Declining groundwater tables threatens several municipalities in Sweden which drinking water is collected from. To ensure a sound drinking water supply, the Geological Survey of Sweden has initiated a groundwater exploration plan. Airborne electromagnetic measurements have seen an uprise in hydrogeophysical projects and have a great potential to achieve high-quality models, especially when combined with drilling data. In 2018, the Geological Survey of Sweden conducted an airborne electromagnetic survey, using the SkyTEM system, in the outskirts of Örebro, Sweden. SkyTEM is a time-domain system and is the most favoured system in hydrogeophysical investigations and was developed especially with hydrogeophysical applications in mind. It is unique by being able to measure interleaved low and high moment current pulses which enables for both high resolution close to surface and increased depth of investigation. During 2019, further drilling in the area including both lithological, and geophysical logging including natural gamma and normal resistivity were carried out. High natural gamma radiation typically indicates content of clay in the rocks. The geology in the area is well explored since the 1940’s when oil was extracted from alum shale in Kvarntorp, located in the survey area. Rocks of sedimentary origin reaches approximately 80 m down until contact with bedrock. Well preserved layers of limestone, shale, alum shale and sandstone are common throughout the area. Combining SkyTEM data with borehole data increases the confidence and generates a model better reflecting the geology in the area. The AarhusInv inversion code was used to perform the modelling, developed by the HydroGeophysical Group (HGG) at Aarhus University, Denmark. Four different models along one single line were generated by using 3, 4, 6 and 30 layers for the reference model in the inversion. Horizontal constraints were applied to all models. Vertical constraints were only applied to the 30 layer model. The survey flight altitude is considered high and in combination with removal of data points being affected by noise, the maximum number of layers in the final model is limited to three. This suggests that the 3 layer model is the most representative model for this survey. The conductive shale seen in the geophysical log is visible in all models at a depth of roughly 40-60 m which is consistent with the geophysical log. No information is retrieved below the shale which concludes that the contact between the sandstone and crystalline rock is not resolved. The lack of information below a highly conductive structure is expected due to shielding effects. This study recommend to carefully assess the flight altitude at quality-control analysis during survey design.

The main focus of the project was the comparison of results from a new towed transientelectromagnetic (tTEM) data set with existing data including airborne transient EM(ATEM), radio magnetotellurics (RMT), cone penetration test with resistivity (CPT-R),geotechnical interpretations and geological observations in a quick clay landslide site atFråstad close to Lilla Edet in south-west Sweden. The tTEM data set was processedand inverted twice in the software Aarhus workbench using different inversion constraintsand settings. The resulting resistivity models where compared with previous geophysicalmodels based on both ATEM and RMT as well as geotechnical information in the form ofborehole logs and CPTR measurements. The results compared well with all other modelsand predicts resistivities in the range of 10−40Ωmin areas of interpreted to hold quickclay by geotechnical methods. As a ground geophysical method the tTEM method is fastand cost-effective, particularly in more open areas with little topographical variations. Inthe example presented in this study tTEM measurements are deemed an effective andaccurate tool to map areas of potential quick clay using the inverted resistivity models incombination with other geological and geotechnical data. / Huvudsyftet med detta projekt är en sammanställning och jämförelse mellan resistivitetsmod-eller från ett nytt markburet TEM data set (tTEM) och tidigare insamlade luftburnaTEM data (ATEM), RMT (radiomagnetotellurik) samt detaljerade resistivitetsmätningari borrhål (CPT-R). Mätområdet ligger i Fråstad vid Göta älv i Lilla Edets kommun isydvästra Sverige. Tidigare undersökningar har visat att området innehåller kvickleraoch där förekommer även skredärr från tidigare kvickleraskred. tTEM datan bearbe-tades,filtrerades och inverterades med hjälp av mjukvaran Aarhus workspace med tvåolika set av begränsningar och inställningar. De resulterande resistivitetsmodellerna jäm-fördes med tidigare geofysiska metoder i ATEM och RMT samt med geoteknisk infor-mation i formen av borrhålsloggar samt CPTR mätningar. Resultatet visar en mycketgod korrelation mellan resistivitetsmodellerna från de olika dataseten. De modelleraderesistiviteterna var 10−40Ωm för de områden som med geotekniska metoder identifieratssom kvickleraområden. Som en markbunden metod är tTEM snabb och kostnadseffektiv,särskilt vid användning i öppna ytor med liten topografisk variation. I exemplen somvisas i denna studie dras slutsatsen att tTEM är ett effektivt och noggrant verktyg föratt hitta områden som potentiellt kan hålla kvickleror. Där kan sedan de resulteranderesistivitetsmodellerna användas tillsammans med annan geoteknisk och geologisk dataför att effektivt kartlägga dessa kvicklersområden.