Performance of light fixed-wing airborne time-domain electromagnetic system for mapping the near-surface cover layer in an alluvial plain context: A numerical study

The mapping of the vertical and lateral variations in the physical properties of the few-meter cover layer over near-surface aquifers is important for hydrogeological modelling, particularly for the quantification of the recharge of groundwater systems. The first ground-based time-domain electromagnetic survey over a small catchment (Avesnelles, France) of the watershed of Orgeval (Seine basin) was carried out to determine discontinuities in the first silt layer as well as in the Brie multilevel aquifer limestone horizon. The results highlighted the following: (1) a good sensitivity of the time-domain electromagnetic survey to the presence of multi-decametric resistive sand lenses, particularly in a location where they were previously identified and (2) the interest in conducting a survey at a fine sampling step but extending to the meso-scale. To overcome the sampling issue over a watershed of several hundred square kilometres, we proposed numerically assessing the use of a prototype of low-cost airborne transient electromagnetic systems towed by light fixed-wing airplanes (with transmitting and receiving loops in the same plane). The present numerical analysis, in 1D for the vertical (i.e. thickness) variation and in 3D for the lateral extensions of localized sandy and resistive units, showed that a conductive few-meter cover can be mapped even with a system flying at 50 m with, however, the need of a priori constraint on the resistivity of the first layer to estimate its thickness variation as accurately as possible. Even if it did not bring more sensitivity to the layer thickness and despite the severe difficulty of practical implementation with a decametric emission loop, the vertical co-planar configuration potentially offered better near-surface lateral resolution (down to similar to 40 m) to delineate the sandy units (discontinuities) within the silt layer (if units are at least 50 m in size) and provided better spatial constraints compared to the classical horizontal co-planar geometry used in the time-domain electromagnetic. Even if not aerodynamically in the plane of the emission loop, the measurement of the H-x component with a vertical dipole emission loop (PERPxz geometry for perpendicular) improved the lateral resolution (down to similar to 20 m; still with at least 50 m size sand units) and confirmed that a geometry different from the classical horizontal co-planar configuration could be valuable.