Controlled-source electromagnetic (CSEM) surveying

Wellbore resistivity has long been the fundamental formation evaluation measurement. Marine CSEM surveying uses the same principles to map resistive bodies such as commercial-scale hydrocarbon reservoirs from the seabed.

As pore fluid resistivity largely determines the EM response, CSEM surveying is the perfect partner for seismic techniques, which generally provide structural information. Together, and with other subsurface information, these methods form a valuable set of exploration tools.

In CSEM surveying, a powerful horizontal electric dipole is towed about 30 m above the seafloor. The dipole source transmits a carefully designed, low-frequency electromagnetic signal into the subsurface.

EM energy is rapidly attenuated in conductive sediments, but it is attenuated less and propagates faster in more resistive layers such as hydrocarbon-filled reservoirs.

Grids of seabed receivers measure the energy that has propagated through the sea and the subsurface. Data processing, post-modelling and inversion are performed to produce 3D resistivity volumes. These datasets are integrated with other subsurface information to enable you to make important drilling decisions with greater confidence. 

 3D EM vessel video  

Magnetotelluric (MT) surveying

In a similar way to CSEM surveying, the MT technique is sensitive to resistive bodies in the subsurface.

Marine MT surveys map subsurface resistivity variations by measuring naturally occurring electric and magnetic fields on the seabed. The sensitivity of our receivers enables us to acquire high-quality MT data inherently as part of a CSEM survey when the controlled source is inactive. The naturally occurring electric and magnetic fields are generated by the interactions of solar wind with the Earth’s magnetic field, which, when strong, are known as geomagnetic storms. The source fields are very low frequency, which offers excellent depth penetration.

The low-frequency, deep-sensing nature of MT surveying makes the technique excellent for mapping and interpreting regional geology. MT technology does not have the same sensitivity towards thin horizontal resistors as the CSEM technique; rather it can penetrate the thicker resistive layers that might otherwise be challenging for CSEM and seismic techniques.

MT surveys have been found most useful in salt and basalt settings where the flanks and/or the base are not well controlled. MT measurements can, therefore, form an excellent complement to other geophysical data, particularly in settings where high-impedance volcanic rocks or salt make the interpretation of other geophysical data challenging. This is why we have been asked to acquire MT data in settings such as the Red Sea, which has complex salt structures.

A typical scenario where MT surveying could be useful is depicted in the schematic above. In a case such as this, CSEM and seismic data would most likely provide excellent information on the structure and reservoir above the salt layer. However, they might struggle to provide information on the depth to basement, base and flanks of the salt, and the presence or absence of sediments below the salt: aspects that relate directly to hydrocarbon prospectivity, the understanding of which may be improved through using MT techniques.