Science & Environment

Goce gravity boost to geothermal hunt

Hellisheidarvirkjun geothermal power plant Image copyright Reuters
Image caption Iceland is famous for its geothermal power stations, but the resource globally is under-developed

The hunt for sources of geothermal energy is getting a boost from new observations of the Earth made from space.

Information about variations in gravity across the planet could help prospectors find promising locations where sub-surface heat can be exploited to generate electricity.

The data comes from Europe's Goce satellite.

It mapped Earth's gravity field from 2009 to 2013 at high resolution.

This keen sensing is now expected to narrow the search for prime spots to put future power stations.

The International Renewable Energy Agency (IRENA) has released the information in a special global atlas.

In parallel, Goce scientists have been discussing the work here at the European Geosciences Union General Assembly in Vienna, Austria.

Down below

Although a large potential resource, geothermal currently accounts for less than one percent of the world’s electricity generation.

Part of that comes down to the huge costs of exploration.

But Goce's maps are expected to shortcut some of the effort by pinpointing regions of the world with the best characteristics, such as where the continental crust is at its thinnest.

As it flew around the planet, the satellite was able to observe very subtle differences in the pull of gravity from one place to the next – a function of the uneven distribution of mass beneath it. This variation in the gravity signal is most obvious over large mountains and over deep ocean trenches.

But by processing Goce's data in special ways, scientists can also tease out details of the different rock layers and structures within the Earth.

Image copyright ESA/IRENA
Image caption The Free Air anomaly map contains information about the rock structure beneath the surface

For IRENA, Goce researchers have provided two types of gravity model: one called "Free Air" and one called “Bouguer”.

The Free Air gravity map gives insights into particular structures that might favour geothermal energy. The Bouguer map, on the other hand, details crustal thickness. Prospectors will still need other types of information to find the heat reservoirs – but the new portal should be a good place to start, especially in some countries where local geological surveys may be very difficult to conduct and are therefore incomplete.

Carla Braitenberg from the University of Trieste has done much of the work to prepare the Goce maps. She told BBC News: "Bouguer tells you where you have crustal thickness variations, and where the mantle is coming up to the surface. For geothermal, you want a thin crust because this is where the temperature gradients will be highest.

"Free Air helps us understand the structures, because to have an exchange of high temperatures at low levels and the surface, you want to use existing fractures, and these fractures are found at contacts between different rock types. Free Air helps us map these contacts."

Image copyright ESA/IRENA
Image caption The Bouguer anomaly map provides information on crustal thickness

Geothermal prospectors have long used gravimeters in the field to aid their search for the right subsurface characteristics. But these are point measurements. Goce now provides the information across the globe and at a resolution never before achieved at that scale.

Rune Floberghagen was the European Space Agency’s mission manger on the satellite. He said: "With Goce, we see the gravity anomalies at a resolution of between 70 and 80km. Towards the end of the satellite's life, we lowered its altitude to improve the resolution, and I’m really pleased we did. This geothermal application shows that it paid dividends."

The prime mission goal of Goce was to acquire gravity field information to investigate how ocean currents move. It has proven to have many solid Earth applications as well.

Image copyright ESA
Image caption Goce's mission ended in 2013 when it fell back to Earth

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