Empirical relation for estimating shear wave velocity from compressional wave velocity of rocks

In situ shear wave velocity is one of the most important parameters for evaluating dynamic elastic moduli and needs special energy sources for generation and receivers for detection. Identification of shear wave phase on a seismic record needs skill as it is not the first wave to arrive and is usually embedded in surface waves. To estimate shear wave velocity for site characterization and for predicting the in situ quality of rock, empirical relations were proposed by Carroll (1969) to predict shear wave velocities from compressional wave velocities of rock samples. His empirical relation was used on the actual field data and the estimated shear wave velocities were compared with in situ velocities measured using cross-hole seismic technique. The shear wave velocities measured at four nuclear power plant sites were used to generate the correlation between compressional and shear wave velocities. A power fit model with a regression coefficient of 0.84 was developed. This correlation is valid for rocks having compressional wave velocity ranging from 4000 m/sec to 6000 m/sec and Poisson's ratio between 0.22 and 0.28. The proposed relation is based on 185 data pairs of in situ compressional and shear wave velocity measurements conducted at four nuclear power project sites. The sites with different host rocks are: Kota (Rajasthan), Kaiga (Karnataka), Tarapur (Maharashtra) and Kakrapar (Gujarat). The empirical relationship proposed in this paper is first of its kind in India. The proposed empirical relation can be used to predict shear wave velocities at similar rock sites anywhere in India.