Faults are common geological structures distributed at various depths within the Earth with different behaviors: from seismic to aseismic. The frictional stability of faults is linked to the properties of asperities that make the contact between fault surfaces. Investigating the interaction between asperities and their link with the frictional stability of faults aims at a better understanding of the intrinsic relationships between the observations of earthquake swarms and the slow local aseismic transient. Here we propose an experimental approach, which allows a customized interface sliding slowly under a well-controlled normal load, to study this problem. This interface consists of asperities modeled by poly-methyl-methacrylate (PMMA) balls in a softer, polymer base representing the parts of the fault that are easily deformed, facing a transparent flat PMMA plate. We employ a high-resolution camera for in-situ optical monitoring of the local deformation of the interface while loaded. We also attach acoustic sensors to capture the dynamics events attesting to local dynamic ruptures. We connect our observations with a mechanical model derived from a high-precision topography of the customized interface. We investigate the effects of various internal parameters of natural fault systems, including the size and density of asperities, their rigidity or the contrast of rigidity compared to the base, on the evolution of the frictional stability under variable normal load and of the behavior of the population of asperities at the transition between seismic and aseismic slip. Our results, bring new observations on the mechanics of swarm and fault transient.

At the Äspö hard rock underground laboratory in Sweden, six in situ hydraulic fracturing experiments took place at 410 m depth. A multistage hydraulic fracturing approach is tested with a low environmental impact, e.g., induced seismicity. The idea is to mitigate induced seismicity and preserve the permeability enhancement process under safe conditions. The fractures are initiated by two different injection systems (conventional and progressive). An extensive sensor array is installed at level 410 m, including simultaneous measurements of acoustic emissions, electric self-potential, and electromagnetic radiation sensors. The monitoring catalog includes more than 4300 acoustic emission events with estimated magnitudes from the continuous monitoring setup (in-situ sensors between 1-100 kHz). The experiment borehole F1 is drilled in the direction of Shmin, perpendicular to the expected fracture plane. Two electromagnetic radiation sensors are installed and aligned to (i) Shmin and (ii) the expected fracture plane with a sampling rate of 1 Hz and a frequency range between 35-50 kHz. The self-potential sensors are installed at level 410 with a distance of 50-75 m from the borehole F1, including nine measuring probes and one base probe. A second self-potential setup is deployed at level 280 m in the far-field with a distance of 150-200 m from F1. The self-potential data were measured with a sampling rate of 1 Hz. For the first time (to our knowledge), the electric and electromagnetic monitoring results of two hydraulic stimulation at mine-scale are presented. The results are discussed, including the different injection types (one conventional and one progressive experiment) and the acoustic emission events. The self-potential results reveal increases in amplitude during both hydraulic fracturing experiments at both depth levels. A second increase in the self-potential was observed only during the conventional injection and only at level 280 m. This is consistent with the results of the acoustic emission catalog, which show a larger number and larger magnitude of events during conventional injection experiments. The changes in the electromagnetic field are predominantly in the direction of Shmin during both the conventional and the progressive injection experiments.

In Central Europe, the largest geothermal potential resides in the crystalline basement rock with important hotspots in tectonically stressed areas. To better harvest this energy form under sustainable, predictable and efficient conditions, new focused, scientific driven strategies are needed. Similar to other geo-technologies, the complex processes in the subsurface need to be investigated in large-scale facilities to ensure environmental sustainability.

The proposed new underground research laboratory GeoLaB (Geothermal Laboratory in the Crystalline Basement) will address the fundamental challenges of reservoir technology and borehole safety. The specific objectives of GeoLaB are 1) to perform controlled high flow rate experiments, CHFE, in fractured rock, 2) to integrate multi-disciplinary research to solve key questions related to flow regime under high flow rates, or higher efficiency in reservoir engineering, 3) risk mitigation by developing and calibrating smart stimulation technologies without creating seismic hazard, and 4) to develop save and efficient borehole installations using innovative monitoring concepts. Planned experiments will significantly contribute to our understanding of processes associated with increased flow rates in crystalline rock. The application and development of cutting-edge tools for monitoring and analyzing will yield fundamental findings, which are of major importance for safe and ecologically-sustainable usage of geothermal energy and further subsurface resources. As an interdisciplinary and international research platform, GeoLaB will cooperate with the German Research Foundation (DFG), universities, industrial partners, and professional organizations to foster synergies and technological and scientific innovations.

GeoLaB is designed as a generic underground research laboratory in the crystalline rock adjacent to the Rhine Graben, one of the most prominent geothermal hotspots in Germany. GeoLaB is an analogue site representative of the world‘s most widespread geothermal reservoir rock, the crystalline basement. In an initial phase, the suitability of a site for GeoLaB located either in the Black Forest or the Odenwald, will be proven by geological, geophysical, and geochemical drilling exploration. At the selected site, a two km long gallery will be excavated, tapping individual caverns, from which controlled, high flow rate experiments will be conducted. The experiments will be continuously monitored from multiple wells, drilled from the underground laboratory or from the surface. This will create a unique 4D-benchmark dataset of thermal, hydraulic, chemical and mechanical parameters. A virtual reality concept accompanies the development of the complex infrastructure concept from the very beginning, supporting the infrastructure set-up and the scientific experiments in planning, documentation and analysis.

GeoLaB will become a cornerstone for the target-oriented development of the enormous geothermal resource. With its worldwide unique geothermal laboratory setting, GeoLaB allows for cutting-edge research, associating fundamental to applied research for reservoir technology and borehole safety, bridging laboratory to field scale experiments and connecting renewable energy research to social perception. GeoLaB comprises a novel approach that will shape research in earth science for the next generations of students and scientists.