In Brief
The BedrettoLab (Bedretto Underground Laboratory for Geosciences and Geoenergies) is a unique research infrastructure run by ETH Zurich making it possible to take a close look at the Earth’s interior. It is located in the Swiss Alps 1.5 kilometres below the surface and in the middle of a 5.2 kilometres long tunnel connecting the Ticino with the Furka railway tunnel.
Equipped with the latest technology, the BedrettoLab offers ideal conditions to conduct experimental research focusing on the behaviour of the deep underground when accessing and stimulating it. Such an access is required to advance scientific knowledge in various domains including geothermal energy and earthquake physics. It is also of relevance to develop novel techniques and sensors for these purposes.
VALTER stimulations to continue this week
After a break filled with analyses and preparations, the BedrettoLab team is conducting a new series of stimulations in continuation of the VALTER project and in preparation of the FEAR project. The first stimulation is starting this week with relatively small volumes of water being injected during a few hours.
The main aim of these stimulations is to investigate stress preconditioning. More specifically, the behavior of individual fractures by increasing the fluid pressure will be tested. These investigations are an important prerequisite for the upcoming FEAR experiment. Later stimulations, to be conducted in March and April, will focus on the increase of transmissivity of the host rock which is essential for the generation of a geothermal reservoir.
As for all activities in the BedrettoLab, safety is the top priority, and this also applies to the upcoming stimulations. We have deployed several safety layers to ensure that the seismicity does not reach crictical levels. Since we closely monitor the stimulations with our exceptionally dense and highly sensitive measuring system, we can identify in near real-time changes within the rock volume of interest. This enables continuously updated hazard re-evaluations. If predefined safety thresholds in magnitude or vibration are reached, the injection is halted and bleed off initiated. 28 February 2023
Sensor calibration in a salt mine
A group of scientists of the BedrettoLab team recently conducted experiments in the Merkers salt mine in Germany with the aim of calibrating a variety of seismic sensors. Calibration of these sensors is required to infer quantitative physical measurements from the recorded ground motions originating from seismic activity at the BedrettoLab. The sensors will later be installed in boreholes for the FEAR project aiming to measure small magnitude earthquakes, so-called “pico-seismicity”, that range between magnitudes of Mw -6 to -2. Special about these small magnitude earthquakes is that they are usually not felt by humans and that they radiate high-frequency energy (i.e., 1 – 100 kHz), a frequency bandwidth that was targeted with the recent experiments. The results gained are allowing us to calibrate the installed sensors and physically quantify pico-seismic earthquakes.
The saltmine offered an ideal setting for in-situ calibration of these sensors as the salt rock has a simple homogenous structure and low attenuation. In general sensor calibration requires an ideal setting meaning that the measurements of specific physical values should work without disturbances. In granite, like the one in the BedrettoLab, the velocity of the waves travelling through the rock volume is disturbed due to fractures and faults which makes calibration very difficult.
For the calibration experiments a variety of sensors were installed around a salt pillar measuring approximately 40 m by 40 m. Different artificial sources were deployed, most informative being a steel ball pendulum were used to initiate seismic waves in the pillar. The recorded seismic waves at a sensor were then compared to a reference laser sensor as well as models that predict the seismic wave field, ultimately leading to the calibration.
The salt mine experiment (SaMiEx) benefited from multiple partners, most notably Gesellschaft für Materialprüfung und Geophysik (GMuG) and the German Research Centre for Geosciences (GFZ) but also the Institute of Mine Seismology (IMS), Kistler Instruments AG and Elsys Instruments AG.
14 February 2023