Author(s): Ovuru Cyril¹, Diepriye Okujagu², Amgbare Peace Ebikomboere³

Abstract:

Sustainable exploitation of groundwater resources in complex deltaic environments cannot rely on the usual practice of “blind drilling” but must instead be informed by scientific principles. In many parts of the Niger Delta, the heterogeneous nature of the Quaternary Benin Formation makes it difficult to differentiate between freshwater aquifers, brine pockets and aquitards using surface geophysics alone. This paper demonstrates how Vertical Electrical Soundings (VES) data can be correlated with nearby borehole lithological logs in order to improve the hydrostratigraphic characterization of the subsurface in Odi (Kolokuma/Opokuma LGA, Bayelsa State). The VES survey employed Schlumberger configuration with current electrode half-spacing (AB/2) ranging from 1 to 100 meters and achieved penetration depths exceeding 35 meters. Partial curve matching of the resulting resistivity sounding curves followed by one dimensional computer iteration using standard filter coefficients was used to interpret the VES data. The geoelectrical models were then constrained by comparing them with borehole logs obtained at the same site through rotary drilling. Data from VES, borehole logs and previous hydrogeological studies suggest that there are four to five layers in the stratigraphy including conductive topsoil (30–60 Ωm), clayey sand to silty clay aquitards (140–180 Ωm) and distinct fine to medium grained sand aquifers. The study found strong positive correlations between the coarse-grained sandy aquifers encountered at depths of 20–36 meters in borehole logs and areas of high resistivity (>800 Ωm) from the VES survey as well as longitudinal conductance values that are less than 0. 1 ohm-1 (indicating weak to moderate clay confining layer protection). It also revealed that non-uniqueness still affects geoelectrical inversion although boreholes reduce ambiguity in interpretations but do not eliminate it completely. In Odi (and other locations where similar conditions prevail), sustainable exploitation of groundwater in the Benin Formation requires an integrated approach to subsurface characterization involving both electrical resistivity soundings AND borehole lithological logs among other data. This study provides a framework for such integration, identifies depths at which good yields are likely to be obtained from discrete aquifers AND enhances understanding of local groundwater flow in deltaic sediments.

Keywords: Hydrogeophysics, Vertical Electrical Sounding, Aquifer Characterization, Lithostratigraphy, Niger Delta, Benin Formation, Longitudinal Conductance.

Download Full Article from below:

Hydrogeophysical Characterization of Aquifers in the Odi Depocenter, Niger Delta: An Integrated Approach Using VES and Lithology

Pages:1-11