The storage cavern field at Epe has been brined out of a salt deposit belonging to the lower Rhine salt flat, which extends under the surface of the North German lowlands and part of the Netherlands. Cavern convergence and operational pressure changes cause surface displacements that have been studied for this work with the help of SAR interferometry (InSAR) using distributed and persistent scatterers. Vertical and East-West movements have been determined based on Sentinel-1 data from ascending and descending orbit. Simple geophysical modeling is used to support InSAR processing and helps to interpret the observations. In particular, an approach is presented that allows to relate the deposit pressures with the observed surface displacements. Seasonal movements occurring over a fen situated over the western part of the storage site further complicate the analysis. Findings are validated with ground truth from levelling and groundwater level measurements.

Since the end of 2022, two ground motion services that cover the complete area of Germany are available as web services: the German Ground Motion Service (Bodenbewegungsdienst Deutschland, BBD) provided by the Federal Institute for Geosciences and Natural Resources (BGR), and the first release of the European Ground Motion Service (EGMS) as part of the Copernicus Land Monitoring Service. Both services are based on InSAR displacement estimations generated from Sentinel‑1 data. It would seem relevant to compare the products of the two services against one another, assess the data coverage they provide, and investigate how well they perform compared to other geodetic techniques. For a study commissioned by the surveying authority of the state of Baden-Württemberg (Landesamt für Geoinformation und Landentwicklung Baden-Württemberg, LGL), BBD and EGMS data from different locations in Baden-Württemberg, Saarland, and North Rhine-Westphalia (NRW) were investigated and validated against levelling and GNSS data. We found that both services provide good data quality. BBD shows slightly better calibration precision than EGMS. The coverage provided by EGMS is better than that of BBD on motorways, federal roads, and train tracks of the Deutsche Bahn. As an example, where both services have difficulties in determining the correct displacements, as they cannot be described well by the displacement models used for processing, we present the test case of the cavern field at Epe (NRW). Finally, we discuss the implications of our findings for the use of the products of BBD and EGMS for monitoring tasks.

Time series of interferometric SAR (InSAR) images offer the potential to detect and monitor surface deformation with high spatial resolution, even for slow deformation processes. However, many different sources contribute to phase changes which are used in InSAR to estimate displacements. Complex displacement mechanisms or strong atmospheric contributions can complicate the separation of these contributions and even cause problems when unwrapping the phase. A preliminary model of expected displacements can support this process but requires information about all involved deformation processes. However, as these processes are often the main subject of the investigation, they are not sufficiently understood in advance. In this contribution, we approach this issue by analyzing InSAR time series results of regions with complex deformation behavior with the established statistical methods of principal and independent component analysis to identify dominant displacement patterns. We study Sentinel-1 InSAR data from 2015 to 2022 above the storage cavern field Epe in North Rhine Westphalia, Germany. Epe displays a spatially and temporally complex surface deformation field, which was described in previous studies as consisting of a linear signal relating to the cavern convergence as well as of seasonal and cavern pressure-dependent contributions. Our resulting displacement components can be clearly separated and appointed to different sources. This is supported by ground truth data and supplemental measurements of cavern pressure levels and groundwater levels. We also find that the previously described linear parametrization of displacements related to cavern convergence is no longer sufficient for longer time series. Our results show that we can obtain source-dependent displacement models from long and complex InSAR time series when using ICA. These can then either be used to refine time series processing or to describe the physical processes causing to the surface displacements with a geophysical source model. Both will be the subject of future investigations.

The cavern field at Epe has been brined out of a salt deposit belonging to the lower Rhine salt flat, which extends under the surface of the North German lowlands and part of the Netherlands, and is used to store e.g. natural gas, brine and petroleum. Cavern convergence and operational pressure changes cause surface displacements that have been studied for this work with the help of SAR interferometry (InSAR) using distributed and persistent scatterers. Vertical and East-West movements have been determined based on Sentinel-1 data from ascending and descending orbit. Simple geophysical modeling is used to support InSAR processing and helps to interpret the observations. In particular, an approach is presented that allows to relate the deposit pressures with the observed surface displacements. Seasonal movements occurring over a fen situated over the western part of the storage site further complicate the analysis. Findings are validated with ground truth from levelling and groundwater level measurements.For porous storage sites the geomechanic response can be described as elastic: displacement is almost proportional to reservoir pressure and displays the same pronounced seasonal behavior. At Epe the visco-elastic response of the salt layer has to be considered. The general appearance of the surface displacement is that of a strongly smoothed and shifted version of the cavern pressure curve. To deal with this situation a temporal model for displacement with pressure changes (pressure response) is derived that relates cavern pressure with observed displacement based on the theory for visco-elastic behavior of a Kelvin-Voigt body.In order to deal successfully with the challenging displacement field at Epe several algorithmic improvements were implemented. To obtain a more complete picture of the displacement field DS pre-processing has been combined with StaMPS. Furthermore, StaMPS was modified in order to support unwrapping with a phase model composed of linear trend, pressure response and a seasonal component (caused by ground water level changes). Finally, refining the iterative estimation scheme of StaMPS helped avoiding leakage of the displacement signal to the spatially correlated noise term.Determining vertical and east-west displacements from InSAR line-of-sight displacements is fundamental for interpretation and integration with levelling data. In this study, a basic method of orbit combination and another one supported by a simplistic geophysical model were applied in order to obtain 2D-displacements. For the basic method the north-south component was handled as if it were zero, while the geophysical model predicts the LOS effect of NS displacements. It assumes that caverns act as spherical pressure/volume sources embedded in an elastic half space (“Mogi” sources). To incorporate the visco-elastic component, each cavern is encompassed by a spherical salt shell that obeys the Kelvin-Voigt differential equations. The model is used here to describe either the parameters of the linear component of the displacement model or of the pressure response. A novelty of the orbit combinations implemented for this study is that the different components of the phase model are combined separately. This allows for a better understanding of the phenomena that contribute to the displacement field.

Raw material depletion, uneconomic production, and government policies will result in the abandonment of hundreds to thousands of underground mines worldwide within the next years. Hence, mine drainage becomes operationally and economically dispensable. Consequently, the mine flooding is initiated by the mine operator. This increases the subsurface pore fluid pressure potentially inducing seismicity and ground displacements. Here, we present a synthesis on published and unpublished research on three abandoned hard coal mining districts in Germany (Ruhr, Saar, Ibbenbüren), which are currently undergoing flooding. The interdisciplinary approach (3D fault modelling and stress analysis, seismological, ground displacement, and outgassing monitoring) suggests that flooding correlates with the temporal occurrence of post-mining microseismicity (mainly M _L < 2) and ground displacements (centimeters-decimeters). Flooding-induced microseismicity in the Ruhr area is focused in pillar areas below the mine panels and is not correlated with the activation of large-scale fault systems. Anomalous outgassing into soil is a temporally variable process in the Ruhr area. Ground displacements from InSAR measurements show complex spatio-temporal patterns that correspond to developing mine water levels in different parts of the mines. The ground displacements analyzed vary during different phases of the mine life cycle: 1. Strong subsidence during production; 2. Residual subsidence after end of production; 3. Acceleration of subsidence with beginning rebound of mine water; 4. Reverse to uplift after the mine water reaches a certain impoundment depth; 5. Waning uplift after the final mine water level has been reached.