3D modeling of geological and geotechnical soil characterization using the MASW method: a case study in Southern Ijaw LGA, Bayelsa State, Nigeria

Abstract

The Multichannel Analysis of Surface Waves (MASW) method is pivotal for non-invasive subsurface shear-wave velocity profiling, essential in geotechnical and seismic investigations. This study aims to model 3D geological and geotechnical soil properties in Southern Ijaw LGA, Bayelsa State, Nigeria, using MASW. Data collection involved a 12-channel ABEM Terraloc Mark 6 geophone system across nine survey points, complemented by nine boreholes and two Standard Penetration Tests (SPT). The results revealed a shallow water table at 0.30 m and stratified soil layers ranging from soft clay to dense sands. SPT N-values increased from 2 at the surface to 34 in deeper layers, reflecting significant soil densification and strength enhancement with depth. Shear-wave velocities (Vs) ranged from 207.11 m/s at 0.87 m to 502.88 m/s at 12.9 m, while compressional-wave velocities (Vp) ranged from 388.71 m/s to 948.98 m/s. Poisson’s ratio was consistent at 0.3, indicating uniform properties across depths. Elastic moduli, including shear modulus (µ), Young’s modulus (E), and bulk modulus (K), increased with depth, indicating greater subsurface material stiffness. The 3D geological model delineated distinct layers: soft clay (0–3.61 m), silty clay (3.61–5.19 m), fine sand (5.19–6.55 m), medium sand (6.55–7.83 m), and medium to coarse sand (7.83–12.90 m). These layers exhibited specific engineering properties, with ultimate and allowable bearing capacities ranging from 154.38 to 543.71 kPa and 51.46 to 181.24 kPa, respectively. MASW-derived N-values showed a strong correlation with traditional SPT N-values (R2 = 0.9401) and shear wave velocities and SPT N-values (R2 = 0.961), confirming MASW's effectiveness for geotechnical characterization. This enhances the precision of 3D soil models and supports more informed engineering decisions. The findings significantly advance the understanding of soil properties, particularly in the Niger Delta.