CSEM and Magnetotelluric Surveying

Wed, 01/25/2012 - 17:16

The development of magnetotelluric (MT) surveying has moved along at a rapid pace over the last decade, overcoming various challenges and demonstrably improving the reliability of results.

Some of the most interesting issues have come in ramping up technology to commercial scale. Magnetotelluric surveying works on the principle of resistivity, in a similar way to controlled source electromagnetic (CSEM) surveying. MT data is gathered simultaneously, along with CSEM data, when the controlled source is inactive. Whilst MT technology does not have the same sensitivity to thin horizontal resistors, it can penetrate thicker resistive layers than CSEM and seismic, which makes it a valuable tool in areas that have salt or basalt layers.

After deploying receivers on the seabed, a transmitter is towed close to the seabed in order to measure the electric field. The resulting data shows areas of resistivity buried in the seabed that aect the results compared to the reference area (see diagram below). In the early stages of magnetotelluric survey, this data would simply be handed to the operator. Today, companies work to produce a subsurface resistivity models that are delivered in 3D, and which can then be interpreted alongside seismic data.

During the field-testing stage, receivers are dropped to the seabed using concrete blocks. When the survey is completed, an acoustic signal activates an electronic release that sends the receivers back to the surface and leaves the blocks behind. This might be acceptable when only 10 or 20 receivers are dropped, but when a full survey complement of 1,000 receivers is considered, the environmental impact of leaving so many concrete blocks on the seabed becomes a greater issue. In order to overcome this, EMGS, a magnetotelluric survey company, created and patented a method of compressing sand into dense blocks that would drop the receivers to the seabed, and biodegrade in six to twelve months after a survey is completed.

The main limitation of resistivity data is that it cannot currently be used as a standalone data set to determine where to drill. Both CSEM and MT uncover resistors rather than reservoirs, thus it cannot be 100% certain that a resistor discovered by the survey is indeed indicative of hydrocarbons. Rather, magnetotelluric surveying companies prefer to talk about their technology as complementary to seismic (please see the article on the previous page for more information about how MT and CSEM can complement traditional seismic surveys). One of the main reasons for this is that MT technology utilizes extremely low frequencies, which translates into lowresolution data. This can be complemented with highresolution seismic data, but will never fully replace it.

In such a long-term industry, the relatively short history of this technology means that its full potential of its technology cannot yet be viewed in tangible results. CSEM and MT survey companies report a drilling success rate of between 50-70% when their technology is utilized. However, over the last two years many surveys have been completed where CSEM and MT technology was utilized, and where wells have not yet been drilled. This should mean that the drilling success rate improves rapidly, as the latest generation of the technology should help companies to be more successful in their exploration drilling. Indeed, it may soon be the norm for companies to complement their seismic surveys with magnetotelluric data.