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Groundwater-yielding capacity, water-rock interaction, and vulnerability assessment of typical gneissic hydrogeologic units using geoelectrohydraulic method

Akingboye Adedibu Sunny, Bery Andy Anderson, Kayode John Stephen, Ogunyele Abimbola Chris, Adeola Adedapo Oluwasanu, Omojola Olumide Olubusayo, Adesida Adeola Samuel

Acta Geophysica

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2023

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Vol. 71, no. 2

697--721

EN

Geohydraulic parameters, namely hydraulic conductivity (K), transmissivity (T), effective porosity (φ), permeability (kp), anisotropy coefficient (λ), and longitudinal conductance (S), of aquifer units in Etioro-Akoko, southwestern Nigeria, were evaluated using the Schlumberger vertical electrical sounding (VES) technique. This study aimed to understand the hydrodynamics and water–rock interaction of the near-surface crustal architecture to determine the groundwater yield and vulnerability of the aquifer units in the study area. A total of 7 model curve types were generated for fifty-two geoelectrical surveyed points, with percentage distributions in the order of HA>AA>H>KH>A>HK>AK. The VES curve models constrained the subsurface layers into topsoil, weathered units, weathered/fractured bedrock units, and fresh bedrock. The weathered and fractured aquifer zones occurred at the depths of 8 m and>16 m (with depths exceeding 26.5 m for some sections). The K and T values for the aquifer units varied from 0.1901 to 0.6188 m/day and 0.7111 to 6.3525 m2/day, respectively. These parameters coupled with the aquifer φ (18.03–23.35%) and kp (0.028–0.089 µm2) classified the delineated aquifer units as low to moderate groundwater-yielding capacity aquifers, with recorded resistivity values between 85.1 Ω-m and<613.0 Ω-m. The observed positive correlations and R2 values with>32–100% prediction rates affirmed the dependence of K on T, φ, and kp for effective water–rock interactions and groundwater transmissibility. The recorded S values (0.0146–0.162 mhos) and low logarithm hydraulic resistance, Log C (0.89–1.75 years), suggested poor to weak aquifer protective capacity ratings, resulting in high aquifer vulnerability index delineated across the study area. As a result, deep-weathered/fractured aquifers should be exploited for sustainable potable groundwater supplies. However, intended wells/boreholes in the study area must be developed properly for long-term groundwater abstraction to alleviate potable groundwater deficit and optimize future operational drilling costs.

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Development of novel velocity–resistivity relationships for granitic terrains based on complex collocated geotomographic modeling and supervised statistical analysis

Akingboye Adedibu Sunny, Bery Andy Anderson

Acta Geophysica

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2023

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Vol. 71, no. 6

2675--2698

EN

The significance of velocity-resistivity relationships has been utilized in various geologic terrains and under different conditions. The approach is yet to be exploited in tropical granitic terrains, with no definitive empirical relationships being developed. The empirical relationships are critical for rapidly delineating subsurface petrophysical, geomechanical, hydrogeological, and soil-rock features. As a result, a novel approach has been used in this study to develop velocity-resistivity relationships for tropical granitic environments, combining complex collocated velocity (Vp) and resistivity (ρ) models with simple linear regression analysis. The granitic terrain of Penang Island, Malaysia, was chosen as the study area. The geotomographic results delineated three layers, which include the residual soils (topsoil and completely weathered granite), highly to relatively weathered granitic unit (including fractures), and integral/fresh granitic bedrock. Due to the complexity, ruggedness, and varying weathering and fracturing conditions of the subsurface lithologic units in tropical regions, the supervised regression modeling successfully developed a unified and other three specific velocity–resistivity empirical relations for the lithologic units. The derived velocity-resistivity empirical relations have high practical prediction accuracies to predict Vp data. The predicted Vp data and models from the velocity-resistivity relations had good lithological and structural correlations with their observed models. The overall performance of the results indicated that the velocity-resistivity empirical relations could delineate the subsurface geologic variabilities distinctively because they are resistivity-dependent. Hence, the developed comprehensive methodological and SLR workflows and the velocity-resistivity empirical relations were posited for use in granitic terrains with similar geology to the study area, especially in areas with shallow overburden.

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Electrical resistivity tomography for geo-engineering investigation of subsurface defects: a case study of Etioro-Akoko highway, Ondo State, southwestern Nigeria

Akingboye Adedibu Sunny, Osazuwa Isaac Babatunde, Mohammed Muraina Zaid

Studia Quaternaria

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2020

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Vol. 37, Nr 2

101--107

EN

The durability of roads is dependent on the proper screening of the variations in subsurface geological characteristics and conditions through geo-engineering investigations and good construction practices. In this study, electrical resistivity tomography (ERT) technique was used to investigate the subsurface defects and potential failures along the substrate of Etioro-Akoko highway, Ondo State, southwestern Nigeria. Results of the inverse model resistivity sections generated for the two investigated traverses showed four distinct subsurface layers. The shallow clayey topsoil, weathered layer, and partially weathered/fractured bedrock have resistivity values ranging from 4–150 ohm-m, 10–325 ohm-m, and 205–800 ohm-m, with thickness values of 0–2 m, 0.5–12.5 m, and less than few meters to > 24 m, respectively. The fresh bedrock is characterised by resistivity generally in excess of 1000 ohm-m. The bedrock mirrored gently to rapidly oscillating bedrock troughs and relatively inclined deep penetrating multiple fractures: F1–F'1, F2–F'2 and F3–F'3, with floater in-between the first two fractures. These delineated subsurface characteristic features were envisaged as potential threats to the pavement of the highway. Pavement failures in the area could be attributed to the incompetent clayey sub-base/substrate materials and the imposed stresses on the low load-bearing fractured bedrock and deep weathered troughs by heavy traffics. Anticipatory construction designs that included the use of competent sub-base materials and bridges for the failed segments and fractured zones along the highway, respectively, were recommended.