Natural stone assessment with ground penetrating radar (GPR) The 15th Meeting of the Association of European Geological Societies 16 20 September 2007 Tallinn, Estonia Hannu Luodes 18.9.2007 1
Location of the study areas 2
The ground penetrating radar instrumentation GSSI SIR 2000 control unit Antennae between 40 MHz and 400 MHz 200 MHz antenna Control unit 3
Natural stone assessment with ground penetrating radar (GPR) Study area 1 Porphyritic granite situated in the south-eastern Finland It belongs to the Central Finland granitoid complex and is described postkinematic in relation to the deformation of the area 4
Study area 1 5
Study area 1 Typical feature of the rock is a strong horizontal exfoliation type fracturing (height of the bench about 5 m) 6
Study area 1 GPR measurement traverse of 65 m in length (vertical and horizontal scale are equal) The profile was corrected with altitude data to present the actual surface and fractures orientation 7
Study area 1 The GPR was able to detect the major horizontal and sub-horizontal fracturing 8
Study area 1 Vertical or near vertical fractures were difficult to detect due to their small reflection surface towards the measuring antenna 9
Study area 1 The horizontal fractures near the surface were sometimes strongly weathered providing space for surface water to collect Those spots were detected as individual targets on the radar measurement profile The spots can be thought to be reflections from a subhorizontal fracture zone near the surface 10
Natural stone assessment with ground penetrating radar (GPR) Study area 2 Soapstone situated in North Karelia It belongs to an Archaean greenstone belt, in the border zone between the Archaean area and the Palaeoproterozoic North Karelian Schist Belt 11
Study area 2 Typical North Karelian soapstone Sample size 260 mm x 200 mm 12
Study area 2 Soapstone quarry in North Karelia 13
Study area 2 With the soapstone, the measurements revealed more of the internal structure of the rock, making the detection of the fracturing complex Especially internal lineation was quite well visible in those sections where the stone contained more conductive minerals. 14
Study area 2 Large open horizontal fractures were detectable also in this stone type, which can be helpful in planning of the quarrying operations. 15
Conclusions Ground penetrating radar (GPR) is suitable for large-scale quality assessment of natural stone deposits and quarries The measurements are easy and quick to do and the preliminary results can be evaluated immediately The fracture detection level varies depending on the used equipment, mainly on the frequency The development in GPS technology has made it possible to position the measurements accurately and thus trace back the results into the quarry to help extraction planning The differences in the measurement results and their interpretation found among these two rock types indicate the importance of knowing the electromagnetic properties and mineralogy of the stone Also knowing the geological structure and other characteristics of the stone beforehand helps to avoid false interpretations 16