Integrating TDEM and MT methods for characterization and delineation of the Santa Catarina aquifer (Chalco Sub-Basin, Mexico)

Abstract

Magnetotelluric (MT) and time domain electromagnetic (TDEM) surveys were undertaken in the region of Santa Catarina, located in the Chalco Sub-Basin of the Mexico Basin. The objective was to constrain the geometry of the fresh water aquifer and confirm the continuity of the basaltic flows between the volcano and the sedimentary basin. In order to define the stratification at depth with an emphasis on the geometry of the main aquifer, 11 MT and 5 TDEM soundings were recorded along a north–south profile. Interpretation of MT soundings show that the bedrock is located at a depth of at least 800–1000 m. Using TDEM apparent resistivity curves to constrain the high frequency MT data, three main layers were defined overlying the bedrock. These layers are, from the surface to bottom, a 20- to 40-m-thick layer of sands, ash and clay, followed by a very conductive 200-m-thick layer of sand and ash, saturated with highly mineralized water and, finally, a zone with gradually increasing resistivities, corresponding to the main aquifer. The TDEM data, the magnetic transfer functions and the 2D MT model also indicate that a shallow resistive structure is dipping, from the northwest, into the lacustrine deposits of the basin. This feature is likely to be highly permeable fractured basaltic flows, evidenced also in one of the water wells. To verify the presence of fractured basalts below the volcano ranges, 38 TDEM soundings were collected on the flanks of the Santa Catarina range. Layered models obtained from the TDEM soundings enabled an assessment of a major conductive zone (1–10 Ω m) at depth. Two hypothesis are envisaged and the nature of this zone is attributed either to a clayey layer or to fractured basaltic flows. If the latter possibility is confirmed, this continuous zone could provide a channel by which the water contaminated by the Santa Catarina landfill may leak into the basin.