STUDY OF THE SANTA-CATARINA AQUIFER SYSTEM (MEXICO BASIN) USING MAGNETOTELLURIC SOUNDINGS

A tensor magnetotelluric test survey was carried out in the region of Santa Catarina, located in the Chalco sub-basin of the Mexico Basin. T he objective was to define the stratification at depth with an emphasi s on the geometry of the main aquifer of that region which is partiall y known from DC resistivity soundings and drilling. High-quality magne totelluric soundings could be recorded in the immediate vicinity of la rge urban zones because the sub-surface is very conductive. Interpreta tion shows that the solid bedrock is located at a depth of at least 80 0 m to the south and 1300 m to the north; it could, however, be much d eeper. Using complementary DC resistivity sounding and well-logging da ta, three main layers have been defined overlying the bedrock. These l ayers are, from surface to bottom, an unsaturated zone of sand, volcan ic ash and clay about 10 m thick, followed by a very conductive (1.5 o hm.m) 200 m thick layer of sand and ash with intercalated clay, satura ted with highly mineralized water, and finally a zone with resistivity increasing gradually to 60 ohm.m. The investigated deep aquifer const itutes most of this third layer. It consists of a sequence of sand, gr avel, pyroclastites and mainly fractured basalts. MT resistivity sound ings and magnetic transfer functions also indicate that a shallow resi stive structure is dipping, from the northwest, into the lacustrine de posits of the basin. This geologic feature is likely to be highly perm eable fractured basaltic flows, which provide a channel by which water contaminated by the Santa Catarina landfill may leak into the basin.