Introduction
Marine electromagnetic (EM) is one of the new geophysical methods. To broaden its use from exploration to appraisal and production applications, the system and services cost must be reduced significantly. Novel deployment options, for example as ocean bottom cables, create the need for smaller magnetic field sensors. To provide state of the art sensitivities using a much smaller form factor a different concept for the sensing of magnetic field components was needed. Fluxgate sensors with exceptional noise performance and high bandwidth have been created that can be used for MMT as well as for CSEM measurements in time and frequency domain. In comparison to traditionally used induction coil magnetometers they only use a fraction of the space allowing for tight packaging needed in OBS or streamer like systems.
Complementary to seismic methods marine electromagnetic methods have become important to the oil industry. While seismic data provides the best stratigraphic and reservoir shape information EM methods provide information on fluid properties and allow a distinction between brine or oil filled areas of a reservoir. Based on the electrically resistive nature of oil & gas reservoirs controlled source electromagnetic methods (CSEM) have been used as direct hydrocarbon indicator (Eidesmo et al., 2002) or together with constraining information in more advanced interpretation schemes (Hu et al., 2007, Johansen et al. 2007).
Deep penetrating EM methods like marine magnetotellurics (MMT) have been proven to provide highly valuable stratigraphic information in areas that are hard to penetrate by seismic signals. Improved processing results in the delineation of salt bodies have been achieved by combining seismic and magnetotelluric information in a joint interpretation (Wu et al, 2008).
Independent of the methodology used almost all commercial marine EM measurements are carried out using autonomous node recording systems that are released into the water from a crane on the back deck of a vessel. After positioning the vessel over the designated receiver location the EM recording node is released and autonomously sinks down to the bottom of the ocean. Associated with this deployment mode is a lack of positioning precision, as water currents influence the passage of the node, combined with a total loss of control in regards of sensor orientation. While these problems can be overcome in an exploration scenario the more stringent positioning and orientation requirements of time lapse measurements (Orange et al., 2009) are hard to achieve with autonomous nodal systems.
For tighter acquisition, appraisal and production applications KMS-Technologies presently develops a cable based EM acquisition system. It combines time domain (transient) electromagnetic methodology with the benefits of a densely sampling measuring system.
In time domain controlled electromagnetic (tCSEM™) direct current is transmitted into the Earth via a large electrical dipole. The return path of the resulting current density pattern includes the water body as well as the sedimentary layers beneath.
At certain predefined times the current is switched off and the collapsing magnetic field causes eddy currents to flow in the subsurface (Strack, 1992, 1999). Transient responses of these eddy currents are measured using electric and magnetic sensors.
