The field measurements as part of the module "Hydrogeophysics" of the master's degree programme in Physics of the Earth and Atmosphere took place in Whitsun week in 2024. The multi-day excursion took the students and lecturers to the East Frisian island of Spiekeroog. The car-free island is located in the Lower Saxony Wadden Sea National Park and can be reached via a ferry connection from Neuharlingersiel.
The Hydrogeophysics module aims to familiarise students with the use of geophysical techniques to characterise hydrological structures and processes, and to apply this knowledge in the field exercise. This includes learning about the operation of geophysical instruments and the subsequent evaluation and interpretation of the measured data.
The available instrumentation was to be used to analyse flow and transport processes in the shallow subsurface. The exact question was related to the freshwater-saltwater interface of the island's groundwater and its tidal dynamics.
The methods used were geo-electric resistance tomography (ERT), ground penetrating radar (GPR) and electromagnetic induction (EMI). This year, the measurement area for the field exercise covered the north and north-west of the island.
In addition to the student-led measurements, our research assistant and doctoral student Kristoffer Kerkhof has further expanded our self-potential measurement network. The aim of these research activities is the minimally invasive monitoring of tidal dynamics in the subsurface.
Background of the measurements:
Fresh water is highly relevant for the environment and the population. On the island of Spiekeroog, the drinking water supply is ensured by a freshwater lens located below the island. This is created because fresh water has a lower density than salt water. The lens is fed by precipitation that seeps through the dunes on the island. With a maximum vertical thickness of 45 to 50 meters, the freshwater lens extends under the main dune arch in the north-western part of the island. A hydrodynamic equilibrium is established between the existing fresh and salt water. As many factors, such as vegetation, precipitation but also excessive extraction rates, can disturb this equilibrium, the freshwater lens under Spiekeroog is constantly monitored.
Interesting in this context are the flow patterns of precipitation that penetrates the ground and the influence of the tides on the currents in the subsurface.
