This post is part of the GeoConnect³d blog.

Managing the subsurface in a sustainable way is a challenge, which will not become easier with the discovery and implementation of innovative technologies in the future (see also this earlier Geoconnect³d blog post). One particular problem associated with the subsurface is that exploring it, e.g. for geothermal energy, and thus knowing what to take into account for its exploitation, requires quite some investments, not only financially but also in the expertise aspect. Therefore, optimizing the cost-efficiency and reliability of subsurface-exploration remains a major challenge, yet essential to be tackled. Another policy need in the topic of subsurface management that requires more dedicated research is that of predicting and managing hazards, e.g. in the context of CO₂-storage, both natural and subsurface activity-induced. The concept of geomanifestations provides a creative though unconventional way to approach this conundrum. Within the Geoconnect³d project, geomanifestations were defined as anomalies. Not just any anomaly, but those that specifically express ongoing or past geological processes (see also this blog post). One of the goals of Geoconnect³d is to inventory geomanifestations, and, moreover, to show stakeholders, especially policy makers, how we can learn from them for future development and management of our subsurface.

Let us illustrate that with an example. Probably, most people are familiar with the well-known thermal springs in and to the south of the city of Aachen (Germany; Figs. 1 & 2). Two series of springs showing temperatures between 40 and 70 °C are aligned along two NE-SW-oriented Variscan thrust faults. A study of Herch (2000) indicated that both thermal and chemical anomalies increase towards a NNW-SSE-oriented normal fault, the Laurensberger Fault, related to the Roer Valley Graben, part of the Rhine Graben rift system. It is proposed that the springs originate from the crucial intersection of these two fault systems and consequent mixing of waters: topography-driven meteoric water circulates along permeable layers in the Variscan structures, while warmer, mineralized water rises up along deep-seated graben-related faults.

Going some 15 km to the NW, we arrive in Heerlen (the Netherlands), part of the South Limburg coal mining district (Figs. 1 & 3). Mining activities in one particular zone of the Oranje-Nassau-I mine were disturbed several times in the past by unforeseen inflows of hot brine at about 250 m depth, where the stone-drifts and galleries crossed a satellite fault from the Benzenrader Fault. The water showed surprisingly anomalous temperatures up to 50 °C, chloride contents up to 25000 mg/l and gas contents of CO₂, H₂S and He, indicating a juvenile origin (Van Rooijen Adviezen, 1998). After the mines were abandoned and flooded, the influence of this geomanifestation unfortunately was erased. But where did this hot water actually originate from? It is probably not a coincidence that the mine gallery crosses the NW-SE-oriented Benzenrader fault system. Likely, hot brine migrates upward relatively fast along this fault, which lines up with the Laurensberger Fault, responsible for the anomalies in the Aachen springs.

Now, why would we (VPO, Flemish Government, Belgium) as policy makers be interested in geomanifestations observed in Germany and the Netherlands? This interest departs from the fact that if we move another ~20 km NW-wards, we arrive in Flanders in the Campine Basin (Fig. 1). The Rhine Graben rift system crosscuts the eastern part of the Campine basin and even continues more northwards, again in the Netherlands. A similar fluid circulation might be active today below our feet, unnoticed due to dilution by other water sources. This possibility certainly deserves our attention in future follow-up research, as it could guide prospection for geothermal energy. A story to be continued thus, but with firm roots in the Geoconnect³d geomanifestations inventory.

Helga Ferket and Johanna Van Daele
Vlaams Planbureau voor Omgeving (VPO)

References

Herch, A. (2000) The thermal springs of Aachen/Germany – what Charlemagne didn’t know. Environmental Geology. 39. 5. 437 – 447 https://doi.org/10.1007/s002540050450.
Van Rooijen Adviezen (1998) Het mijnwater, stand van zaken voorjaar 1998. Samenstelling: Van Rooijen Adviezen, Staatstoezicht op de Mijnen, Provincie Limburg. Maastricht.

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