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In a case history from the Mahanadi basin (India), we demonstrate the use of resistivity data from electric-log soundings and from borehole logs to discriminate between saltwater and freshwater aquifers. We use interpreted data from eight surface-based vertical electrical soundings (VES) and electric well logs from three boreholes in this study. We establish a quantitative relation among longitudinal unit conductance S (obtained from VES), water resistivity [Formula: see text], and layer thickness h. We show that ambiguities in resistivity data interpretation limit its ability to distinguish between freshwater and saltwater aquifers. Electric well-log data interpretation is much more accurate but requires boreholes, which are not cost effective when exploring for groundwater. Integrating well-log-based estimates of [Formula: see text] into resistivity interpretation of surface-based soundings improves its ability to discriminate freshwater aquifers while maintaining cost-effective exploration.

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The resolution of the freshwater and saline water aquifers in a coastal terrain (Mahanadi Basin, India) is updated. We analysed electrical borehole log data at four sites and compared the water resistivity regime of the freshwater and saline water zones obtained from electrical borehole logging, with the resistivity regime obtained by interpreting vertical electrical sounding (VES) data. The multilayer VES data interpretation is modified to a simple model, containing only the freshwater zone and the saline water zone. The composite geophysical parameters of the freshwater and saline water zones, in particular the resistivity and longitudinal unit conductance regime, are identified. The resolution obtained from the composite geophysical data analyses is very clear and convincing. The composite longitudinal unit conductance regime of the saline water zones is very high compared to that of the freshwater zones. This makes the identification of the two aquifers easy and increases its reliability. A technique which enables analysis of composite geophysical data of freshwater and saline water zones at VES sites in the vicinity of the borehole log sites is proposed. The significance of longitudinal unit conductance in resolving the freshwater and saline water aquifers is illustrated graphically. The proposed technique is validated by correlating the longitudinal unit conductance and resistivity with the total dissolved solids. The efficiency of the technique is validated by carrying out discriminant function analysis.Key words composite water resistivity; composite resistivity; composite longitudinal unit conductance; composite total dissolved solids (TDS); vertical electrical sounding (VES) Caractérisation d'aquifères d'eaux douce et salée par des méthodes d'analyse de données géophysiques composites Résumé Cet article améliore la caractérisation des aquifères d'eaux douce et salée dans une zone côtière (bassin de Mahanadi, Inde). Nous avons analysé les diagraphies électriques de quatre sites et avons comparé les régimes de résistivité de l'eau (Rw) des zones d'eau douce et d'eau salée obtenus par diagraphie, avec les régimes de résistivité obtenus par interprétation de sondages électriques verticaux (SEV). L'interprétation des données SEV multicouches est modifiée en un modèle simple contenant seulement la zone d'eau douce et la zone d'eau salée. Les paramètres géophysiques composites des zones d'eau douce et d'eau salée, en particulier la résistivité et la conductance longitudinale unitaire, sont identifiés. La solution obtenue à partir de l'analyse des données géophysiques composites est très claire et convaincante. Le régime de conductance longitudinale unitaire composite de la zone saline est très élevé par rapport à celui de la zone d'eau douce. Cela rend l'identification des deux aquifères facile et plus fiable. On propose une technique permettant l'analyse des données géophysiques composites des zones d'eau douce et d'eau salée pour les sites de SEV à proximité des sites de diagraphie. L'importance de la conduc...