Circulating water through the geothermal reservoir will create pressure changes that can cause small movements to take place along some of the natural fractures in the rock. These movements are called ‘microseismic’ events. They are, in effect, micro-earthquakes; a bit like normal earthquakes but thousands of times smaller.
The events are so small that we will need a network of sensitive instruments to detect them. Specialised software will allow us to pinpoint where they happened and measure their size. This is a very useful tool to help understand the shape of the geothermal reservoir deep underground. The same thing was done during the Hot Dry Rock project at Rosemanowes Quarry in the 1980s. Some of the located events from that project are shown in the figure on the right and they clearly showed how most of the injected water moved downwards.
There may be hundreds of these events during the UDDGP project but it is very unlikely that more than a handful will be noticed at the surface; they will be too small and too deep.
We have already installed some of the network in order to collect background data and we will be completing it soon. Part of the network will measure surface vibrations, and some of these instruments will be located in local schools, offering a chance for children to get involved with the project and to learn about natural seismicity.
Data from the monitoring system will be managed by the British Geological Survey and made available through their website.