Electromagnetic Fields

Introduction

Electromagnetic fields (EMFs) are considered to be a potential Stressor in the context of ocean wave energy development because they can be perceived by, and may influence or alter the behavior of, a fairly broad array of species. The Receptor groups known or thought to be sensitive to electric or magnetic fields include the lampreys and hagfish (agnatha), the sharks, rays and skates (elasmobranchs), the salmonids, the sturgeons, the sea turtles, and Dungeness crab. Further, there is uncertainty about the electromagnetic signatures of the wave energy conversion (WEC) devices, the strength and variability of local ambient fields on the US West Coast, and the thresholds of behavioral change in the susceptible biota. The targeted workshop on wave energy ecological effects conducted in 2007 by Oregon State University (Boehlert et al. 2008) concluded that there was the potential for impact on individuals of all these groups during the Operational Phase of a wave energy project (see Table 1).

Table 1.  EMF stressor table (from Boehlert, et al. 2008).

Brief Review of Information

EMFs have two primary components: an electric field, or E-field, and a magnetic field or B-field. The motion of conductors through the magnetic field produces a secondary, induced electric field, which is called the I-field (see Figure 1). EMFs would presumably be present or produced only during the Operational Phase of a wave energy project. Electromagnetic fields are given consideration in the prospective literature concerning the possible environmental effects of wave energy development. This attention stems from both the novel nature of EMF as a stressor in the marine environment, and from the uncertainty about EMF's actual environmental effects. The literature is based, in part, on existing evidence and studies from subsea high voltage cables, and, in part, on assumptions about the expected electromagnetic signatures of various WEC devices in conjunction with the potential effectiveness of EMF containment in those devices by a metallic shield called a Faraday Cage.

Figure 1. Schematic representation of the structure of an electromagnetic field (Gill et al. 2005).

Units and Measurements

The planet earth has positive and negative magnetic poles, and is thus surrounded by a magnetic field (B-field) that reaches relatively far (i.e., thousands of kilometers) into space. Ambient ls of this magnetic field in the northeast Pacific Ocean are on the order of 50 microTeslas (µT or millionths of a Tesla; 1 T = 10,000 Gauss), but E-fields are often reported as nanoTeslas (nT or billionths of a Tesla). Seawater, with a global salinity averaging about 3.5%, is an Electrolyte. Its motion via currents through the earth's magnetic field causes induction of a weak electric field, with higher current speeds inducing stronger electric fields. The ambient levels of this i-field strength in the northern California Current are on the order of tens of microVolts per meter (µV/m).

Ambient Levels of EMFs in the Oceans

The study of naturally occurring EMFs is an active area of study for geophysicists, who use a variety of specialized instruments, including many designs of a Magnetometer for its measurement. There is significant spatial and temporal variation in ambient magnetic fields in the ocean (and induced electric fields, reflecting similar variability in the speed and direction of ocean currents). For example, measurements of magnetic background variation were performed in 1990 offshore Cape Blanco, Oregon, related to the exploration for heavy sands containing large amounts of iron. The modal background levels in the ocean overlying the substrate were estimated to be 52.7 µT, with diurnal variability on the order of ~20 nT (Clifton, et al. 1991). Modeling results for a 0.85 knot current flowing through a 48 µT B-field resulted in calculated i-fields of 39 to 42 µV/m (Scottish Executive 2007).

Note: An E-field can be produced by electrochemical interactions of metals with sea water in a process called Galvanic Corrosion, as noted in Table 1. Such a field could be a stressor in the operational phase of a wave energy project, as there is evidence that Dungeness crab will avoid crab pots with such a galvanic field when the insulating wrap over the metal is compromised (Boehlert, et al. 2008).

Receptor Sensitivity and Known Thresholds

Marine invertebrates and vertebrates known or suspected to be sensitive to electromagnetic fields include Dungeness crab (Cancer magister), Sea Turtles, Green Sturgeon (Acipenser mediarostris), Sharks, Rays and Skates (Elasmobranchs), anadromous fishes, importantly Pacific Salmon (Onchorhynchus spp.), and tunas, including Albacore Tuna (Thunnus alalunga). Animals are thought to use their sense of electric or magnetic fields for orientation, navigation, and prey location. Laboratory studies have directly demonstrated electric field sensitivity in Elasmobranchs, whereas sea turtles, salmonids and tunas are suspected to be sensitive to magnetic fields based either on the presence of Magnetite in the brain (e.g., salmonids) or on their observed navigational abilities and behaviors.

Laboratory experiments with fishes have demonstrated both a positive and negative Taxis, or behavior, in response to E-fields. Elasmobranchs have been shown to be sensitive to both direct current (DC) and alternating current (AC), but once the AC signal reaches a higher frequency than about 8 Hertz (cycles per second), sensitivity to the AC E-field is apparently lost (standard AC line voltage in the United States is 60 Hertz). Laboratory experiments have established threshold levels for a variety of species, and some are able to perceive AC or DC electric field strengths well under 1 µV/m. In terms of biological significance, this means that the animals can use small differences in field strength to locate prey by sensing the i-field produced by motion through the water or even by blood flowing through the organism. A summary of known electric field sensitivity for marine species is presented in Table 2, taken in its entirely from the Reedsport OPT Wave Park (2010) FERC license application.

