Use of Airborne TEM, Ground RMT and borehole data to answer hydrogeological questions: A case study in Gotland, Sweden

Airborne Time Domain Electromagnetic (ATEM) measurements are conducted over four areas in the Island of Gotland in Sweden to identify and study underground water resources such as paleo-valleys and fracture zones in bedrock. Inversion of ATEM data contains new information about potential underground reservoirs. ATEM models are compared with more detailed ground RMT models and borehole data. The RMT and ATEM resistivity models demonstrate a considerable correlation in resolving layered models with lateral and vertical variations that reflect changes in geology and physical properties such as porosity and salinity. Borehole data collected close to the RMT profiles were used to calibrate the models against geology. The formation resistivity from borehole measurements was used as link between the borehole measurements and modeled ATEM and RMT data. Interpretation of ATEM models in areas 1 to 3 shows that towards the southeast the resistive limestone becomes thicker. Low resistivity zones, probably due to fractures within the limestone, are identified and can be counted as new potential underground water resources. Using a 40 m threshold, the saltwater surface found to be deeper towards the east in areas 1 to 3 and shallower in area 4.