One of the many renewable energies being developed, allows for the extraction of heat from the upper crust of the earth to produce electrical power. This is called Engineered Geothermal Systems (EGS) which is now being taken up commercially. Initially a borehole is drilled to a depth to reach a defined temperature (in Western Europe around 5000m will produce a temperature of 200ºC) and water is injected to open up natural fractures which generate seismic sound. The opening up of fractures is monitored using an array of seismic sondes. These seismic sounds are triangulated to find out exactly where and when natural fractures are opened and then slipped by shear mechanism. This gives an insight into what is happening at great depth and an image is produced to show the area where the water has migrated. A second borehole is drilled about 600m away based on the image returned by the seismic sounds. Cold water is then pumped into the first well which flows through the opened fractures and is returned to the surface as steam. This is then converted to electrial power.
Current Installations of the Sonde at Geothermal Plants
Eden Project, Cornwall, UK
“We’re currently using an array of these sondes to monitor any seismic activity, whether natural or induced, while injecting water to open up the natural fractures in the rock. These highly sensitive accelerometer-based seismic sondes are a product of over 30 years of development within DJB Instruments, which started at the UK EGS project at the Rosemanowes site in 1980s and continued at the European site in Soultz in France until recently. They’ve developed a unique product which is being adopted worldwide and EGS Energy are proud to use them at the proposed EGS site at the Eden Project.” Roy Baria, Technical Director of EGS Energy
In 2007 the first geothermal power plant with the focus on commercial electricity production was put into operation in Germany.
The Geysers, California
In 2009 the DJB Sonde was installed in a pilot site in California
Advantages of Seismic Sondes over other Seismic Monitoring Instruments
- Slimline diameter of 76mm means the instrument can be deployed in small bore holes.
- Can be deployed at a depth of up to 400m
- Does not require a clamping mechanism as it rests at the bottom of the hole.
- No moving parts, thus, more reliable.
- Large accelerometes and low noise amplifiers give a high output which is very sensitive to even the smallest of signals.
- No deterioration of signal over time.
- Easily removable for redeployment.
- The internal accelerometers have a large bandwidth compared to other geophones or seismometers, this faciliates additional analytic treatment.
- No spurious responses within the bandwidth of interest.
- Down-hole amplifiers to improve signal to noise ratio.
- Can be operated off 12v batteries.
- Low power consumption provides a long battery life, even on small batteries in remote areas.