Workreport 2011-90

Geological Signatures of Drillhole Radar Reflectors in ONKALO

Döse, C. & Gustafsson, J.

English

44

The geological signatures of radar reflectors in ONKALO have been evaluated as a sub-activity within the Joint Work Programme “Rock Suitability Criteria – strategies and methodology” between Svensk Kärnbränslehantering AB and Posiva Oy. In addition to the geological signature, the usage of geophysical data to predict large fractures was evaluated.

Pilot hole radar loggings were carried out using a RAMAC GPR-250 MHz dipole antenna. The radar data were evaluated and reflectors with known position and intersection angle to the pilot hole were correlated with fractures or foliation in the pilot hole and with Tunnel Crosscutting Fractures in the tunnel. This data served as in-data for the evaluation of the geological signatures of radar reflectors.

The result of the evaluation is not univocal. Half of the reflectors could be explained by fractures in the pilot hole, but only about 10 % of the reflectors can be explained by Tunnel Crosscutting Fractures. Of these 10 %, 2/3 can also be explained by foliation, leaving only some 3 % of the total reflectors more unambiguously correlated with Tunnel Crosscutting Fractures.

The fractures correlated with radar reflectors do not diverge much from other fractures. Fractures having intersection angles of 30°-60° are more likely to be detected by radar relative to other. Other properties that seem to be overrepresented in fractures correlated with radar reflectors are quartz and/or graphite content, width =0.8mm and higher alteration (Ja=3), but the data is not unambiguous.

Radar reflector, TCF, FPI, fracture, pilot hole, drill hole, correlation.

Geological Signatures of Drillhole Radar Reflectors in ONKALO (pdf) (933.2 KB)