Revealing the geological materials properties by a shallow seismic method for investigating slope site effects: a case study of Qiaozhuang town, Qingchuan County, China

Abstract

This article presents a description of the lithology and weathered features of the subsurface under Qiaozhuang Town through a 2-D interpretation based on a shallow seismic reflection geophysical survey. To obtain the overburden, phyllite, and limestone geological parameters for the dynamic effect assessment of the slope site, twelve shallow seismic profiles were collected in the target area at slope sites where intensive ground damage was caused by the Wenchuan earthquake. To expose the geological features of rock masses with different lithologies, six shots field recordings for each profile were performed. The results showed reflection signals of a seismic wave that exhibited four to five weathered layers within a shallow depth range. The interpretation of velocity analysis of the geological imaging shows that the elastic velocities of the reflection layers increase with depth. The overburden, the first layer of slope surface, had velocities in the range of 724 m/s to 1024 m/s with the thickness changing in a large domain from 5 to 25 m, which was partially revealed in an exploration shaft and in a borehole. In deeper bedrock, the limestone was characterized by a mean velocity of 1470 m/s to 3760 m/s and the phyllite was characterized by a mean velocity of 1440 m/s to 3580 m/s. Moreover, the geological imaging of partial profiles showed that a weak and discontinuous reflection seismic wave may be interpreted as a fractured zone. The interpretation results reveal that the elastic velocity changing with lithology material is clear presence, leading to the impedance contrast varies from the slope inside to the surface. In particular, the presence of lower velocity layer, such as the overburden, which big material impedance contrast (> 1.0) likely formed strong slope site effects. Therefore, the geological material property of the study area was probably another factor causing the slope failure during the Wenchuan earthquake.