3D Resistivity Interpretation of Helicopter-borne Frequency-domain EM (HEM) Data from Ramså Basin and Around in Andøya,

Summary The Ramsa Basin on Andoya, Northern Norway is the only known Mesozoic basin onshore Norway. The primary basement rocks in the area are granodiorite. The Mesozoic sedimentary strata of the Ramsa basin comprise sandstones and different clay stones of Cretaceous and Jurassic ages. Resistivity of the subsurface can be investigated by electric and electromagnetic (EM) geophysical methods to delineate the extension of the Ramsa basin. Borehole logging, electrical resistivity tomography (ERT) and helicopter-borne frequency-domain EM (HEM) surveys were performed in the area in the frame of different projects to gain a better understanding of the basin geology and structural settings. A quasi-3D inversion of HEM data shows a good agreement with borehole resistivity and ERT resistivity. HEM interpretation presents a 3D subsurface resistivity image of a larger area in comparison to what could have been drawn from few ERT lines and borehole logging. HEM interpretation together with other ERT and borehole resistivity indicates extension of Ramsa basin boundary beyond presently mapped boundary. Two new highly conductive areas are identified by HEM data interpretation and one of the areas is confirmed with presence of graphite and sulfide minerals by geological observations.