May 24 2011

Electromagnetic Field Stressors

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Electromagnetic fields are used by marine organisms for several purposes, from navigation to communication to predation. One potential biological threat of installing energy-generating technologies is that the fields will disrupt the behaviors of marine organisms. Electromagnetic fields are created when electricity is transported. Currently, there is a debate about whether AC or DC cables to offshore energy generating technologies would be best. For this reason, the environmental impacts of both technologies are important to understand.

Diagram indicating the origin of electric and magnetic fields that emanate from transmission cables

Magnetic Fields

The Earth generates its own baseline magnetic field which has a strength of 25-60 μT. In comparison, most people are exposed to magnetic fields that are 2000 times stronger on a daily basis (NaiKun 2009). Marine organisms, however, are far more sensitive than people to magnetic indicators because they are not able to rely on vision as well as land animals. Every year, researchers discover new details about how that marine organisms use the Earth’s magnetic field.

Man-made technologies can interfere with marine animals’ information gathering processes. Both AC and DC transmission cables create magnetic fields. While DC cables create a magnetic field that depends on the amount of current transmitted, AC cables create an oscillating magnetic field. The magnetic field created by DC cables increases dramatically with the distance between the bipolar cables. Burying the cables can significantly reduce the field strength.

Plot comparing AC magnetic field strength with distance from cable. Graph from a 50 km, 3-phase 33 kV HaidaLink cable that extends to an offshore wind farm from Graham Island (NaiKun 2009)


Plot comparing DC magnetic field strength with distance from cable. Graph from a 90-km, 200 kV cable that extends to an offshore wind farm from Ridley Island (NaiKun 2009)


Electric Fields

By themselves, electric fields are not created by underwater transmission cables because of the grounded metal wire sheath. However, an electric field is induced when marine organisms move through the magnetic fields created by the current. The strength of these fields is usually really small (on the order of 20 to 40μV/m). Certain species, particularly elasmobranchs, are able to detect these induced electric fields (Gill 2009). In general, these induced fields are greater for DC submarine transmission cables than for AC cables.

There are precautionary measures that can be taken to reduce the electromagnetic fields. Choice of electromagnetic field is extremely important. While DC cables are more efficient at transmitting electricity, they typically create a stronger surrounding magnetic field as well.  Another consideration is burying the submarine cables, which reduces the magnetic and induced electric fields in the surrounding area. The table below describe the differences in induced electric field created by a 200,000 kV DC cable when buried instead of laid on the seabed (NaiKun 2009):

HVDC Cable Location Induced Electric Field in 5 knot Current
On seabed 10,000 µV/m
Buried 1m 51 µV/m

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