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In this paper the distribution of P-wave velocity in regionally metamorphosed rocks of the Zayandeh-rud dam has
been investigated. The data collection has been carried out by a 24 channel digital device. Data analysis shows that
P-wave velocity in metamorphic rocks increases with depth continuously following the linear equation v(z) = 3200+6.5 z . The P-wave velocity in accordance with metamorphic grades from slate to schist changes from 3200 m/s at surface up to 5475 m/s at 350 m depth.
High velocity gradient in metamorphic rocks of the study area is a consequence of changes in metamorphic grades from surface to the investigated depth and the effects of surface weathering. The calculation of porosity based on Wyllie (1965) equation indicated that the porosity has changed from 5.5% at the surface to less than 1% at the 350 m depth continuously, which is in agreement with litho logical changes from slate to schist.