The objective of this research is to investigate the added value and robustness of InSAR data for monitoring subsidence due to solution mining in Twente (Haaksbergen/Enschede/Hengelo area). Main uncertainties to research are the InSAR and subsidence data reliability and accuracy, the way to use the data to calibrate salt cavern motion models.
Salt solution mining in the Haaksbergen/Enschede/Hengelo has continued for more than 60 years. The salt layer being exploited is relatively shallow (400-500 m). Various mining strategies have been deployed, always resulting in some subsidence, and one sinkhole incident. Currently applied mining methods are designed to also minimize subsidence risks. A regular subsidence monitoring strategy is in place using geodetic surveying every 1 to 5 years. There are 61 caverns in Twente which are or might become unstable (Zoutwinning in Twente | Zoutwinning | Staatstoezicht op de Mijnen (sodm.nl)). To monitor the caverns the micro seismic network was installed in 2016 as well as obligatory periodic sonar measurements of the caverns. Additionally, a lot of the potential unstable caverns are being filled with slurry to prevent instability. In 2022 an investigation revealed that one cavern had become unstable and has migrated to the surface (underneath the Twence terrain). Independent InSAR measurements of subsidence rates would possibly give more insight into which caverns are unstable and need filling. InSAR is a relatively new satellite-based measurement method that is capable of monitoring ground motions in the area at much shorter time intervals, allowing for early detection of irregular subsidence behavior. On the other hand, the measurement locations are not easily defined, and quality control of interpreted subsidence data is not fully robust. The method has potential, but so far has not been applied to this area, particularly focusing on salt mine subsidence and stability.
Specific research questions to ansered are:
1. What are the quality control procedures of the InSAR data and what accuracy and precision can be expected? Research question
2. What are the worldwide experiences with salt cavern monitoring using InSAR? Research question
3. Which locations over the last 10 years show subsidence rates which correspond to possible unstable salt caverns? Research question
4. What is the coupling between (known) cavern deformation and subsidence data? Which caverns may be unstable?
Based on answers to these research questions, recommendations should be given on how InSAR data can be of added value to subsidence monitoring (including early warning for instability of the caverns) and model calibration.
The research started in 2025 and is carried out by Sensar and TNO.