Three scalar AMT campaigns were conducted in the mid-summer, late-fall, and late-winter of 1990 in two nearby localities of southern Rio Grande do SuI State, Brazil. Comparisons of the results obtained at each season highlighted the problem of seasonal variation in the upper crustal conductivity distribution in the area. The most striking fact in these temporal conductivity variation is the presence, in the last two campaigns, of a new deeper conductive layer (conductance of 6 to 7 S) at depths from 500 to 600 m, which was not observed in the mid-summer campaign. Given the general good quality of the analyzed AMT data, it seems reasonable to discard the possibility of source effects on the magnetotelluric fields as responsible for the observed seasonal variation of the apparent resistivity curves. Careful observation of the regional rainfall data indicated that another likely explanation for the seasonal conductive layer could be the occurrence of a transitory deep aquifer that would grow or wane in time in accordance to the climatological conditions.


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