The Hydraulic Characteristics Of Mbano And Environs, Imo River Basin, Nigeria Using Geoelectric Data

ABSTRACT

Hydro-geophysical study of Mbano and environs, Southeastern Nigeria was carried out with the objective of estimating aquifer hydraulic parameters from resistivity data. Thirty-six (36) vertical electrical resistivity (VES) sounding data using the Schlumberger array were acquired with a maximum current electrode spread of 1000m using the ABEM Terrameter SAS 4000. Out of the thirty-six (36) VES soundings, fourteen (14) parametric soundings were carried out at the locations of existing boreholes where pumping test data are available for the purposes of calibration and correlation. The acquired data were processed in other to generate layer parameters using a combination of curve matching and computer iterative modeling techniques. Geo-electric curve types identified in the study area ranges from simple KQ, AQ, and QH curves to complex KQH, KHQ types. The general sequence of the curves suggests alternate sequence of resistive-conductive layers. Results of the study revealed that the aquifer resistivity in the area varies from 772 Ωm to 5000 Ωm with a mean value of 2521 Ωm. The depth to water table varies from 25 m around Umuduru Egbeaguru to 150m at Umunumo 1. The aquifers of the study area highly variable in thickness with values ranging from 5 m at VES 35(Umuduru Egbeaguru) to 97.8 m at VES 18(Ezumoha) with average value of 41.78m. Estimates of the Dar-zarrouck parameters revealed that the transverse resistance values ranges from 3860 to 171,171 Ωm2 with a mean value of 116,386.6 Ωm2 . Similarly, the longitudinal conductance ranges from 0.0072 Sm to 0.01884 Sm with an average value of 0.0478Sm. Information extracted from resistivity and Darzarrock parameters were used to estimate aquifer hydraulic parameters. The hydraulic conductivity across the area ranges from 1.04 m/day to about 12.74m/day, while the transmissivity ranges between 5.2 m2 /day at Umuduru Egberaguru to 564.85 m2 /day at Ikpa Mgbeke. Information extracted from iso-resistivity models, geo-electric cross-sections, litho logs, and electric logs of the study area revealed three distinct aquiferious zones: an unconfined aquifer at a depth of about 25m—34m, semi-confined aquifer at a depth of 50— 70m and a confined aquifer at a depth of 150—165m within the limit of the probe. The north-eastern part of the area is believed to have geo-materials with high aquifer potentials.