Precious resources from the depths

As our cities con­tin­ue to grow, more and more resources are being used under­ground, for exam­ple in the con­struc­tion of sub­way and urban rail tun­nels, through geot­her­mal ener­gy or for drink­ing water sup­plies. Researchers at the Tech­ni­cal Uni­ver­si­ty have now cre­at­ed a three-dimen­sion­al under­ground mod­el of Munich. And they have made some sur­pris­ing dis­cov­er­ies, as Dr. Kai Zosseder from the Depart­ment of Hydro­ge­ol­o­gy explains.

Dr. Zosseder, why do we need to manage the subsurface?

Fore­casts pre­dict that by 2050, more than 70 per­cent of the world’s pop­u­la­tion will live in cities. At the same time, cities are grow­ing not only in area and height, but also in depth. For exam­ple, through the expan­sion of the sub­way net­work or the con­struc­tion of under­ground park­ing garages. The sub­soil also pro­vides impor­tant resources for the city, includ­ing ground­wa­ter, which is need­ed as ser­vice and drink­ing water, or geot­her­mal ener­gy as a renew­able ener­gy source. The sub­sur­face also offers stor­age poten­tial for heat, which is urgent­ly need­ed to make heat sup­ply more flex­i­ble. These many demands nat­u­ral­ly give rise to con­flicts, as var­i­ous uses are mutu­al­ly exclu­sive. On the oth­er hand, they could also com­ple­ment each oth­er. Sus­tain­able spa­tial plan­ning is there­fore nec­es­sary in order to make the best use of the potentials.

How can such sustainable spatial planning succeed?

In the “GeoPot” project, which is fund­ed by the Bavar­i­an Min­istry of the Envi­ron­ment, we have cre­at­ed a 3D sub­sur­face mod­el of Munich. This shows the poten­tial in detail. This gives the city a tool that is invalu­able for civ­il engi­neer­ing or ground­wa­ter plan­ning. Stadtwerke München is already using the mod­el to bet­ter plan its sub­way expan­sion. At the same time, the tun­nel walls can be used for geot­her­mal ener­gy at cer­tain points. But that’s only pos­si­ble if we know what the sub­sur­face looks like.

How exactly did you create the 3‑D underground model?

We col­lect­ed the data in a team of sev­en researchers over five years. To do this, we eval­u­at­ed around 20,000 bore­holes. These were already avail­able to the city of Munich, but had only been used in iso­lat­ed cas­es for spe­cif­ic projects. For these bore­holes, there are descrip­tions of the geol­o­gy, i.e. the soil prop­er­ties. We trans­late these descrip­tions into so-called grain dis­tri­b­u­tion curves. We can tell which rock types, for exam­ple clay, grav­el and sand, are present in that sec­tion of the bore­hole in per­cent­age terms. If there is a lot of grav­el, it means that there is also a lot of water, because of the high­er water per­me­abil­i­ty. If there is a high per­cent­age of sand, for exam­ple, the ther­mal con­duc­tiv­i­ty is high­er and we have bet­ter stor­age poten­tial. Uncer­tain­ties are of course includ­ed in the mod­els. You can cre­ate con­ser­v­a­tive mod­els for plan­ning to be com­plete­ly safe, or you can cre­ate some­what more opti­mistic models.

Did you make any surprising discoveries when evaluating the model?

The ques­tion of how the ground­wa­ter bod­ies are con­nect­ed is very rel­e­vant. For exam­ple, to be able to cal­cu­late how far a cer­tain pol­lu­tant spreads. Or even if, for exam­ple, a drink­ing water well is to be approved, it is impor­tant to know whether ground­wa­ter bod­ies are inter­re­lat­ed here. It has always been assumed that these areas of inter­ac­tion are only found in iso­lat­ed places. But in real­i­ty they occur much more fre­quent­ly, as we have now been able to show — and which has also sur­prised us to some extent.

Will the model be developed further?

New data that we obtain from bore­holes, for exam­ple in the course of the con­struc­tion of the new main line, are con­stant­ly being entered into the sys­tem to make the mod­el even more accu­rate. We are also cur­rent­ly work­ing on iden­ti­fy­ing stor­age poten­tial. Under­ground heat stor­age facil­i­ties are of great inter­est to ener­gy sup­pli­ers, and only an under­ground plan­ning tool can make effi­cient use of them. In addi­tion, we would like to use the mod­el to go even deep­er into where oth­er poten­tial uses are located.