Geophysical investigations in sweden for the characterization of a site for radioactive waste disposal — An overview

Abstract

The geophysical measurements carried out for the investigations of prospective sites for radioactive waste disposal in Sweden are described. The investigations comprise detailed surface measurements with magnetic, electromagnetic, electric and seismic techniques. Measurements are made with a line spacing of 40 m and a station spacing of 20 m in an area of 4–6 km 2. At each site 7–10 deep boreholes are drilled. A number of geophysical logs are run in the boreholes, including several electric logs, providing data on temperature, salinity, and gamma ray. These geophysical data are evaluated together with the results from geological and hydrological investigations to make a three-dimensional geological and tectonical model of the site. This model defines the geometry used in the numerical modelling of the groundwater flow. In the case of thin, resistive overburden the surface investigations proved to be effective in the detection of dipping fracture zones. A line spacing as low as 40 m is needed to get adequate mapping of the fracture zones. The methods used cannot be considered efficient for mapping sub-horizontal fracture zones. The borehole logs provide data for the identification of fracture zones as well as different lithological units and give qualitative information of the hydraulic conditions. The temperature and salinity logs have provided the best hydraulic information, whereas, after testing, the induction log (borehole slingram), the differential resistance log and the VLF-log were found to be not effective for our investigations. The borehole techniques used, normally give information only about a relatively small volume of rocks around the boreholes. To obtain a detailed description of the bedrock between the boreholes, development of cross-hole geophysical, high-resolution techniques of considerable range, is needed.