Estimation of porosity and specific yield from vertical electrical sounding (VES) measurements in Anambra State, Southeastern Nigeria

Abstract

Estimation of porosity and specific yield was carried out from Vertical Electrical Sounding (VES) measurements in Anambra State, Southeastern Nigeria. The study area lies within longitudes 06° 38I 00IIE and 007° 15I 00IIE and latitudes 05° 42I 00IIN and 006° 45I 00IIN with an area extent of about 4844 km2 (1870 ml 2 ), underlain by formations within two geological basins. It falls within the Anambra River Basin. The dominant aquifer is the Nanka Sands with Ameki Formation, Ebenebe Sandstone and Amenyi Sands as sandy lenses. A total of four hundred and fifty-two (452) VES was carried out in over one hundred and seven towns (107) within the study area, employing the Schlumberger array configuration. Layer resistivities, thicknesses and depths were obtained using the computer program INTERPEX. Lithological inference from layer resistivity shows dominantly sand/sandstones with moderate shaley facies. The 2D regional maps for Iso-resistivity, Isopach, depth, formation resistivity factor (F), groundwater resistivity, porosity and specific yield were constructed. Iso-resistivity 2D map shows high layer resistivities within the central part of the study area, with corresponding appreciable thicknesses. These are possible potential groundwater zones. Based on the groundwater resistivity, values range from 350 to 800 Ωm for fresh water and 50 to 250 Ωm for non-fresh areas, possibly iron water. Formation resistivity factor F is variable and dimensionless. F ranges from 2 to 40. Porosity ranges from 10 to 30% within the sandy formations and 35 to about 60% within the shale/clay formations. Specific yield is varied in the study area with values ranging from 5 to 90%. The constructed 2D regional maps can serve as a useful guide for groundwater exploration, development and management.