Helga Wiederhold

Glacial structures in northern Germany revealed by a high‐resolution reflection seismic survey

In recent years, the sediments filling Pleistocene glacial structures have become of increasing importance to paleoclimate research. Climatic changes are documented by the deposition cycles in small, closed, trough or bowl-like structures. A 2-D, high-resolution, shallow reflection seismic survey was conducted in 1996 over such a structure near Tostedt in northern Germany. The objective was to obtain a more accurate picture of the structure and the underlying geology. Designed especially for shallow surveys, a newly developed impulse source provided sufficient energy to observe reflectors as deep as 400 m. Signals are characterized by a high-frequency content with maximum energy between 80 and 180 Hz. The migrated section shows a distinct reflection pattern revealing local glacial dynamics. Reflectors at depths of 30, 40, and 45 m from inside the Tostedt structure correlate well with three interstadials of the Weichselian period. Weak reflections define the bottom of the structure, with a maximum depth of 70 m. The structure is imbedded in a much larger, previously unexpected depression of similar shape. The fill of this larger depression is seismically nearly transparent. High-amplitude reflections delineate its lower boundary with a maximum depth of 130 m. Two major reflectors at depths of 170 and 270 m correlate with lower Miocene and middle Oligocene units. They indicate that subrosion, which might have been expected from the presence of a nearby salt diapir, is absent. This confirms the purely glacial origin of the two bowl-like structures.

GPR investigations of an island aquifer as a contribution to groundwater modelling

The freshwater resources of the North Sea Islands are fed only by precipitation. In the subsurface of the islands, a freshwater lens floats on the saltwater that fills the pore space in this coastal region. The aim of the GPR investigation was to map the shape of the groundwater table as well as sedimentary structures as input parameters for hydrogeological simulation. In total, 20 km of constant offset (CO) radar profiles were measured with centre frequencies of 80 and 200 MHz. Wave velocities were determined by common midpoint (CMP) measurements and vertical radar profiling (VRP) in a monitoring well. The 80 MHz data show a clear reflection at the groundwater table whereas the reflection is blurry and shifted to lower frequencies for the 200 MHz data. This is caused by the gradual increase of water content above the capillary fringe. The GPR results are in good accordance to the observation wells in the area. In the centre of the island the groundwater table is found to be up to 3.5 m above sea level and it sinks down to sea level towards the coast line. Local depressions are observed in the region of the pumping stations of the local water supplier. A sharp horizontal reflection below the water table can be seen on most of the profiles and was identified as a silt loam layer by hand drillings. However, GPR data indicate some scattered gaps in this layer so that hydraulically it acts as an aquitard with leakage. GPR is useful to map the shape of the groundwater table on large areas of the island as addition to point information at observation wells. It gives valuable complementary information that enables an improved model for hydrogeological simulations. These simulations show that with predicted climate change until the year 2100, the salinity of the freshwater in the deeper aquifer near the pumping wells of the local water supplier will increase and thus a relocation of the pumping wells is recommended.

Calibration of Density Driven Flow Model for the Freshwater Lens beneath the North Sea Island Borkum by Geophysical Data

With the aim to be able to estimate climate change impacts on coastal aquifers and especially the situation of barrier islands in the Wadden Sea a numerical model is set up for the North Sea island of Borkum. The work is done in the frame of the EU Interreg project CLIWAT. The database includes information from drillings, seismic measurements, an airborne electromagnetic survey (HEM) and monitoring data from two vertical electrode chains as well as measurements of groundwater table, pumping and slug tests, water samples. The groundwater model has been set up by the finite-element programme FEFLOW. The density driven flow model can be calibrated successfully on the base of hydraulic, hydrological and geophysical data. Verification runs with the calibrated model show a good agreement for the measured and computed hydraulic heads and the measured with computed TDS-data. This is the case as well on a large scale for the whole freshwater lens as also on a small scale in the area of the well fields.