![]() Such low velocity range is typical of unconsolidated materials and, thus, cannot rationalize the geoelectrical attribution (ignimbrite). Tomographic analysis of the seismic arrival times revealed that below a depth of 45 m, (tunnel zone), the velocity substratum is marked by a range, (1200–1800 m/s). Following unexpected encounter during excavation, refraction seismic and core drilling were carried out for additional insights. ![]() Based on limited available geological information, the geoelectric horizon was attributed to weathered and variably saturated ignimbrite. The corresponding value on 2D model, (>350 Ω∙m), is also compatible. The 1D geoelectric model indicates that the formation resistivity at tunnel zone varies from 50 to 500 Ω∙m. Initially, geoelectrical mapping involving 12 Vertical Electrical Sounding (VES) and a short Electrical Resistivity Imaging (ERI) line, was carried out. As an exemplary case study, we present the geophysical survey conducted along the surface projection of a tunnel in the quaternary volcanic terrain of the Main Ethiopia Rift. The paper highlights the potential drawback of mapping a single geophysical property for subsurface characterization in potential engineering sites. ![]()
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