Velocity Analyses of the Reference Rock in Korea

The reference rock is the rock expected to show little variation of seismic properties at different locations, therefore, can be used as a basis for site response analysis. The P-wave velocity (Vp,ref) and S-wave velocity (Vs,ref) of the reference rock were analyzed using bore-hole velocity measurements in Korea. The velocity measurements had been obtained by the down-hole test, the cross-hole test, or the SP logging.

The minimum S-wave velocity of 2,000 m/s was applied to the Vs,ref analysis, while the minimum P-wave of 3,500 m/s to the Vp,ref analysis. The velocity gradient along depth was limited not to exceed 25 (m/s)/m in the reference rock. The Vs,ref was identified and estimated in 108 S-wave velocity profiles of total 345 profiles. The Vp,ref was identified and estimated in 94 P-wave velocity profiles of total 212 profiles.

Depth ranges and velocity estimates of the reference rock were found to have a clear correlation with weathering grades, but no correlation with rock types. The velocity measurements affected by weathering were excluded in the estimation of the reference velocities. The estimates of Vs,ref and Vp,ref shows no systemic differences according to sites as well as velocity-measurement methods. This enabled us to assume that the whole estimates of the reference velocities were sampled from the same populations.

The averages of Vs,ref and Vp,ref are 2,529 ms/s and 5,152 m/s, respectively. The distribution test revealed that the velocities measured in the depth range of the reference rock follow neither a normal distribution nor a log-normal distribution with a significance level of 5%. The distribution test also revealed that Vs,ref and Vp,ref follow neither a normal distribution nor a log-normal distribution with a significance level of 5%.