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Geoelectric Field Monitoring

 

Figure 1: Installation of the measurement electrodes at Lerwick observatory in the Shetland Islands.
Figure 2: Electrode and signal termination & digitiser enclosure at Hartland observatory.
Figure 3: EMI-701 electrodes installed at Eskdalemuir.

We are currently installing surface electric field monitoring systems at the UK observatories (Eskdalemuir, Lerwick & Hartland) to measure the changing electric field generated near the Earth's surface.

Monitoring the electric field will allow validation of models derived by the team and help improve predictions of potentially damaging geomagnetically induced currents in the power grid.

Data are recorded at 1 Hz along East-West and North-South electrode lines and automatically transmitted back to our office in Edinburgh for analysis and near real-time display.

Background

To understand how space weather impacts the grid, BGS has a model of the surface electric field, induced by rapid geomagnetic variations, coupled to a DC model of the UK's power transmission system. The electric field is the source of electrical currents flowing in the Earth's crust (Telluric currents) and these can damage transformers in the power grid.

However, the electric field is not routinely measured anywhere in the UK.

We are now installing electric field monitoring equipment for long-term measurements at the three UK magnetic observatories. These data will be used to help constrain electric field models and, as a result, improve the prediction of induced currents in the UK power system.

Hardware

Measurements of the electric field are made by recording the voltage difference between two points in the ground, separated by a known distance in a given orientation.

Two electrode pairs are used in a North-South and East-West configuration to capture the strength and direction of the surface electric field vector. The electrodes are spaced approximately 100m apart and are located as far as possible from sources of cultural noise and non-natural conductive structures (e.g. buried pipes and cables).

Non-polarising electrodes (LEMI-701, CuSO4 Chemistry) are used to make the voltage measurement as they prevent any self-potential effects caused by the interaction of the electrode conductor with the surrounding soil and offer very good stability for long term measurements. They are buried at a depth of at least 70 cm to reduce temperature fluctuations and transient soil-conductivity changes due to heavy rainfall.

Voltage measurements are made via a 24-bit analog to digital convertor (Earthdata, PS-2400) that has an output rate of 50 Hz and data resolution of 1 μV. These data are then digitally filtered (using a Windowed-Sinc kernel) to produce 1 Hz samples.

These are collated in hourly files and automatically transmitted, every 10 minutes, back to a BGS data server in Edinburgh.

Deployment

Installation at Eskdalemuir observatory was completed in November 2012, Lerwick observatory in March 2013 and Hartland in May 2013.

The completed systems at Eskdalemuir and Hartland have run reliably since installation with 100% data availability. Unfortunately damage caused by a lightning strike at Lerwick caused the loss of some data at the end of July to mid-August 2013.

The signal to noise ratio at Eskdalemuir is not as good as the Lerwick site, likely due to nearby equipment producing contaminating the measurements, and a naturally weaker signal due to the lower latitude of the site. A new location for the electrodes, further away from existing equipment has been identified to reduce this.

The three systems have only been recording data for a short amount of time, but indications are that the data are of sufficient quality to reliably capture changes in the electric field during periods of increased magnetic activity.

Data

Comparison of orthogonal magnetic (B) and electric (E) field components at Eskdalemuir observatory. The E-W direction is denoted 'y' and the N-S direction 'x'. dB/dt is the time rate of change of the magnetic field.
Plots of the electric field data are generated alongside magnetic data from the UK observatories for comparison and analysis. These data are available online.

National Astronomy Meeting 2013

This work was presented at the Royal Astronomical Society (RAS) National Astronomy Meeting 2013 at St. Andrews, Scotland.

Poster

RAS Press Release

Podcast

Acknowledgements


This project was funded by the BGS Opportunities Fund 2012. Ongoing operations are supported by NERC National Capability funding.

Contact

For more information please contact Dr Gemma Kelly.