Integrated geophysical methods to constrain subsurface structures of Tulu Moye-Bora-Berecha axial volcanic complex, main Ethiopia rift: Implications for geothermal resources.

The Main Ethiopian Rift (MER) is a well-known continental rift whose axial sector is characterized by the occurrence of regularly spaced silicic caldera complexes and central stratovolcanoes, interspersed with large fields of fissural basalts, small mafic scoria cones and numerous young normal faults and fissures. The Tulu Moye-Bora-Berecha volcanic complex is found in the central portion of the MER and includes the Tulu Moye geothermal prospect area. A combination of gravity and magnetic methods was used to better constrain the subsurface volcanic stratigraphy and tectonic structures. Regional and residual anomaly maps were produced from the gravity data and the magnetic data were corrected to produce anomaly and enhanced maps. A complete Bouguer anomaly contour map was produced after the necessary reduction was applied to the gravity data. Due to the geomagnetic field's dipolar nature and since the study area is in the equatorial region (<15°), the reduced-to-equator (RTE) technique was used to minimize external effects and correct the data as if the body had been laid at the magnetic equator. The anomalous source's depth was calculated using a Euler depth solution and spectral analysis approach. Joint 2D forward models on three profiles were developed using GM-SYS of Oasis Montaj software. From the interpretation of the geophysical results, the following conclusions have been reached: (1) the crystalline basement is more raised around Salen ridge and west of it; (2) the main heat source of the geothermal system appears to be a central region of the studied area near Salen ridge and is estimated to be at 4-5 km depth, (3) under Gnaro obsidian dome, the basaltic and silicic volcanic horizon is thin, and the deep-seated regional fault serves as the main conduit for the passage of hot fluid to the surface and (4) the gravity and magnetic anomaly plots, regional-residual maps and enhanced data plots all indicate towards the existence of a major geological feature-a large caldera-comprising of Tulu Moye, Bora and Berecha volcanic centers.

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.