Hydrogeologic, Electrical and Electromagnetic Measurements for Geotechnical Characterization of Foundation Beds at Afunbiowo, near Akure, Southwestern Nigeria

In order to evaluate layer competence for civil engineering foundation, engineering geophysical surveys (electromagnetic profiling and resistivity soundings), and hydrogeologic measurements, involving determination of depths to water table across wells, were conducted at Afunbiowo, near Akure, Nigeria. The EM raw real values vary from -44.0 to 49.9, while filtered real values vary from -43.8 to 48.5 across the study area. The Karous-Hjelt and Fraser filtered (KHF) pseudo-sections suggest the presence of clay and weathered materials in the central parts of the area. The interpretation of the geoelectrical data reveals that the near surface materials overlying the crystalline bedrock exhibit significant variations of resistivity values, ranging from 29 to 1400 ohm-m, while the bedrock resistivity ranges from 96 to 24729 ohm-m. This interpretation correlates with the high EM anomalies across the area. A pronounced low resistivity zone in the shallow subsurface is associated with saturated sands or clay rich sand. Depth to water table varies from 0.3m to 7.5m, with an average level of 4.5m. Water table occurs at shallow depths in the southwestern, central and north central segments of the area. Since a raised water table, or clay saturated with water may create a wet foundation and consequently engender geotechnical instability, building foundations in the southwestern, central and north central segments of the area must be founded on watertight concrete base.