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Synergie passiver seismischer Methoden zur Anwendung in der Geothermie (SynPaTh)

Antragstellerin Dr. Katrin Löer
Fachliche Zuordnung Physik des Erdkörpers
Förderung Förderung von 2016 bis 2021
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 298820846
 
Erstellungsjahr 2021

Zusammenfassung der Projektergebnisse

We used a so-called beamforming technique to analyse seismic noise, that is, the tiny but permanent vibrations of the ground, with the aim to infer information about the sources of ambient noise as well as the subsurface structure it as travelled through. By considering three components (3C) of ground motion, rather than just one as in conventional beamforming, we can identify the contributions of different wave types and analyse them in more detail. For example, from seismic noise data recorded at the Parkfield network located on top of the San Andreas Fault (SAF), we observed that so-called Love waves travel slower in the direction of the SAF (a phenomenon called “anisotropy”), which confirms assumptions previously made that beamforming can indeed be used to identify subsurface fractures and their orientations. We also found, however, that the geometry of the network has to be taken into account, as it can cause erroneous velocity measurements. We made a suggestion of how to do this using “Surface wave array response functions” (SARFs), that is, a function unique to each network that describes how well the true properties of a recorded surface wave can be recovered. We then applied the beamforming technique to an ambient noise data set recorded at the superhot geothermal system in Los Humeros, Mexico. For geothermal heat to be extracted from the ground, fractures in the rock are required that allow fluids to circulate and heat up before being pumped to the surface. Fractures only occur when the rock is brittle; in very hot areas, however, rocks tend to be less brittle and thus less fractured. Here, we estimate the depth to which brittle structures occur below the LHVC. The beamforming method was able to image deeper structures of the LHVC, which are typically not found by standard methods. Our results indicate that rocks in this area are mostly brittle, and thus potentially fractured, down to a depth of 10 km. This is much deeper than has been assumed before, considering the high subsurface temperatures in the area, and increases the rock volume potentially available for geothermal exploitation. Beamforming can also be used to estimate the composition of a recorded wavefield and while it is often assumed that ambient seismic noise consists predominantly of surface waves, our results indicate that body waves can make up a significant share, depending on frequency range and location. Understanding more about the components of ambient noise is crucial for techniques that try to suppress it in the data as well as those that attempt to use it to create images of the subsurface. To our knowledge, the significance of “apparent anisotropy” introduced in beamforming results by an uneven distribution source and/or station, had not received much attention before this study. It thus came as a surprise to us and lead us to dedicate more research to this topic than initially intended. - The presence of a shallow magma body (between 5 km and 10 km depth) at Los Humeros geothermal field, as assumed by other studies, was not confirmed by our results. Instead, indications for a deep brittle-ductile transition zone point towards a fractured structure down to 10 km depth. For an exhibition on geothermal energy exploration (“Vom Kohlebergbau zum Wärmebergbau”) in the German Mining Museum Bochum (Deutsches Bergbau-Museum Bochum) in collaboration with the International Geothermal Centre Bochum (GZB) Katrin Löer was interviewed about her research on seismic noise and how it can help to characterise geothermal fields.

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