Application of the joint MT-CSEM inversion algorithm for field data interpretation

In offshore hydrocarbon explorations, both the magnetotelluric (MT) method and the controlled-source electromagnetic (CSEM) method have been widely used. The MT method uses the electromagnetic fields due to natural sources that can be modeled by using plane waves. It has a relatively large depth of investigation and can be effectively used to obtain rough conductivity estimate of the subsurface. On the other hand, the CSEM method measures the field generated by a low-frequency electric dipole that is towed by a ship at several tens of meters above the seabed. The CSEM data are mainly sensitive to resistive objects, which makes it capable of providing useful information of the locations and shapes of hydrocarbon reservoirs. It is well known that MT and CSEM data provide complementary information: MT may provide information on the background conductivity structures, while CSEM may identify resistive targets (hydrocarbon reservoirs). Constable and Weiss also pointed out the practical importance of MT data: The same receivers can collect both CSEM and MT data, hence MT data come at a very low cost since they can be recovered from time series data when the CSEM source is turned off.