Digital Earth - Journey to the Center of the Earth

Understanding what goes on in the deep underground has applications in geothermal energy, finding groundwater reservoirs, deposition of CO2, and prediction of earthquakes.

Which child hasn’t at some point wondered what would happen if you were to dig a hole all the way to the opposite side of the Earth? The famous French novelist Jules Verne (1828-1905), by many considered the founder of the science fiction genre, took things a step further as he let his imagination send a crew on a “Journey to the Center of the Earth”.

Today, we know well that such a mission wouldn’t get far. We understand that below our planet’s relatively thin solid crust, hot streams of fluid stone and metal make direct human exploration suicidal. Still, we cannot stop speculating about the conditions in the deep underground. Not only are we born curious, but developments in the underground influence our life in many ways. Millions of people live in areas that are subject to earthquake hazards, and knowledge about where to find groundwater resources is key to sustain living conditions in many areas.

A relatively young field is exploitation of geothermal energy. Due to the elevated temperatures in the deep underground, there is plenty of energy to be harvested. Moreover, geothermal energy does not produce CO2 nor pollute. Identifying the best geological sites for geothermal energy is a discipline of growing interest. Another emerging field is identifying geological structures which are suited for CO2 storage. Given the urgency of the climate crisis, increasingly more governments opt for capturing CO2 from energy production and industrial plants and store it underground.

“How do we investigate these topics, when we cannot send neither humans nor robots into the deep underground? The answer is to go there digitally.”

Seismic waves travel the underground

How do we investigate all these topics, when we cannot send neither humans nor robots into the deep underground? The answer is to go there digitally. This is the scope of the Solid Earth and Computational Geoscience group at NBI.

Unlike Jules Verne, who could leave things to his rich imagination, the researchers need to be sure that their visualizations reflect the true conditions. And since, again, it is not possible to probe these conditions with on-location sensors, scientists have to make do with so-called seismic investigations. Investigators will send acoustic waves through the geological layers, or initiate controlled explosions which cause shock waves to travel the underground. The way these seismic waves are distributed will yield information about the geological formations.

For many years, the fundamental equations were not able to directly predict local phenomena – such as for instance earthquakes – in a highly complex system like the Earth. However, the advent of high-performance computing and new algorithms for efficient data analysis has provided a turning point. The digital representation of local Earth properties allows accurate simulation of complex wave forms in Earth’s interior.

Solving inverse problems

Further important advances have been made in a related field. Seismic information is indirect information. Often different geological structures could, in principle, cause the observed patterns. In science, this is known as the inverse problem: the Earth’s structure is calculated backwards from the observed seismograms.

The group at NBI has contributed to solving inverse problem challenges in recent years, culminating in 2021 with the development of the first computational method for high-speed calculation of Earth models with more than 1,000,000 unknown parameters, and at the same time quantifying the uncertainties of the solutions.

The method will enable us to generate high-resolution images of the deep Earth, while also contributing to a range of important practical applications: Mapping of sub-surface reservoirs for geothermal energy, discovery of groundwater reservoirs, and search for geological layers suitable for deposition of CO2.