Using DCIP and TEM to investigate the geology of the Alnarp Valley in southwestern Scania, Sweden

Abstract

In southwestern Scania there is a paleovalley, the Alnarp Valley, which holds a confined aquifer called Alnarpsströmmen. Its geology consists mainly of sand and gravel and it is generally confined by one or more layers of clayey till above, and the limestone bedrock below. In collaboration with Region Skåne and SGU, this thesis aims to gather information about Alnarpsströmmen using two geophysical methods Direct-Currect and Induced Polarization (DCIP) and Transient Electromagnetic Method (TEM), both relying on measuring the ground’s resistivity to electrical current and using those measurements together with information gathered from nearby boreholes to make geological interpretations. Before resistivity data could be interpreted, it had to go through inversion, where a mathematical model is generated that outputs data that are as similar as possible to the ones gathered in the field.
It was decided to focus mainly on the area around Grevietäkten, a groundwater supply area that today provides about a quarter of Malmö’s drinking water. Some sites were chosen outside of this area to offer comparison. Sites were selected by examining data gathered via helicopter borne TEM measurements, or SkyTEM, as well as data gathered from boreholes, and they were selected to be interesting from a geological standpoint or to fill a gap in the datasets.
Fieldwork was carried out for three weeks. For two weeks starting on the 19th of April 2021 and concluding on the 30th of April 2021 DCIP measurements were conducted. Further fieldwork was delayed, and TEM fieldwork began on the 15th of November 2021 and concluded on the 19th. In total, 11 different profiles were investigated.
Inversion of DCIP data was done in the software Res2DINV, and TEM data was inverted in SPIA. Resistivity profiles including DCIP, TEM, and SkyTEM data, as well as nearby boreholes, were created by importing data into GeoScene3D. While interpreting, some general observations were made. The clayey till often appears as a low resistivity layer compared to the high resistivity layer that denotes the aquifer sediments. The bedrock is not reached via DCIP in most cases, but TEM and SkyTEM show that it generally has a very low resistivity and decreases with depth, indicating heavy fracturing and/or high ionic content. After interpretation, a table with the most commonly occurring materials and their resistivity values was created and is presented below.
Material, interpeted Resistivity (Ohm∙m)
Alnarp sediments 40-100
Clay 10-40
Gravel 300-400
Limestone 10-100
Sand 100
Till 30-80

Popular Abstract

Investigating the geology of the Alnarp Valley – with electricity

Do you know where your water comes from? If you live in southwestern Scania, in Malmö for example, there’s a decent chance the water in your tap comes from the massive underground Alnarp Valley, and the aquifer it hosts. This thesis set out to delve deeper into the geology of this valley, especially around the main water extraction area, Grevietäkten.

The methods of choice for this thesis were TEM and DCIP, two methods that both rely on sending electricity into the ground and measuring the grounds resistivity. Resistivity is a measure of how strongly a material opposes the flow of electric current. The higher the resistivity, the harder it is for electricity to flow. Since every material responds differently, this can be used to get an idea of what lies beneath our feet.
One interesting finding was that the limestone bedrock near Grevietäkten showed remarkably similar resistivity values to the profiles near the coast, at Abbekås. In coastal areas, saltwater will intrude into the groundwater. Being rich in ions, the saltwater is a great conductor of electricity, which reduces the resistivity of the material it is present in. We suggested that the low resistivty near Grevie could be a result of ionic content as well, since groundwater extraction has been known to draw up deeper groundwater, containing more ions, closer to the surface.
Being an important regional water source, the geology of the Alnarp Valley has already been extensively studied, but there are still gaps is many areas, some of which this study set out to close. And while the valley is important today, it may well be even more important in the future in case of drought. Most of the valley is covered by, to water, impermeable clay, but it is also important to study where it is not, and to prevent pollution in these areas from infiltrating into the groundwater.
Another stated goal of the study was to try and get an idea of the resistivity values of the materials that exist in the aquifer. This is presented in the form of

• author: Berthelsen Gustafsson, Simon and Larsson, Holger
• supervisor: Matteo Rossi; Alfredo Mendoza
• organization: Civil Engineering (M.Sc.Eng.); Engineering Geology
• alternative title: Användning av DCIP och TEM för att undersöka Alnarpsdalens geologi i sydvästra Skåne, Sverige
• course: VTGM05 20232
• year: 2024
• type: H3 - Professional qualifications (4 Years - )
• subject: Technology and Engineering
• keywords: DCIP, TEM, SkyTEM, geophysics, Alnarpsdalen, Alnarp Valley, hydrogeology, Alnarpsströmmen, Alnarp stream
• other publication id: LUTVDG(TVTG-5182)/1-87/(2024)
• language: English
• additional info: Examiner: Gerhard Barmen
• id: 9173544
• date added to LUP: 2024-09-09 11:22:33
• date last changed: 2024-09-09 11:22:33