Integrated electromagnetic and seismic methods for petroleum exploration

During the past decade several deep electromagnetic techniques have been successfully applied to hydrocarbon exploration problems. Case histories for magnetotellurics (MT) and long offset transient electromagnetics (LOTEM) have shown both the strengths and the limitations of the individual techniques when applied in a production mode under real field conditions.When exploring for hydrocarbons it is essential that the resistive units are as well resolved as the conductive ones. This means that one must use an electric dipole transmitter and an electric field receiver because this is the most direct way to measure vertical current systems in a layered earth. Furthermore, to extend the LOTEM results to greater depth and to improve the resolution of MT at shallower depth, a combination of both techniques is required. When using MT and LOTEM together for mapping of lateral changes in resistivities, a priori information such as the interfaces from reflection seismics is needed. This type of integration of the different techniques significantly increases the reliability of the interpretation because the strengths of the different methods compensate for each other’s weaknesses. This concept is underlined using field data from different case histories around the globe.A typical petroleum exploration problem is a resistive unit at great depth which simulates porosity variations within carbonates. This kind of earth model is investigated using a synthetic data set. The joint interpretation of LOTEM-MT does not yield sufficient resolution. When adding seismic information to the EM interpretation the resolution becomes significantly better and the porosity changes can be resolved.