Abstract

Seismic compressional, P- and shear, S-wave measurements were done on the unconsolidated top-soil with depth of 1 m at the different locations of the study area. In case of working with metallic P-wave source, the effect of the air-filled zone and disturbance of the air-wave are received by geophones located near the source (short offset). It is possible to reduce the effect of the disturbance of air-wave and to obtain correct P- and S-wave velocities by using source equipment consisting plastic hammer and wooden cone for very short offsets. In this study, a 12-channel seismograph with signal stacking ability was used together with high frequency (100 Hz) geophones on the top-soil. The geophone intervals were set to 20 cm at the all locations. Two dimensional models were constructed by applying two dimensional inversions to the first arrival time of direct and refracted waves. It was seen that compressional wave velocity (Vp) values of very-near-surface soils are lower than the velocity of the P-wave in air (330 m/s). Additionally, it should be noted that the ratio between P and S wave velocity is about 1.5 in the dry unconsolidated top-soils.

Key words: near surface seismic, top-soil, shear and compressional waves, air-velocity, Vp/Vs ratio.