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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