the current switch represents time zero, it can be robustly integrated with seismic data. Additionally, by recording in the absence of the active source, the airwave phenomena can potentially be isolated from the subsurface response in shallow water and useful data collected at much closer offsets.
To date, multiple tCSEM surveys have been collected in such areas as the Caspian Sea, the Mediterranean and the Gulf of Mexio. Inversion results for transients from these surveys have shown a strong correlation to the anticipated subsurface resistivity. For tighter acquisition and appraisal and production applications the nodal system is augmented by a cabled system, presently under development. The tCSEM ocean bottom cable system will record both electric and magnetic field measurements and will provide denser sensor spacing than is typically afforded by nodal systems alone. Better repeatability of sensor positioning make the cable an ideal system for time lapse surveys. The goal is now to integrate nodal and cable based acquisition systems with down hole measurements and seismic to provide the optimal set of EM measurements.
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