Modelling and interpretation of VTEM data from Soppero, Sweden

Sammanfattning: The geological and geophysical knowledge about the northernmost part of Sweden has recently increased due to the Barents project, which includes acquisition of modern geophysical and geological information on behalf of the Swedish Geological Survey (SGU). During August 2013, a helicopter-borne versatile time domain electromagnetic (VTEM) survey was performed by Geotech Ltd, in the Soppero area northeast of Kiruna. From the VTEM measurements, a number of TEM anomalous zones have been identified and two of them are located south and southeast of the Lannavaara village. The main conductive features in the Lannavaara area can be explained by the presence of graphitic schist, which is spatially associated with a number of sulphide and iron oxide mineralisation occurrences. In this project, Maxwell thin sheet modelling and EM Flow conductivity-depth-imaging (CDI) software have been applied to selected anomalies in the Lannavaara area, for the purpose of extracting geometrical parameters of conductive features. This information has been used in order to confirm the structural framework of the area and evaluate the utility of VTEM measurements in this geological environment. In general, Maxwell thin sheet models of anomalies with small amplitudes show a better correlation with existing drill holes than models of anomalies with large amplitudes. The use of small amplitudes managed to confirm the structural model in the central part of the investigated area, which is an anticline. However, the use of different models and their distribution across the area is limited. Compared with Maxwell, CDIs from EM Flow provided a better way of confirming the general structural model in the area, although they include artefacts due to strong lateral gradients in conductivity. The Lannavaara area has also been investigated by VLF, Slingram and magnetic measurements and based on these data, multivariate analysis in SiroSOM reveals a strong correlation between VTEM and Slingram data, while VLF data appears to have much less or more complicated correlation with the other data sets. In summary, the results from the various software raise a question about the geological complexity in parts of the Lannavaara area, which may include multiple layers of graphitic schist, possibly expressed as smooth transitions in conductivity when represented by data from electromagnetic methods.