Groundwater investigation research is increasing owing to its prominence in setting development plans for livelihood, industrial and agricultural sectors. The rapid population growth has led to the development of new methods for groundwater exploration since it has become one of the world's primary water supply sources. Due to variations in geological and hydrogeological conditions, groundwater resources are not shared evenly across the globe, particularly in shallow formations, where groundwater frequently occurs (Farrag et al. 2019). In Khartoum, groundwater covers more than 52% of the total water supply deficiency, mainly for irrigation (Mohammed et al. 2022). The remaining needs are provided by surface water sources, mainly from the Nile River. Since Nile River water is highly turbid and the cost of pumping Nile water to agricultural lands is relatively high, people rely heavily on groundwater. The absence of a public water delivery system resulted in several challenges, such as ineffective groundwater discharge, decreased production dependability, and degrading groundwater quality (Farah et al. 2000). Therefore, groundwater aquifer delineation and parametrization are crucial for the sustainable management of groundwater resources.
Petrophysical and hydraulic parameters of groundwater aquifers are crucial elements that must be considered for managing groundwater extraction. Precise assessment of these characteristics can aid in improving the prediction of aquifer behavior under hydraulic stresses and discharge and gives reliable inputs for the simulation of groundwater flow and contaminant movement (Amiri et al. 2022). The most frequent parameters for groundwater aquifers evaluation are hydraulic conductivity, porosity, permeability, and shale volume. Hydraulic conductivity, as an essential factor, measures how easily water can flow via the pores and voids of the geological formation. Generally, the hydraulic conductivity in porous materials is correlated with grain size and porosity. Additional textural characteristics, including cementation exponent or tortuosity factor, are frequently considered (Szabó, 2015a). The ideal manner for estimating hydraulic and petrophysical parameters in hydrogeological problems is by taking core samples (Rajaobelison et al. 2021) from groundwater aquifers during wells drilling and analyzing them using one of the available and frequently applied methods for parameter estimation. Another approach is pumping tests using either constant discharge or step drawdown tests. Unfortunately, employing lab analysis or pumping test for hydrogeological and petrophysical parameters estimation is costly and time consuming. Pumping test methods are dependent on a series of assumptions that occasionally may not be valid. (De Clercq et al. 2020). It assumes that the aquifer is homogenous, isotropic, and has infinite extent and neglects groundwater storage during the test. Moreover, in heterogeneous aquifer systems, the estimation of these parameters is restricted to the measurement point; therefore, the error is inevitable in result generalization. To address these drawbacks, geophysical methods are widely applied for groundwater exploration and aquifer characterization (Alabi et al. 2021; Amiri et al. 2022; Ebong et al. 2014; Farrag et al. 2019) since they provide a continuous estimate for hydrogeological and petrophysical parameters. In this research, geophysical well-logging techniques are used to fulfill these goals.
The primary purpose of open-hole wireline logging, as a component of in-situ assessments, is to identify variations in petrophysical and hydrogeological parameters along groundwater wells and to match these differences with those of surrounding wellbores. Geophysical inversion or deterministic techniques can be used to determine the petrophysical parameters. Most deterministic methods are based on the separate examination of well-logs. A more accurate estimate can be made by combining all available data from several well-logs and invert them together in one interpretation procedure (Szabó et al., 2015b). Three well logs are presented in this paper. The spontaneous potential measurements capture the electrical potential created by two electrodes, one fixed on the surface, and the other moved through the geological formation due to water formation and ion-selective membrane (shale). Natural gamma-ray logs measure the radioactive decay of atoms and rocks. The gamma ray intensity of rocks is depending on the quantity of the radiating element and the density of geological materials. It is influenced by the presence of potassium, uranium, and thorium. Spontaneous potential and natural gamma ray logs are used to distinguish between non-pervious materials such as shales and potentially pervious rocks like sandstone, thus, setting up lithological boundaries. It can be further used to estimate shale volume to help in lithological correlation. Resistivity logs measure the capacity of the rocks and soils to prevent the passage of direct electrical current through them. Various resistivity techniques with varying depths of analysis and resolution can be employed to identify the mud infiltration pattern and calculate water saturation (Szabó et al., 2015c). Non-focused resistivity tools are often suited for hydrogeological problems. The resistivity of the mud-invaded zone is measured using short resistivity tools, whereas the non-invaded zones are observed with long resistivity probes. The aforementioned well logs satisfy the criteria in a typical situation; however, several other cutting-edge methods could provide additional details. For instance, neutron porosity measures the amount of hydrogen ions by employing a neutron source emitting high-energy neutrons directed toward the geological formation. It is always used for total porosity determination. Gamma-gamma density logging records the rocks' bulk density, which is highly dependent on the porosity and densities of the rock matrix and pore fluids. Nuclear magnetic resonance logs directly estimate permeability, pore-size distribution and hydraulic conductivity (Walsh et al. 2013).
Hydrogeological investigations frequently use non-invasive methods of surface electrical resistivity. These methods are typically implemented in exploratory investigations to install groundwater wells to prevent breakdowns, typical for wells constructed using geological data or nearby existing boreholes (Hezzi et al. 2021). Vertical electrical sounding is the most popular resistivity profiling approach for defining potential groundwater zones and calculating hydraulic parameters (Arétouyap et al. 2019). This technique was efficiently used to quickly and affordably determine the depth of the water table, aquifer thickness, vertical and horizontal geological variation, and hydraulic characteristics (George et al., 2015; Mohammed, 2016; Szűcs et al., 2021; Tijani et al., 2021). Based on the variations in petrophysical and hydrogeological parameters, VES techniques are used in groundwater investigations to identify abrupt changes in the underlying geological strata.
Due to urbanization and population growth, a severe freshwater supply shortage has been faced in the current circumstances. It became expensive to purify Nile River water, especially during the rainy seasons. As a result, the government agency represented in ministry of water resources is currently focusing on improving groundwater reserves to cover the water supply deficit. Therefore, accurately estimating hydraulic parameters by considering the groundwater aquifer complexity is crucial for sustainable groundwater supply. Considerable studies have been conducted to evaluate hydraulic parameters of the Nubian aquifer; nevertheless, these studies rely solely on pumping test analysis. This study aims to look into and provide additional details about the potential for groundwater in various zones concerning their lithological, petrophysical, and hydraulic features. This goal is meant to be achieved by comprehensively examining and interpreting all available lithological, hydrogeological, and geophysical data.