Investigating the Depth and the Geometry of the Quarzitic Panafrican Basement Using Near-Surface 3D Seismic Refraction Tomography: Case Study of the Locality of Bakel (Senegal)

Seismic refraction investigations have been carried out in Bakel, Eastern Senegal. The purpose was to map geometrical relationship between the existing rock types and the Panafrican quarzitic basement, which is valuable information for the project of the Bakel fluviatile port construction. Four seismic refraction profiles were acquired. The obtained data have been processed by inversion. The obtained four seismic P-wave velocity profiles have been integrated to obtain a 3D model. By comparing the outcropping geological formations with the observed seismic data at the surface, it was possible to identify the lithology corresponding to each measured range of seismic velocity for the alluvium, the weathered bed rock, and the fresh rock. The results showed that the depth of the fresh rock of the basement varies from 0 to 18 meters above the sea level, with a deepening toward the Senegal River and toward the Northern part of the studied area. The presence of alluviums and their thickness are linked to the existence of bays and gulfs. The results of this study give valuable information for the river bed dredging cost assessment prior to the port construction phase.

Soil and Subsurface Sediment Microzonation Using with Seismic Refraction Tomography for Site Assessment (Case Study: IKIA Airport, Iran)

The site effects relating to the amplification of ground motion under earthquake loading are strongly influenced by both the subsurface soil condition and the geologic structure. In this study, for site characterization at the Imam Khomeini International Airport (IKIA) area in south of Tehran, in-situ seismic refraction tomography were carried out as a part of site investigations project, in addition geologic setting, borehole drilling, ground waters information and measurements. Based on seismic refraction studies, three layers are separable which with increasing in depth the S and P wave velocity is increased and this indicates increasing in compaction of soil and geologic materials. In the second and third separated layers, the zones with low and high seismic shear wave velocity is approximately equal, and northeast and southwest of the airport site has the low velocities, in addition to containing loose soils, highly weathered stones, and low depth to groundwater. In terms of Poisson’s ratio, the most important and key installations of airport site are located in suitable positions. According to Iranian Seismic Code, most of the lands around the airport are in class 2 and 3. It seems that a fault or a discontinuity is passed from northwest to the southeast of the study area. This site, according to geological, subsurface geophysical, and geotechnical boreholes studies, is high risk-earthquake prone.