The interleaf esker structure and associated hydrogeological features of Virttaankangas in southwestern Finland are derived from a combination of high-resolution reflected seismic and refractive tomography
A new high-resolution (2-4 m source and receiver spacing) reflection and refractive seismic survey was carried out to characterize the aquifer and confirm the existing sedimentary model of the Virttaankangas interleaf esker, part of the Säkylänharju-Virttaankangas glacier esker - a chain complex in southwestern Finland. The interleaf esker complex, which hosts the Aquifer Recharge (MAR) plant, is the entire source of water for the city of Turku and its surrounding cities. Accurate delineation of aquifers is therefore essential for long-term MAR planning and the sustainable use of Esker resources. In addition, another goal is to solve the little-known stratigraphic problem, namely 70-100 meters thick glacial sediments that cover fractured bedrock areas. Bedrock surfaces and fault zones were confirmed by combining reflected seismic and refractive tomography results and further validated based on existing borehole information.
High-resolution seismic data proved successful in accurately depicting the Esker core and revealing complex formations from fan blades to kettle holes, providing valuable information for potential new pumped wells. This study illustrates the potential of geophysical methods in rapid and cost-effective Esker studies, in particular the digital-based Landstreamer and its combination with geophone-based wireless recorders, where the cover sediment is quite thick. High-resolution seismic data proved successful in accurately depicting the Esker core and revealing complex formations from fan blades to kettle holes, providing valuable information for potential new pumped wells. This study illustrates the potential of geophysical methods in rapid and cost-effective Esker studies, in particular the digital-based Landstreamer and its combination with geophone-based wireless recorders, where the cover sediment is quite thick. High-resolution seismic data proved successful in accurately depicting the Esker core and revealing complex formations from fan blades to kettle holes, providing valuable information for potential new pumped wells. This study illustrates the potential of geophysical methods in rapid and cost-effective Esker studies, in particular the digital-based Landstreamer and its combination with geophone-based wireless recorders, where the cover sediment is quite thick.
Eskers, defined as layered sediments of gravel and sand deposited by glacial meltwater, are hydrogeological settings of great significance in regions that have experienced glaciation, such as Finland, Sweden, the British Isles, the United States and Canada. They are extraordinary aquifers and are often also used as building aggregates, so an accurate understanding and description of the large structures and hydrogeological units in these precious underground reservoirs is essential for their environmental and economic use; Eskers owns marked groundwater aquifers and aquifer recharge for managing groundwater production.
Eskers are also associated with tunnel passages (or tunnel valleys), which are erosive manifestations of subglacial water flow channels that typically truncate the subglacial bed shape and/or until. Some particularly large eskers, such as the Säkylänharju-Virttaankangas complex in southwestern Finland, have been attributed to temporal sea intrusion sedimentation within the interleaf joint between two different interleaf ice streams The interleaf esker complex has a wide range and is characterized by glacial sediments up to 100 meters thick consisting of large sedimentary units with extensive internal structures. Therefore, these complexes require deep infiltration survey methods that can establish connections between bedrock surface topography and large-scale esker elements in order to reliably model hydrogeological units and associated groundwater flows.
An effective way to recharge aquifers for environmental or economic benefits, such as providing a freshwater supply to an entire community, MAR applications benefit from a growing number of multidisciplinary studies combining geophysical methods and hydrogeology. Near-surface geophysical methods are commonly used for engineering, geotechnical, environmental, or hydrogeological applications, but over the past 30 years, high-resolution reflected seismic methods have also been used in near-surface surveys as the primary choice for oil and gas exploration; Several case studies prove their effectiveness. Like other geophysical methods, seismic methods (refraction and reflection) are non-invasive and provide an effective tool to characterize shallow (< 150 m) but heterogeneous, geologically complex targets such as underground reservoirs, aquifers, and structures that carry or control their location. There is a gradual shift to obtaining high-resolution seismic data, especially in loose or normally consolidated sediments, by employing fast and cost-effective methods such as seismic land tractors.
The continuous development of acquisition systems and sensor types has led to the use of Landstreamer in many near-surface surveys, including groundwater and glacial geomorphological studies, proving their time-efficient acquisition with high data quality. By definition, a landstreamer is a set of seismic sensors that can be towed behind a vehicle without the need to install sensors. While most ground tractors use geophones (simulation systems) for data acquisition, the newly developed advanced MEM (microelectromechanical) based broadband seismic ground tractor was used in this study to delineate the main inter-leaf esker system and the MAR aquifer in Virttaankangas in southwestern Finland.
