Seasonal Variation in Apparent Resistivity Probably Associated with Drastic Rainfall Changes in Southern Brazil

Three scalar AMT campaigns were conducted in the mid-summer, late-fall, and late-winter seasons of 1990 in two nearby localities of southern Rio Grande do Sul State, Brazil. Comparisons of the results obtained at each season highlighted the problem of seasonal variation in the apparent resistivity measurements in the area. From 1-D inversions, the most striking fact in this temporal variation is the presence, in the last two campaigns, of a new deeper conductive layer (conductance of 6 to 7S) at depths of the order of 500m, which was not observed in the midsummer campaign. Careful observation of the regional rainfall data indicated that the seasonal conductive layer could be generated by the occurrence of a transitory deep aquifer that would grow or wane in time in accordance to the climatological conditions. Another likely explanation could be in terms of a misinterpretation of the geoelectrical section due to the distortion of the MT curves by induction effects generated by a saturated surficial layer.