Introduction

At the future dam site (MIRTOFITO in northern Greece) it was estimated from the result of the preliminary geological survey and borehole data that the granite basement at the dam axis location had subsurface fractures and faults. These fractures had no evidence at the ground surface so the fracture zones had to be determined in order to plan the grouting of the dam foundation.

One Seismic Refraction - Shallow Reflection Survey, carried out from GEOSERVICE has localized with high accuracy the geometry of the fracture zones in the area. These results have been used as a guide to the engineers for the final dam axis construction.

Field works

Seismic lines have been carried out with a TERRALOC MARK II, 24 ch seismograph. Geophone spacing was 5 m and outshots were 15 and 30 m for all lines. Small 2m holes have been constructed with a COBRA hammer to be used as seismic sources for the profiles. All field data have been digitally recorded and used for the following interpretation.

Data interpretation

All data have been interpreted using the following concept:

Initial Model
Synthetic seismogram calculation with raytracing
Comparison with field records
Acceptable?
Yes. Then go to 7: Final Model
No: Then model improvement: go to 3
Final Model

Conclusion

Using modern seismic methods, dam construction engineers have been informed with accuracy about the location, depth and inclination of all the fracture zones in the granitic basement. Seismics have guided a few "direct sampling "holes that followed, at the axis location. All seismic results have been verified from the rock samples taken from the field.