We report Q-estimates from borehole-to-tunnel seismic experiments
at Nagra's ''Grimsel Test Site'' (GTS). The probed granitic rocks
are cut by brittle faults but do not show any signs of pervasive
weathering. Q-values estimated from P-wave first arrivals by the
spectral ratio and rise time methods range from about 20 to 60 with
a median value of 35 and a standard error of 20% or less. The surprisingly
low Q-values inferred indicate that even outside the weathering zone,
seismic attenuation in some parts of the uppermost crystalline crust
may be very high.