Taking the temperature of the underground

Where is the safest place to dispose of our nuclear waste? What will happen to the mountain landscapes when glaciers melt? Where is the best place to build a tunnel through a massif? And what is the best location to recover geothermal energy?

Many of the answers to these very different questions can be found using a new method that reveals the average temperature of the underground much faster than existing methods. In addition, the new method offers access to the more recent thermal history of our underground—i.e. the past 100,000 years, where the existing methods do not contribute with any information.

“It is important to know the average underground temperature over the past thousands of years, as it gives us an idea of the area’s stability, and whether it has been exposed to major changes or thermal influences. This is good to know, for example, when choosing where to deposit nuclear waste or build a tunnel,” explains Senior Researcher Mayank Jain from DTU Nutech.

DTU Nutech has headed an international collaboration through which the method has been developed over several years. The work of the researchers was published in Elsevier Earth and Planetary Science Letters in 2015.

Ubiquitous mineral remembers thermal history

The new method is quicker compared to the direct measurement, for estimating the true underground temperature. You often have to drill deep in order to get access to the underground resources, but during the drilling, temperature changes occur, and it can take up to six to seven years before the original ground temperature is re-established.

However, instead of waiting so long, researchers behind the new method use the mineral feldspar, which is the most common mineral in the two upper-most kilometres of the Earth’s surface;  this mineral stores information on the thermal history of the area.

Millennium after millennium, the feldspar will be exposed to background radiation which comes from the radioactive elements present in the underground such as uranium and thorium. The background radiation ‘knocks loose’ electrons inside the feldspar. The loose electrons are captured elsewhere inside the feldspar, typically at the sites of crystal defects, where they get trapped for geological time periods. Only light or heat can release these trapped electrons. Deep in the underground, the feldspar is not exposed to light, and it is thus only the heat from the surroundings, which is able to release the electrons inside the mineral.

The rate of release depends upon the ambient temperature. “The higher the temperature, the faster the release” explains Mayank Jain.

Luminescence reveals past temperatures

The researchers are able to leverage this knowledge in the laboratory. By exposing the feldspar to infra-red light, they force the mineral to release the ‘captured’, accumulated electrons. The amount of released electrons reveals how much heat the feldspar has been exposed to: If only a few electrons are released, the underground has been relatively warm. The larger the number of electrons released, the greater the accumulation of electrons over the years, which implies that the feldspar’s surroundings were relatively cool.

When the electrons are released inside the feldspar, surplus energy in the form of a light particle, a photon, is also released. It makes the feldspar emit a very weak light. This is a well-known phenomenon called luminescence. In the case of the feldspar, the light is so weak that it cannot be seen with the naked eye, but can only be captured by a highly sensitive apparatus—a so-called OSL (Optically Stimulated Luminescence) reader.

“Once we have measured the light signal from the feldspar with our OSL reader, we get experimental data that we use in our mathematical model. Our model takes several parameters into account, including how much radiation the feldspar has been exposed to. In the end, we have an estimate of the historical average temperature in the rocks or sediments from which feldspar was taken,” explains Mayank Jain.

During the development of the new method—also called OSL thermochronometry based on infrared stimulation—the researchers dedicated a significant effort to understand electron behaviour inside the feldspar, and to validate their method. The validation is carried out by examining rock samples from Germany's deepest borehole, KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland), which is almost 10 kilometres deep, and more than two decades old. Here, underground temperatures are well-documented, giving the research group a set of data, which they could compare with the results from their new method

Better predictions of future landscapes
Validation of the new method will continue in Switzerland, where, for example, the geologist Georgina King has spent the past six months comparing the data from the new method with well-known data from Himalayan rocks. As a geologist, she is thrilled at the prospect of using the method to gain more knowledge about the history of the underground through the past 100,000 years:

“The last ice age belongs to the Earth's recent history, where many landscapes have seen dramatic changes. If we can obtain more knowledge about what happens, for example with a rural area when glaciers melt and disappear, we can better predict the effect of the current global warming on our landscapes. Can we, for example, expect increased erosion of mountain areas when the glaciers are gone? In a mountainous country such as Switzerland where there are many villages in glacier valleys, it may have a direct impact on people, even though it will not happen for many years,” says Georgina King.