Mapping and characterisation of Swedish bedrock by DC resistivity and transient-field electromagnetic measurement

Abstract: DC electrical and electromagnetic methods have been used for a long time to detect and characterise fracture zones in site investigations and other engineering projects. However, the methods have not been used very much in basic geological mapping of crystalline rock on a regional to semi-regional scale. One reason for this is that measurements with traditional instrument set-ups used, for example, in ore and water prospecting are quite time- consuming. Three alternative methods for DC resistivity measurements are presented in this thesis. Vertical soundings can be performed efficiently with a modification of the traditional Schlumberger-array and large areas can be covered quickly with gradient array measurements. Resistivity anisotropy can be estimated with the new so called x-configuration. Nine electrodes, arranged in such a way that they form the letter x, are connected to an instrument in the centre. The apparent resistivity and apparent anisotropy can be found by least-squares fitting using different combinations of current and potential electrodes. Resistivity measurements using the new procedures have been performed in a number of areas. Systematic differences in resistivity were observed between different rock types indicating different intensities of fracturing. Marine volcanics for example have lower resistivity than terrestrial volcanics in the Arvidsjaur area. This means that it is possible to use resistivity measurements to characterise rock types and to detect boundaries between rocks of different character. The anisotropy was found to coincide with the foliation or bedding direction of the rock. Deep electrical soundings were performed in four areas in northern Sweden. The depth of investigation was several kilometres. The electric resistivity of the near-surface rocks was determined and their depth extent was estimated in three of the areas. A large lateral anomaly probably caused by marine volcanics was found in the fourth area. Transient-field electromagnetic (TEM) sounding is a method that has been used in mapping of sedimentary basins, geothermal exploration etc. The fundamental theory of TEM soundings and the processing of TEM sounding data are described in the thesis followed by examples of TEM soundings on crystalline bedrock. It is necessary to use large transmitter moments when measurements are made on highly resistive rock due to the weak secondary field. Bulk resistivity estimates made with DC measurements and TEM soundings were consistent in the Revsund granite area. TEM soundings offer a possibility to make bulk resistivity determinations to large depths in crystalline bedrock without the need for large transmitter-receiver separations. Fracture density and pore-water salinity are two important factors in the choice of a suitable location for a repository for highly active nuclear waste. A case history is presented where DC resistivity measurements were used in combination with TEM soundings in order to gain knowledge about the above mentioned factors. Two different types of granite were investigated and the resistivity of the younger Götemar granite was found to be around five times higher than the resistivity of the Småland granite, indicating a considerably lower fracture density in the former. The depth to saline groundwater was determined primarily with the TEM soundings and good agreement was found with data from deep drillholes.