What’s new in exploration

Electromagnetic (EM) survey techniques can identify differences in the resistivity of subsurface rocks that can help discriminate between water and hydrocarbons, and reduce exploration risk. Marine EM surveys are conducted, using a towed electrical source and either towed or seabottom-based receivers. The sources are typically towed near the bottom in water depths greater than 500 m. However, new techniques allow towing within 10 m of the surface, in waters as shallow as 30 m.

IAGC study. In November 2011, the International Association of Geophysical Contractors (IAGC) announced the completion of an environmental impact assessment (EIA) of electromagnetic techniques used in oil and gas exploration, and production, in marine environments. The study was funded by the member companies of IAGC’s EM subcommittee and prepared by LGL Limited environmental research associates.1

The study looks at EM technologies that are used to search for resistivity anomalies worldwide: controlled-source electromagnetics (CSEM) and multi-transient electromagnetics (MTEM). The authors summarize more than 400 publications that address potential effects of EM survey technologies and naturally-occurring EM fields on marine life.

The EIA focuses on those marine animals that are of prime interest (commercial, social, cultural and scientific) to society and have at least some potential to be affected by EM surveys. This includes marine mammals (cetaceans, pinnipeds, polar bears); seabirds; sea turtles; and fish. The main areas of concern are EM emissions, electrolysis, underwater noise, light, ship strikes and accidental spills.

All animals are electrical conductors. Biological organisms continually generate internal voltage gradients and electrical currents, such as those associated with the nervous system; all types of biochemical reactions, ranging from digestion to higher brain functions; sensory and motor mechanisms; reproductive processes; and membrane integrity. According to the EIA authors, sufficiently strong EM fields have the ability to induce micro-currents within an organism and possibly disrupt these normal electrical functions. The EIA study emphasizes elasmobranches (sharks, rays and skates) and chimeras (deep sea ratfish), a group of electroreceptive fish that are potentially most affected by electromagnetic emissions.

Operations. The predominant EM survey technology in use employs an electric dipole antenna (source) towed behind a vessel. Electric and magnetic fields are measured and recorded by an array of receivers. The receivers are usually deployed on the seafloor and retrieved later to access the data. Work is underway to commercialize a system that tows receivers underwater, behind the vessel, similar to most marine seismic surveys.

Equipment, materials and activities characteristic of EM surveys include underwater noise emissions; light emissions; accidents (e.g., small-scale spills from flotation fluids, such as Isopar); EM source emissions; and electrolysis at the electrodes (chlorine emission).

Fish and seabirds are “not believed to be particularly sensitive” to noise from vessel operations. But marine mammals, especially cetaceans (whales, dolphins, porpoises), are much more sensitive. Noise-induced stress has not been studied in sea turtles. Some researchers speculate that sperm whales in the Canary Islands have lost hearing sensitivity to low frequencies generated by ship engines and propellers because of heavy marine traffic.

Survey vessels tow at such low speeds that collisions with large marine animals are “very unlikely.” General vessel traffic has affected behavior of whales, and semi-continuous activity in certain areas has caused whales to abandon those regions.

Of greatest concern are vessels that employ dynamic-positioning thrusters to maneuver while deploying or retrieving large numbers of bottom-mounted receivers. The use of thrusters has been noted to decrease the number of marine mammals sighted during operations.

Artificial lighting on ships at sea, offshore structures and coastal communities, attracts nocturnally active seabirds, and migrating land and water birds, particularly when there is low cloud cover, fog or rain. This attraction results in bird fatalities from collisions and strandings.

In the laboratory, long-wavelength visible light was found to disrupt magnetic orientation in birds. Reducing, shielding or eliminating skyward radiation of artificial lighting at sea was found to reduce the number of birds stranded. A 2008 study, in which white and red lights on an offshore platform were replaced with green lights, showed a large reduction in the number of birds attracted to the structure.

Results. There are ways to mitigate the effects of offshore EM operations. CSEM receivers can be fixed to the seabed with compacted-sand anchors that break down in six to nine months, reducing the number of thruster-assisted recoveries.

The EIA concluded that EM sources, as presently used, have no potential for significant effects on fish, seabirds, sea turtles and marine mammals. In addition, cumulative effects from EM surveys are negligible compared to natural EM anomalies, induced fields from natural water currents, and anthropogenic EM sources, such as those originating from undersea equipment. The authors point out that these are generic, rather than specific predictions, and determination of the significance of environmental effects may “vary considerably by jurisdiction.”  

LITERATURE CITED
1. Buchanan, R.A., R. Fechhelm, P. Abgrall, and A.L. Lang, 2011, Environmental Impact Assessment of Electromagnetic Techniques Used for Oil & Gas Exploration & Production. LGL Rep. SA1084. Rep. by LGL Limited, St. John’s, NL, for International Association of Geophysical Contractors, Houston, Texas. 132 pp. + app.