Rock mass quality characterization through integrated outcrop, petrographic, geomechanical, and geophysical methods of gabbronorites of Nawagai Mélange

Reliable rock-mass characterization in structurally complex terrain requires spatial information beyond conventional drilling and surface mapping. This study presents an integrated framework combining outcrop mapping, petrography, synthetic and field electrical resistivity tomography (ERT), and laboratory testing to characterize gabbronorites in the Nawagai Mélange. Outcrop mapping reveals a two-layer gabbronorite structure: an intensely weathered, locally deformed upper layer (L1) overlying a relatively fresh, competent layer (L2). Synthetic and field ERT models corroborate this stratification, with L1 defined by low resistivity and L2 by high resistivity. L1 is characterized by resistivity < 1000 Ω m, Kv of 0.15–0.55, RQD of 25–75%, UCS of 40.4–55.0 MPa, porosity of 0.19–0.28%, and water absorption of 0.07–0.10%, while L2 shows resistivity > 1000 Ω m (locally exceeding 10,000 Ω m), Kv of 0.55–1.00, RQD of 75–100%, UCS of 70.5–97.9 MPa, porosity of 0.05–0.27%, and water absorption of 0.02–0.09%. Petrographic analysis supports these results, revealing extensive mineral alteration, clay formation, and microfracturing in L1 as against the intact mineral fabrics preserved in L2. Overall, the fresh gabbronorite (L2) with its higher UCS, RQD, Kv, and resistivity alongside lower porosity reflects good rock-mass quality, whereas the weathered L1 remains mechanically weak. This multi-scale approach enhances subsurface characterization in data-scarce, drilling-limited regions and highlights the value of integrating multiple methods for robust rock-mass assessment in complex tectonic settings.