Using surface waves for estimating shear wave velocities in the shallow subsurface onshore and offshore Israel

摘要

Analysis of data from conventional (P-wave) seismic surveys conducted in various areas of Israel shows that in many cases these data can be successfully used to retrieve information on the shear wave velocity (V_s) in the shallow subsurface. The V_s estimation procedure is based on a dispersion analysis of the different kinds of surface waves contained in the seismic records. Examples from numerous land surveys demonstrate that dispersion curves of Rayleigh waves can be clearly detected in a wide range of frequencies varying from 5 Hz to 100 Hz. The estimated V_s values usually correspond reasonably well with the available results of S-wave refraction surveys. The V_s estimation procedure was also applied to data from shallow water marine surveys carried out in the Dead Sea, the Sea of Galilee and the Mediterranean Sea areas. The data were acquired by hydrophones located at the sea bottom, using either explosives or an airgun as the energy source. The corresponding seismic records contain a well defined low-velocity, low-frequency dispersive wave train identified with Scholte waves. The dispersion curves of the waves are located within a narrow frequency range of 3 - 17 Hz. The resulting V_s distribution below the sea bed correlates reasonably well with borehole and land refraction data from the vicinity of the marine surveys. Inversion of the dispersion curves from both land and marine surveys enables us to obtain estimates of the V_s down to depths of 40 -50 m.