Improving Groundwater Resource Mapping in Complex Geological Regions Using Constrained Inversion of Seismic and ERT Data

Groundwater resources are under increasing pressure from climate change and growing demands, making the application of advanced exploration methods crucial, especially in areas like northern Germany, where complex subsurface geology leads to significant challenges. This study integrates seismic reflection methods and Electrical Resistivity Tomography (ERT), employing constrained inversion with PyGIMLi, where seismic interfaces guide ERT to enhance subsurface imaging.

The study area is located in Hude, Lower Saxony, Germany, within the Oldenburg-East Frisia Water Board (OOWV) region. The subsurface geology predominantly consists of Plio-Pleistocene unconsolidated sediments. The Miocene sequence, characterized by widespread clayey and silty deposits, forms the aquifer base. During the Pliocene, the Baltic River System deposited a substantial succession of sands over the Miocene strata, followed by at least two Quaternary glaciations. These glaciations introduced glaciofluvial sediments, including thick sands and gravels interspersed with impermeable till layers and clay-rich units. Glacial processes also created channel structures infilled with cohesive sediments, such as the Lauenburger Clay, resulting in significant heterogeneity in hydraulic conductivity and petrophysical properties.

The Elsterian meltwater sand aquifer, located beneath the Lauenburger Clay, is a key groundwater-bearing unit with a significant thickness of approximately 60 meters. The variability in the thickness and distribution of the Lauenburger Clay, which acts as a confining layer, has been interpolated through geological modeling and further investigated with geophysical measurements. This complex geological framework, characterized by alternating permeable and impermeable layers and significant variability, emphasizes the value of non-invasive geophysical methods in reducing reliance on exploratory boreholes while effectively identifying aquifers and improving understanding of subsurface conditions.

We acquired two intersecting seismic profiles (N-S and W-E), using both P-wave and S-wave reflection techniques. The N-S P-wave profile (1,560 m) runs parallel to the Quaternary channel and covers two newly drilled exploratory boreholes, while the W-E profile (840 m) crosses the channel. Corresponding S-wave profiles (1,440 m N-S and 842 m W-E) provided higher-resolution images, particularly in shallower regions where P-wave data alone may lack sufficient detail. Vertical seismic profiling (VSP) was performed in the boreholes to refine velocity models. Complementary ERT profiles of 1,430 m (N-S) and 950 m (W-E) were acquired along the seismic lines using Wenner and dipole-dipole configurations.

The results highlight the advantages of combining seismic and ERT methods through constrained inversion, leading to enhanced imaging of the subsurface. This approach, guided by seismic interfaces, enables precise identification of geological formations, closely matching borehole lithological data. These findings demonstrate the potential of integrating seismic and ERT methods to optimize groundwater exploration, enabling more precise identification of aquifers and reducing the number of required exploratory wells, particularly in areas with complex geological conditions.