P-S converted wave Survey in Quaternary Plain with Severe Statics Problem

Shear (S-) wave velocity structure is one of the key elements for the estimation of strong ground motion generated by earthquakes. Reflection Survey is one of the best technologies to image the underground structure and the velocity. In Japan, the most populated areas are in Quaternary plains, especially in alluvial plains. In these alluvial plains, the S-wave velocity of the surface strata is very slow and varies quickly place to place, especially near rivers or ponds. Their thickness also varies too. These things make the statics of the detector side statics (S-wave statics) in the P-S converted wave processing very complicated sometimes. We conducted two 9-component surveys. In one survey, S-wave statics are larger than ten times of those of P-wave. This survey shows a good example of rapid and large change of the S-wave statics. P-wave statics and S-wave statics have some similarity, but they differ in spatial trend. Because of the rapid change of the S-wave statics, it was difficult follow the S-wave first break at first. By re-picking of the first break after rough statics application, the refraction statics result gives a much better image of P-S converted wave. This example shows the importance of the detector side statics corrections.