Marine EM in GOM: Advances and outlook

Marine electromagnetic (EM) sounding methods provide valuable complementary information to conventional seismic exploration methods and success stories have been claimed by several oil companies: 1) as indicator of hydrocarbon presence derived from strong resistive anomalies 2) as complimentary tool in structural exploration. While 3D seismic identifies geological structures, it does not directly reveal the fluid content (hydrocarbons). Marine EM sounding exploits variations in electrical resistivity, and is directly sensitive to fluid saturation and thus resistive hydrocarbons. Under the right circumstances it can confirm the presence of hydrocarbons by identifying their resistive characteristics. This means that the possibility of drilling dry exploration wells is significantly reduced, as is the need for extensive appraisal drilling. EM data is used to resolve ambiguities in the structural interpretation of seismic data. For example, whereas the top of a diapiric salt body is often well constrained by seismic data, the position of the lower boundaries is often more elusive. Carbonate (or salt blankets, or resistive basalt) layers complicate the detection and characterization of deeper structure because of diffusive scattering in the layer. However, the resistivity contrast between these layers and the sediments below is an ideal target for EM sounding methods. Recently, two marine EM methods have become popular: The controlled source EM (CSEM) method and magnetotellurics (MT). The CSEM method uses an electric dipole source to transmit low frequency electromagnetic signals to an array of receivers that measure the electromagnetic field at the seafloor. Variation in amplitude and phase of the received signal as the source is towed through the receiver array yield the resistivity structure of the sub-surface to depths of several kilometers. The MT method uses naturally occurring electromagnetic source fields to determine the resistivity of the sub-surface. Thus, by studying the variation in response as a function of frequency, the variation in resistivity as a function of depth may be determined. These methods give complementary information about the resistivity structure of the sub-seafloor. Whereas CSEM data are primarily sensitive to resistive structures, and in particular to layers that are thin compared to their depth of burial, MT data can constrain larger scale conductive structure. By combining natural and controlled source methods better constraints on the geometry and properties of the seafloor can be gained than from either data type alone. Several case histories with large salt structures in the section illustrate that the techniques are useful for future exploration in the GOM. We see the technology moving from its present focus of deep water to include shallower water depths (where CSEM sounding is presently restricted). In addition, we envision the integration of complimentary EM techniques to get a better constrained resistivity image of the subsurface.