Geological mapping of borehole breakouts observed in acoustical and optical borehole imagery of deep drillholes of Olkiluoto, Finland

The Finnish spent nuclear fuel repository is currently under construction by Posiva Oy, and the repository will be located in Olkiluoto Island, western Finland. After intensive research of the site from surface drillholes, construction of underground rock characterisation facility named ONKALO began in 2004 in the same location for the purposes of detailed site characterization. Additionally, to ONKALO and research drillholes, 58 deep drillholes provide a large database for the geological, geochemical, geophysical and rock mechanics site characterisation. One of the key aspects of rock mechanics site characterisation is to determine the in situ stress field prevailing at Olkiluoto. Mathematically, the stress field is represented as a tensor with both direction and magnitude. The effect of excessive magnitudes of the tensor can be manifested by oriented fractures on the walls of the drillholes forming a phenomena of borehole breakouts along the drillhole length. The breakouts form when compressive stress, induced by the concentration of the in situ stress around the drillhole, exceeds the rock strength. The breakouts occur on the opposite sides of the walls. Due to the breakouts occurring in the areas of maximum compression around the drillhole walls, both the direction of the minimum (σh) and maximum (σH) horizontal in situ stress components can be inferred along the drillhole. This Master's thesis was set to identify and map the breakouts in the deep drillholes in Olkiluoto using the acoustic and optical imaging data of the deep drillholes. Acoustic and optical televiewers are two of the several geophysical measuring methods used in the drillholes. They produce images of the walls of the drillholes by wave reflection. During the study, different types of breakouts were identified along the drillholes in Olkiluoto: dotted, zipper-like and a mix of the two previous types with different intensities. The locations of the identified breakouts and other features (direction of σh, rock type, foliation) were recorded in a mapping form developed during this work. The mapping results show that in the total of 19 imaged drillholes, 40 breakout sections exist in only 5 drillholes. Geophysical data show that the breakouts are highly dependent on the geology and the majority of the breakouts occur in veined gneiss and in less competent rock types such as deformed or fractured rocks. Most of the breakouts were observed below 600 meters depth. Major group of directions of calculated σH from the mapped breakouts is oriented NE-SW (60-90°/245-270°) and secondary groups are oriented 107-150°/285-325° and 10-40°/190-220°. In some drillholes, the directions are rotated about 40° due to a significant brittle fault zone separating the site of Olkiluoto into stress field domains.