Study of riverine deposits using electromagnetic methods at a low induction number

We conducted electromagnetic EM profiles along the Po River in Turin, Italy. The aim of this activity was to verify the applicability of low-induction-number EM multifrequency soundings carried out from a boat in riverine surveys and to determine whether this technique, which is cheaper than aircarried surveys, could be used effectively to define the typology of sediments and to estimate the stratigraphy below a riverbed. We used a GEM-2 handheld broadband EM sensor operating with six frequencies to survey the investigated area. Ground-penetrating radar GPR, a conductivity meter, and a time-domain reflectometer were used to estimate the bathymetry and to measure the EM properties of the water.A global positioning system, working in real-time kinematic mode, tracked the route of the boat with centimetric accuracy. We analyzed the induction number, the depth of investigation DOI, and the sensitivity of our experimental setup by forward modeling — varying the water depth, frequency, and bottom-sediment resistivity. The simulations optimized the choice of the frequencies that could be used reliably for the interpretation. The 3406-Hz signal had a DOI in the Po River water 27 m of 2.5 m and provided sediment resistivities higher than 100 m.We applied a bathymetric correction to the conductivity data using the water depths obtained from the GPR data.We plotted amap of the river bottom resistivity and compared this map to the results of a direct sediment sampling campaign. The resistivity values 120–240 m were compatible with the saturated gravel and pebbles in a sandy matrix, which resulted from direct sampling and with the known geology.