Refraction seismic complementing electrical method in subsurface characterization for tunneling in soft pyroclastic, (a case study)

The paper highlights the potential drawback of mapping a single geophysical property for subsurface characterization in potential engineering sites. 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. Initially, geoelectrical mapping involving 12 Vertical Electrical Sounding (VES) and a short Electrical Resistivity Imaging (ERI) line, was carried out. The 1D geoelectric model indicates that the formation resistivity at tunnel zone varies from 50 to 500 Ω∙m. The corresponding value on 2D model, (>350 Ω∙m), is also compatible. Based on limited available geological information, the geoelectric horizon was attributed to weathered and variably saturated ignimbrite. Following unexpected encounter during excavation, refraction seismic and core drilling were carried out for additional insights. 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). Such low velocity range is typical of unconsolidated materials and, thus, cannot rationalize the geoelectrical attribution (ignimbrite). In a joint interpretation, the likely formation that may justify the observed range of the electrical resistivity and low P-wave velocity appears to be unwelded pyroclastic deposit (volcanic ash). Eventually, core samples from the tunnel zone confirmed the presence of thick ash flow. However, the unexpected ground conditions encountered at the early phase, due to insufficient information derived from a single geophysical parameter, caused extra cost and considerable delay.