Mapping freshwater lenses with TEM and SNMR: Case studies from Saloum region, Senegal

ABSTRACT Freshwater lenses encompassed in saline groundwater systems can be critical for society and ecology. Application of geophysical methods to discriminate the fresh/saline interface is becoming routine. However, without borehole control, relying on individual geophysical methods can lead to ambiguous interpretation, whether it is between saline pore fluids and clays, freshwater saturated and unsaturated sands, or simply fresh/saline water itself. In this study, we augmented transient electromagnetic (TEM) profiles with surface nuclear magnetic resonance (SNMR) in two regions of Senegal to uniquely identify and map freshwater lenses. The use of steady-state SNMR protocols allowed for dense coverage and robust estimates of the T2 relaxation time, providing a detailed characterization of the aquifer systems in the absence of borehole control. Specific to steady-state SNMR, these measurements marked the first successful demonstration operating in the relatively low geomagnetic fields associated with equatorial regions. Joint interpretation of the TEM and SNMR showed 10–20 m of freshwater perched on a saline aquifer base in one region, whereas the other showed saline groundwater extending over 6 km inland from the saline river. These case studies demonstrate the utility of a multi-method approach in data-scarce regions.