It is well known that deep geothermal energy systems can cause earthquakes. These are usually mild earthquakes that can only be recorded using sensitive measurement equipment. The injection of cooled water appears to be a significant cause. By determining the location of the earthquakes, researchers are able to establish how the substrate responds to differences in pressure in connection with the injection. This allows the researchers to become more familiar with the deep subsoil.

Ben Laenen

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Seismometer network

The injection of water into the deep subsoil causes small movements that can cause earthquakes. Artificially induced earthquakes are a focal point in the development of every deep geothermal energy project and are an important research topic. Small movements occur along the cracks and crevices in the limestone, which then open up further as a result of the enforced flow and overpressures.For this reason, VITO has further developed a dense local seismometer network. The network allows the researchers to pinpoint the soil movements to within several tens of metres. By comparing the locations of consecutive earthquakes, they can calculate the direction in which the pressure effects of the injection continue. Hence, the researchers are able to monitor whether the earthquakes occur in proximity to major fracture systems. By monitoring the injection (pressure and temperature), VITO is aiming to guarantee the safe operation of the geothermal energy plant.

 

Traffic light system

We use a traffic light system in the geothermal power plant, which is based on four parameters:

  • the calculated magnitude of an earthquake;
  • the distance from the fault;
  • the number of movements per time unit;
  • the maximum measured peak ground velocity in the seismometers of the monitoring network

For each parameter, limit values are defined that demarcate a green, an orange and a red zone. If at least one of the parameters is in the orange zone, operational conditions at the geothermal power plant, such as flow and injection pressures, are gradually reduced. If at least one of the parameters is in a red zone, the power plant is temporarily shut down in a controlled manner.

The parameters can be monitored in the dashboard below. Please note that the limits shown below are valid from 19 April 2021 due to a change in the TLS.

For each earthquake, the local magnitude is given. This value is calculated using the seismometers of the monitoring network on and around the Balmatt site. Heavier quakes are also picked up by seismometers located further away. For those quakes, the Royal Observatory of Belgium (ROB) calculates the local magnitude on the Richter scale (ML). The magnitudes of the VITO monitoring network are systematically higher than the magnitudes of ROB. For the Traffic Light System or TLS, we recalculate the local magnitude to take into account the proximity of the seismometers. It is these recalculated or corrected values that are shown.

The position of the most important events (about half) is determined by means of a detail analysis. Only these events are shown on the map.

The time of the events is shown in local (Belgian) time.

Yellow blocks and green dots: seismometers, blue block: geothermal energy plant