Evaluation of DC resistivity and time-domain IP tomography for bedrock characterisation at Önneslöv, Southern Sweden

Abstract: For both construction and exploration purposes, knowledge of the subsurface is important. Geophysical exploration methods can be used to acquire an overview of an area on which further investigations can be based. In this study direct current resistivity and induced polarization data, inverted with a Cole-Cole model in program AarhusInv, are used to characterize the bedrock at Önneslöv in Scania, southwestern Sweden. Önneslöv is situated at the Romele horst, which contain gneissic bedrock intruded by dykes. Resistivity ρ, with the unit Ωm, is a measurement of a materials' resistance to electric current. Induced Polarization, IP, which is measured as chargeability m with the unit mV/V, is the materials ability to charge up as a capacitor due to displacement of charges. Since both are physical properties of a material they can be used to investigate the material in the ground. The Cole-Cole model calculates the chargeability when the charging current was cut m0, as well as the frequency factor C and time constant τ. In contrast, commonly integral chargeability m is used, which uses a certain timeframe of the polarization decay. Magnetometry, percussion drillings, and borehole geophysics are also used to help refine and verify interpretations. The results show several NW trending linear elements in the ground. Many of them are interpreted as dykes or weakness zones. Weathering zones and weathered bedrock surfaces are also possible to identify, especially when adding normalised IP. The ground also contains numerous NE trending deformed and metamorphosed dykes that cannot be seen directly in the resistivity and IP data. However, these dykes most likely contribute to generally low resistivity values for bedrock in the area. C and τ follow the IP in terms of magnitude and are believed to be connected to textural differences in the ground. An additional finding is that the colour of the gneissic bedrock corresponds with changes in the gamma ray borehole logs, most likely due to the amount of potassium bearing minerals.