Groundwater delineation using vertical electrical sounding data in Sargodha, Pakistan.

Abstract

In the realm of hydrogeology, conducting a thorough hydrogeologic evaluation is crucial for the efficient utilization of groundwater resources within an aquifer system. Throughout history, the process of obtaining hydraulic characteristics has heavily relied on costly and labor-intensive methods of collecting borehole data. As a consequence, the spatial coverage of these data has been limited, particularly in vast regions. However, the utilization of non-invasive geoelectric techniques offers a cost-effective and user-oriented alternative, hence reducing the need for several expensive boreholes in the exploration of groundwater reservoirs. A research investigation was carried out in the Sargodha Area of Chaj Doab, employing a total of 28 Schlumberger rigs for Vertical Electrical Sounding (VES) purposes. The primary aim of this study was to assess the hydrogeological characteristics of the subsurface and provide estimations pertaining to the groundwater reserves. The objective of this study was to demonstrate relationships between subsurface resistivity, water quality, and lithology by the correlation of subsurface resistivity with lithological data. The results of the study indicated clear patterns, such as the identification of silty clay with saline water displaying resistivity values below 20 Ω-m, the presence of sand with brackish water exhibiting resistivity values ranging from 25 to 100 Ω-m, and the occurrence of sand with gravel containing fresh water and resistivity values exceeding 100 Ω-m. The utilizations of electrical resistivity modelling in conjunction with borehole data has enabled the creation of cartographic depictions of subsurface lithological variations and aquifer thickness, while also facilitating an assessment of water quality. The existence of both lateral and vertical diversity in the underlying lithology has been identified, suggesting a significant likelihood for the infiltration of saline water into freshwater groundwater resources. The accurate identification and distinction of aquifer zones exhibiting fresh-saline features are of utmost importance for the purpose of accessing and utilizing potable freshwater supplies. Traditionally, the procedure for attaining this segregation has entailed the gathering and examination of groundwater samples from boreholes utilizing various research approaches, which can be both costly and time-consuming. xi The Dar-Zarrouck characteristics, which include transverse unit resistance, longitudinal resistivity, and longitudinal unit conductance, were employed to improve efficiency. Additionally, the Dar Zarrouck characteristics, specifically the Longitudinal Unit Conductance (S) and Transverse Unit Resistance (Tr), provide additional evidence that corroborates the existence of fresh and saline water in regions characterized by high and low resistivity, respectively. The transverse unit resistance (Tr) has a notably higher magnitude in the high resistivity zone, exceeding 6600 Ω- 𝑚2, whereas it exhibits a comparatively lower value in the low resistivity zone, ranging between 0 3000 Ω- 𝑚2. The longitudinal conductance (S) exhibits a modest magnitude. The high resistivity zone is commonly observed to fall within the range of 44 to 60 siemens, whereas the low resistivity zone demonstrates values spanning from 0 to 24 siemens. The enhancement of the conversion process from one-dimensional Vertical Electrical Sounding (VES) data to two-dimensional depictions of the adjacent subsurface has been greatly advanced through the use of modern display technologies. These models provide valuable insights that considerably improve our understanding of alluvium layers as distinct lithologic units within a two-dimensional spatial framework. In summary, the results of the study align closely with the prevailing hydrogeological knowledge pertaining to the particular area being exami