Table 2.  Known thresholds of sensitivity to magnetic and electric fields (from OWET 2010).

Information about B-field sensitivity is not nearly as robust, likely because of the technical difficulties of controlling studies on magnetic fields. As stated in the Reedsport OPT Wave Park (2010) application:

Organisms that can detect magnetic (fields) or B-fields are presumed to do so by either I-field detection or magnetite-based detection. E-field (or I-field) sensitivity is apparently sufficient for some elasmobranchs to locate prey beneath a thin layer of sediment (i.e., a few centimeters - less than an inch) by sensing the I-field created by blood flow through a beating heart.

E-fields are thought to be effectively blocked by a Faraday Cage, as is shown in studies of armored HVAC cables. Studies estimate magnetite B-field sensitivity are on the order of tens of µT, which could be adequate to sense major changes in a background on the order of 50 µT.

Wave Energy Technology Signatures and Existing Proxies

There are presently no publicly available field (i.e., in situ) measurements of EMF for wave energy conversion devices in the U.S. There are three possible components of wave energy projects from which EMF could emanate: 1) the WEC devices themselves; 2) any water column or bottom electrical infrastructure (such as DC/AC inversion or rectification equipment); and 3) transmission lines.  While there are no proxies for conversion devices or for the infrastructure that may be unique to wave energy developments, undersea cables have an established literature and are well-documented in terms of environmental effects.  Studies of undersea HVAC cables tend to show that burying and armoring cables largely mitigates the E-field, but there are still assumed to be weak B-fields (e.g., see Michel, et al. 2007). Figure 2 shows the exponential decay of a B-bield modeled in a 3-phase AC cable. Note that the field strength is reduced by an order of magnitude within about 30 meters distance of the source (USDOE 2009).

Figure 2.  Measures of the modeled exponential decay of a magnetic field (in Amperes per meter) and magnetic flux density (in microTeslas) with differing line Amperages and distances from an armored, but not buried, three-phase AC cable (USDOE 2009).

During 2009 the British-based Collaborative Offshore Wind Research Into the Environment (COWRIE) project released its phase two EMF study (Gill, et al. 2009). COWRIE investigators used Mesocosm enclosures to monitor the behavior of EMF-sensitive elasmobranchs in the vicinity of buried electrical cables mimicking those of an offshore wind farm. These results are the most applicable to date and likely the most valuable for extrapolating to the real environment, but they are also equivocal. Behavioral effects related to the presence of an EMF were documented for some elasmobranch species, and included general attraction and changes in rate of movement, but without definitive results. Behavioral effects are especially important, for example, when humans (e.g., surfers, bathers) share the water with large elasmobranch predators like sharks. The COWRIE study results underscore the complexity and difficulty of determining actual ecological effects in the ocean environment. Further, their results tend to support the need for technology-, site- and species-specific effects studies as appropriate, based on technology-specific EMF signatures and exposure.

Mitigation

Notwithstanding a substantial number of uncertainties, mitigation exists for some of the EMF concerns accompanying consideration of wave energy development. Burying and trenching armored transmission cables decreases the strength of the B-field that reaches the water column (the cable modeled in Figure 2 was not assumed to be buried). The use of multiple conductors and multiple phases alternating current has an effect on the character of an EMF that may mitigate, for example, by cancelling out some of the B-field. Ground Fault Interruptors (GFIs) are also expected to be included in all components of any wave energy array, as their function is primarily to prevent damage to the system.

Remaining Questions

1. Attraction of sensitive species to a wave energy development;

2. Repulsion of sensitive species from a wave energy development;

3. Interference with feeding/foraging of sensitive species; and

4. Interference with navigation/migration of sensitive species.

Key Data Gaps

Filling the following information gaps would help to reduce uncertainty about wave energy development-sourced EMF and its effects on the Ecological Integrity of the West Coast continental shelf:

  • Remaining uncertainty about ambient EMF background and variability on the West Coast continental shelf (i.e., at least in the vicinity of viable wave energy locales);
  • Uncertainty about the risk of exposure to albacore (and possibly sea turtles) to EMF on the US West Coast based on their migration patterns (for albacore, this would be documenting how often suitably warm temperatures develop inshore, for sea turtles it might be a new set of data like the earlier MMS Environmental Studies);
  • Uncertainty about the risk of exposure of anadromous fish (Pacific salmon and green sturgeon) to EMF during key migrations or key life history stages (likely outmigration for Pacific salmon);
  • Major uncertainty about the EMF signatures and variability (across the likely wave energy spectrum) of wave energy conversion devices and electrical infrastructure (e.g., inverters, rectifying devices, etc.)
  • Lack of information on the sensitivity thresholds of key EMF-sensitive organisms on the continental shelf of the US West Coast; and
  • Lack of information on the response of organisms that may sense the i-field developed by seawater moving through the vicinity of the B-field emanating from a subsea cable.

References

For further information and resources on this topic, please visit the Tethys Knowledgebase at http://mhk.pnnl.gov/wiki/index.php/Browse_Knowledge_Base

Works cited on this page are listed below.