Geoelectrical Investigation of Basement Complex Areas of Lokoja, North-Central Nigeria

Geophysical investigation using vertical electrical soundings (VES) was carried out to determine aquifer types in the basement complex areas of Lokoja. The interpretation of vertical electrical sounding (VES) indicates that the aquifer types in the area are weathered basement aquifer and weathered/fractured (unconfined) aquifers. The weathered basement aquifer type was revealed by VES stations 1, 2, 4, 5, 6, 8, and 9 while weathered/fractured (unconfined) aquifer type was revealed by VES stations 3 and 7. Five out of nine (9) sounding curves show 4-layer geoelectric model and the characteristic geoelectric signatures are AA, HA type curves, while the remaining four (4) show 3-layer geoelectric model and the characteristic geoelectric signature is A-type curve. The thickness of the first layer (topsoil composed of clay and laterite) varies from 6m to 16m and the resistivity values vary from 20.00Ωm to 664.00Ωm. The weathered layer has thickness that varies from 3.1 m to 7.5 m and the depth is between 7 m and 18.3 m while the resistivity varies from 41 ohm-m to 469Ωm. This weathered layer constitutes the major aquiferous unit. The weathered/fractured layer, the second type of aquifer has thickness of between 3 m and 3.7 m and depth of between 17 and 19.2 m while the resistivity ranges from 277.6Ωm to 317.5Ωm. The resistivity values for competent/fresh basement vary from 491.17Ωm to 1947.17Ωm. This fresh basement which is the last layer is at a depth of 18-20 m.