Relating seismic velocities, thermal cracking and permeability in Mt. Etna and Iceland basalts

and columnar basalt from Seljadur (Iceland). Measurements were made in a servo-controlled steady-state-flow permeameter at
effective pressures from 5–80 MPa, during both increasing and decreasing pressure cycles. Selected samples were thermally stressed
at temperatures up to 900 1C to induce thermal crack damage. Acoustic emission output was recorded throughout each thermal
stressing experiment.
At low pressure (0–10 MPa), the P-wave velocity of the columnar Seljadur basalt was 5.4 km/s, while for the Etnean lava flow
basalt it was only 3.0–3.5 km/s. On increasing the pressure to 80 MPa, the velocity of Etnean basalt increased by 45%–60%, whereas
that of Seljadur basalt increased by less than 2%. Furthermore, the velocity of Seljadur basalt thermally stressed to 900 1C fell by
about 2.0 km/s, whereas the decrease for Etnean basalt was negligible. A similar pattern was observed in the permeability data.
Permeability of Etnean basalt fell from about 7.5 10 16m2 to about 1.5 10 16m2 over the pressure range 5–80 MPa, while that
for Seljadur basalt varied little from its initial low value of 9 10 21m2. Again, thermal stressing significantly increased the
permeability of Seljadur basalt, whilst having a negligible effect on the Etnean basalt. These results clearly indicate that the Etnean
basalt contains a much higher level of crack damage than the Seljadur basalt, and hence can explain the low velocities (3–4 km/s)
generally inferred from seismic tomography for the Mt. Etna volcanic edifice