Time-lapse electrical resistivity tomography for assessment of seasonal moisture variations in a tropical regolith

Monitoring and quantifying hydrological flows in the vadose zone is complicated to analyze due to the effects of rainfall in the tropics, the dynamic interactions among rains, the vegetation layer, moisture in the soil, and the entire regolith. Quantifying subsurface hydrological flows at specific scales and high resolution presents further difficulties. To overcome these issues, resistivity methods can play an important role. This paper examines the results of gravimetric moisture content monitoring in the Panamanian tropics through time-lapse electrical resistivity tomography analysis. Changes in the electrical properties of soil were quantified through six tomographic tests performed between February 2012 and March 2013 along with a profile. Significant changes in resistivity were identified between February (dry season) and May, and August and October (rainy season), with negative percentages (-60%) indicating the effects of rain infiltration at the surface and positive percentages (60%) linked to moisture absorption in the soil, electrode relocation for each test or inversion processes. Additional laboratory analyses of soil samples were carried out to obtain gravimetric moisture content tomograms. The changes of this parameter in the subsurface horizons, and the percentage differences in the calculated resistivity values, are helpful for determining the impact of rain on the soils.