Scientific research is carried out in the framework of the INSIDE project (supported by the German Federal Ministry for Economic Affairs and Energy, BMWi) to assess the impact of deep geothermal exploitation on induced seismicity in the Munich area (Germany, Molasse Basin). The project involves the research institute Karlsruhe Institute of Technology as well as two geothermal operators, Stadtwerke München (SWM) and Innovative Energie für Pullach (IEP). The research work focuses on three aspects: the monitoring, the modelling and the integration with operations.

With respect to the monitoring, the deployment of a measurement network going beyond the standard for seismological and geodetic observations is considered. Therefore, an extensive and plural monitoring network was designed to monitor high (seismicity) and low (subsidence, uplift) frequency deformation processes of the subsurface. Several types of technologies as well as several types of deployment configurations are involved. Their relative performances are intended to be compared in order to contribute to the development of suitable strategies for deformation monitoring and their data processing.

After presenting the aim and purpose of the project, we concentrate on the status of the seismic measurement network being implemented around the three geothermal sites of Baierbrunn, Pullach and Schäftlarnstrasse. In addition to “standard” monitoring stations installed in the area, we report on the deployment of various innovative technical solutions, among which a seismic mini-array and a monitoring borehole dedicated to Distributed Acoustic Sensing (DAS). We show how these stations complement the existing network in Munich and present their main characteristics, in particular the associated noise measurements. We additionally discuss the data-management system being developed to handle all these new data.

Geothermal energy represents around 30% of the produced electricity in Iceland with a cumulative capacity being equal to 755 MWe (Ragnarsson et al., 2020). In particular, the Theistareykir geothermal plant, which is located on the Mid-Atlantic ridge in North Iceland, produces 90 MWe using two turbines in operation since autumn 2017 and spring 2018, respectively. We will report on the hybrid micro-gravity monitoring and discuss how this technique will contribute to the sustainable management of this renewable energy. Indeed, the gravity method highlights the mass redistribution and, consequently, helps to quantify the recharge/discharge of the geothermal reservoir.

On one hand we show the results of the repetition of the Theistareykir micro-gravity network of 27 stations measured in summer 2017, 2018 and 2019 i.e. before and after the beginning of the geothermal production, with a Scintrex CG5 gravimeter.

On the other hand, we will also show the continuous gravity changes recorded from fall 2017 to summer 2020 at 3 permanent stations with iGrav superconducting gravimeters calibrated with a FG5 ballistic absolute gravimeter.

The combination of these different types of gravimeters defining the hybrid micro-gravity method is then used to investigate the measured gravity changes in relation to geothermal activity parameters like injection and extraction rates.

After correcting the gravity measurements for the effect of the vertical displacements deduced by continuous GNSS measurements at the permanent stations and InSAR analysis by the University of Iceland, we compare the gravity changes due to mass redistribution to what is expected from the injection/extraction rates.

We finally focus on the question of the sustainability of the Theistareykir power plant since the start of exploitation and discuss the discharge/recharge of the geothermal reservoir.

Ragnarsson, Á., Steingrímsson, B. and Thorhallsson, S. Geothermal development in Iceland 2015-2019. Proceedings World Geothermal Congress 2020, Reykjavik, Iceland (2020).

Geothermal district heating networks are among the key options to decarbonize the heating sector in the State of Geneva in Switzerland. But the development of geothermal district heating requires high capital costs and involves risk of not finding sufficient geothermal resources, which make these systems less competitive. On the other hand, building geothermal district heating creates a wider impact on the economy, domestically and overall. But such impact has rarely been evaluated.

