Investigation of a Mid-crustal Conductor in the North Gjallar Ridge, Offshore Norway, Inferred from Magnetotelluric Data

Abstract

In this case study, we present the results of the processing, analysis and 2D inversion of Magnetotelluric (MT) data acquired in the North Gjallar Ridge (NGR), in the outer Voring basin (Norway), in July 2014. Although the primary objective of the survey was to collect Controlled Source Electromagnetic (CSEM) data for hydrocarbon exploration purposes, a good quality MT signal was extracted alongside on 120 EM receivers. MT data in the NGR reveal an unusual drop in apparent resistivity values at long periods. Such behavior was already observed in the outer Voring basin and the Exmouth plateau (offshore Australia), both are volcanic passive margins. Indeed, 2D inversion of MT data shows a consistent recovery of a conductor at mid-crustal depth (8-12 km) along the axis of the ridge. Imaging seismically crustal structures in the NGR has long been challenging due to the presence of a dense network of volcanic sills and dykes. On the contrary, MT signal can diffuse beneath them and sense deep geo-electrical structures. We tentatively interpret the conductor, which correlates with a positive Bouguer anomaly, as related to the presence of deep non-intruded sediments.