Electrical resistivity tomography for the characterization of basement aquifers in a humid tropical environment: the start of the pilot site of the University of Yaoundé I (Cameroon)

Crystalline basement aquifers in tropical environments represent a strategic groundwater resource for rapidly growing urban areas. However, their functioning and vulnerability to contamination remain difficult to assess because deep weathering and inherited structures strongly modify the geometry and compartmentalization of saprolite and fractured horizons. This study presents first hydrogeophysical results from the pilot site of the University of Yaoundé I (UY1-PS) in Cameroon, located within Pan-African gneisses of the Yaoundé domain near the Sanaga Shear Zone. Shallow saprolite wells are widespread but often affected by sewage contamination, whereas springs and deep fractured-basement boreholes provide drinking water for users not connected to the distribution network. To assess long-term groundwater behavior, the monitoring of groundwater levels and hydrochemistry, spring discharges, and hydro-meteorological parameters at three stations was initiated in 2025 at UY1-PS.

High-resolution electrical resistivity tomography (ERT) surveys were acquired between 13 July and 13 August 2025 across the campus using a 4point light 10W resistivity meter in a Wenner–Schlumberger configuration. Inter-electrode spacing ranged from 5 to 10 m, and the spatial distribution of the profiles was constrained by campus infrastructure, including buildings, roads, and buried utilities. A total of thirteen 2-D ERT profiles were collected and inverted using RES2DINV. Most inversions yielded RMS misfits below 10% ; a limited number of profiles located in electrically noisy urban sectors displayed RMS values between 10% and 15%, acceptable for hydrogeological interpretation.

ERT images resolve a stratified weathering profile composed of a conductive saprolite horizon (about 20–200 Ω·m), a transition or saprock zone (approximately 200–800 Ω·m), and a more resistive fractured basement at depth (generally 250–1500 Ω·m), locally juxtaposed with fresh basement blocks exceeding 1800-2000 Ω·m. Sharp lateral resistivity contrasts may delineate vertical to sub-vertical fractured corridors of high transmissivity consistent with inherited tectonic control on aquifer compartmentalization .

These results constrain the internal organization of a tropical urban basement aquifer and support transferable hydrogeophysical workflows applicable to crystalline aquifer systems in humid tropical environment.

Keywords: Hydrogeophysics; Electrical resistivity tomography (ERT); hard rock aquifers; humid tropical environments; tropical urban areas, saprolite.

Acknowledgements

This work was supported by the IRD through the JEAI DELO project https://share.google/8qBqLFSOuMVO4yBIe. The corresponding author was supported by the Make Our Planet Great Again (MOPGA) postdoctoral program funded by Campus France. The authors acknowledge Geoazur (Université Côte d’Azur, CNRS, IRD) and the University of Yaoundé I for their scientific and logistical support.