Our study aims to analyze the competitiveness of geothermal district heating networks and their wider economic impacts using two competitiveness indicators (levelized costs of geothermal district heating and of district heating system as a whole) and two economic impact indicators (economic impact multipliers and share of domestic economic impacts in Switzerland). We construct a decision tree to generate 9’096 decision paths to develop shallow and medium geothermal district heating in the State of Geneva comprising 10 decision parameters: target of heat demand to be supplied (100 GWh/year and 400 GWh/year), number of districts (1,2,3 and 4 districts), share of geothermal coverage in the district heating system (10%, 40%, 70% and 100%), choice of auxiliary heating source, district heating generation (second, third and fourth), linear heat density (2, 4, 6, and 8 MWH/m•year), geothermal well depths (800 m, 1600 m, 2500 m), geothermal flowrates (20 l/s, 50 l/s, 80 l/s), and 3D seismic exploration program (with or without). We quantify the four indicators for each decision path in a decision tree, including applying probability trees to account for geothermal resource risk through assigning probabilities of success. We then identify the most influential decision parameters using a random forest regression and pinpoint the decision paths that lead to low levelized costs of heat and high economic impact multipliers and share of domestic economic impact. Finally, to analyze the synergies among the four indicators, we identify the common key decision parameters and the decision paths leading to synergies between having low levelized costs and high economic impacts.

The results demonstrate significant variation in the values of four indicators of levelized costs and economic impact, depending on the combination of the 10 aforementioned decision parameters. The influence of geothermal coverage is observed in all four indicators, although more strongly in the variation of levelized cost indicators. For the variation of the economic impact indicators, the choice of auxiliary heating source has a stronger influence than geothermal coverage. We identify that synergy could be achieved in scenarios having 40% geothermal heat and 60% heat from centralized waste incineration, deployed together in a district with a linear heat density of 6 or 8 MWh/(m.year), using second district heating generation.

Our study shows the importance of integrating a combination of many decision parameters to understand the competitiveness and economic impacts of geothermal district heating. Focusing on geothermal coverage, linear heat density, district heating generation, and choice of auxiliary heating sources makes the biggest difference when setting up economically meaningful strategies.

The modification of the stress field induced by fluid injection into the ground can generate seismic motions. Their monitoring is a key point to limit the occurrence of impacting events. Generally, this is performed using seismic surface networks, which can be limited by a significant ambient noise level especially in urban contexts. An alternative consists in the installation of stations in the depth of wells to increase the distance with surface ambient noise sources. However, few are the industrial projects fitted with such technologies, because of their cost and complexity of installation. Another possibility is to operate with dense seismic networks (seismic arrays), combined with appropriate data processing, to limit the impact of anthropogenic noise by distinguishing it from earthquakes. Here we investigate the case of the “Strasbourg induced earthquake sequence”, occurring since mid-2018 around the Geoven deep geothermal doublet operated by the Fonroche company in Vendenheim (France). So far, the BCSF-Rénass (national observatory service in charge of the french seismicity monitoring) has recorded 567 induced earthquakes using traditional local and regional seismic networks. Their catalogue has an estimated magnitude of completeness of Mc=0.6 at best, containing event with a local magnitude (Mlv) up to 3.9, including 22 with Mlv>2 and 4 with Mlv>3. These events are organized into two distinct swarms: a first cluster in the vicinity of the Geoven wells and a second one 4-5km South from it. Although the project has been forced to stop because of the felt induced seismicity, the Northern cluster is still very active, with the largest event occurring the 26^th of June 2021. To improve our knowledge of this seismic crisis, we deployed 3 mini seismic arrays of 21 SmartSolo nodes each around the active cluster, recording at a sampling rate of 1000Hz for 4 months starting a few days after the Mlv=3.6 event of 4^th of December 2020. The aperture of each array is around 70m, allowing good wave number resolution in the frequency range of interest for local seismic events. Beamforming and match field processing techniques allow us to characterize the local ambient noise, which consists mostly in surface waves with slow apparent velocities. As the arrays are located roughly on the top of induced seismic events hypocenters, the front waves illuminate the arrays with a significantly higher apparent velocity. Therefore, stacking brutally the waveforms increases drastically the SNR. We improve it even more by considering the signal instantaneous phase as a coherency parameter during the stacking process, what is called phase-weighted stacking. This allows us to detect events down to magnitude -0.5, which leaves us with 4 to 5 times more events than the BCSF-Rénass catalogue. In parallel, we also investigate how much these arrays can improve event location as a complement to traditional networks.