Evaluation of the effectiveness of backfilling and sealing at an open-pit mine using ground penetrating radar and geoelectrical surveys, Kimheden, northern Sweden

Abstract

At Kimheden, a small copper mine in northern Sweden, reclamation of the two open pits was investigated using ground penetrating radar and geoelectrical multiple-gradient array measurements. The pits had been backfilled with waste rock, with a dry cover being applied on top in 1996 in order to reduce the influx of oxygen to the sulphidic mine waste and the subsequent production of acid mine drainage. The dry cover consists of a sealing layer of clayey till and a protective layer of unsorted till. As geochemical sampling in the drainage from the pits had previously revealed the continued release of contaminating oxidation products, the purpose of the geophysical survey undertaken in 2010 was to identify deficiencies in the cover or other pathways for oxygen to reach the waste rock. The radar images did not reveal any damage in the sealing layer but risks of deterioration of the cover in the long term were identified with both the radar and geoelectrical data. The radar localised regions of thinner protective layer where the sealing layer could be exposed to frost action. The geoelectrical measurements indicated the existence of seepage through the dry cover that presented a risk of erosion of the sealing layer. 2-D inversion of geoelectrical data also imaged some pathways of groundwater around the main pit. The results from the geophysical investigations were used together with other site data in order to show that both deficiencies in the cover and superficial fractures in the pit walls may explain an ongoing influx of oxygen to the mine waste.