Characterization of salt marshes subsoil using electromagnetic induction method: case studies in the Venice Lagoon

The study used electromagnetic induction method (EMI), to characterize the subsoil of the northern Venice Lagoon. The study area is a salt marsh located in a highly saline environment. In such conditions, the use of geophysical electrical methods is not deemed practical, as the high salt concentrations present in the environment result in increased electrical conductivity of the ground and reduce the contrast between conductive and resistive features. Despite the challenges posed by this highly saline environment, the use of a CMD mini explorer equipped with a fixed frequency of 30 kHz and three inter-coils spacing was able to effectively map the subsurface electrical conductivity, providing valuable information regarding the various subsurface layers, buried paleochannels, and the morphology of the tidal meander bend. The comparison of the inverted data to the observed data demonstrated the importance of inverting EMI data to obtain accurate estimations of the true conductivity and depth of the paleochannels. The result from the inverted data revealed these subsurface features in greater detail, particularly at the three study sites designated as Marsh 1, 2, and 3. The overall trend of the conductivity data shows that the conductivity values are lower within the channel when compared to higher values in the surrounding areas. The investigation of Marsh 1 and 3 revealed that Marsh 1 had a channel morphology of an abandoned channel with low conductivity values ranging from 0 to 250 mS/m. Meanwhile, Marsh 3 displayed a meander bend with conductivity values ranging from 100 to 400 mS/m within the channel. The plot of the inverted data from Marsh 2, corresponding to the fifth transect of the channel survey area, shows that the conductivity values range from 150 to 400 mS/m. For all three marshes studied, the results of the inverted data indicate that the channel depth is approximately 1 meter. The conductivity values within the channel of Marsh 3 are consistent with the 300 mS/m isosurface of the three-dimensional reconstructed meander bend. The high-resolution and accurate results obtained demonstrate the efficacy of EMI in providing detailed information on the subsurface features in highly saline environments. Additional EMI data can be gathered to broaden the survey area, or EMI data can be calibrated with other methods and ground truth, such as sedimentary cores, to give a more comprehensive understanding of the subsoil.