The subsurface structure of the Chalco sub‐basin (Mexico City) inferred from geophysical data

Abstract

The Chalco sub‐basin, the hydrogeological reserve of Mexico City, is located in the southern portion of Mexico City's basin, an intramontane valley located at 2200 m a.s.l. in the Trans‐Mexican Volcanic Belt (Central Mexico). Several geophysical studies (dc resistivity, seismic refraction, gravity, well‐logging) have been conducted in this area. The objectives of these independent surveys were different and of regional and local scopes. This study represents the first comprehensive effort to assimilate and interpret the data available to date from the area of the Chalco sub‐basin and to infer its main basinal characteristics. According to the topography we obtained of the basement, a northeast‐southwest structural high separates the sub‐basin into two portions. The western portion is asymmetric and deeper than the eastern one. An east‐west half graben‐like structure is located in the western portion. This structure could make possible an east‐west fluid flow at depth, and it also implies a differential subsidence. It was also possible to establish the stratigraphy of the volcano‐sedimentary infill. The gravity models give the interface: (1) between lacustrine sediments (aquitard) and the granular aquifer and (2) between the granular aquifer and the basaltic container. The resistivity boundaries correspond to the contact zone between saline and fresh water in the aquitard and semiconfined aquifer respectively, and they correlate with the existence of a vertical flow of saline fluids from the aquitard to the aquifer. The estimate of the volume of fluids contained in the granular aquifer range between 2 and [Formula: see text].